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prediction of sunspots has been an everlasting interest in the space science community since the discovery of the sunspot cycle. the sunspot number is an indirect indicator of many different solar phenomena, e.g., total and spectral solar radiation, coronal mass ejections, solar flares and magnetic active regions. its cyclic variation can even be used as a pacemaker to time different aspects of solar activity, solar wind and resulting geomagnetic variations. therefore, there is considerable practical interest in predicting the evolution of future sunspot cycle(s). this is especially true in today's technological society where space hazards pose a significant threat, e.g., to satellites, communications and electric grids on ground have been recognized. another interest to predicting sunspots arises from the relatively recently recognized influences of variable solar radiation and solar wind activity on earth's climate system.there are a variety of methods developed for predicting sunspots ranging from statistical methods to intensive physical simulations. some of the most successful, yet relatively simple, methods are based on finding precursors that serve as indicators for the strength of the coming solar cycle. these methods are often based on statistics of all past solar cycles. however, most of these methods do not typically take into account the 22-year hale cycle of solar magnetism, which is well known in different solar and geomagnetic phenomena.here we study the prediction of even and odd numbered sunspot cycles separately, thereby taking into account the hale cyclicity of solar magnetism. we first show that the temporal evolution and shape of all sunspot cycles are extremely well described by a simple parameterized mathematical expression. we find that the parameters describing even sunspot cycles can be predicted quite accurately using the sunspot number 41 months prior to sunspot minimum as a precursor. the parameters of the odd cycles can be best predicted with geomagnetic maximum geomagnetic aa index close to fall equinox within a 3-year window preceding the sunspot minimum. cross-validated hindcasts indicate that our method has a very good prediction accuracy. for the coming sunspot cycle 25 we predict an amplitude of 171 +/- 23 and the end of the cycle in september 2029 +/- 1.9 years. we are also able to make a rough prediction for cycle 26 based on the predicted cycle 25. while the uncertainty for the cycle amplitude is large we estimate that the cycle 26 will likely be stronger than cycle 25. these results suggest an increasing trend in solar activity for the next two decades. | prediction of even and odd sunspot cycles: implications for cycles 25 and 26 |
to reliably predict the space weather due to the impact of solar transients, it is imperative to accurately model the solar wind background into which these solar transients propagate. the alfven wave solar atmosphere model (awsom) within the space weather modeling framework (swmf) is a physics-based 3d extended mhd model to simulate the solar wind conditions. awsom includes radiative cooling, heat conduction and turbulence due to non-linear interaction between forward-propagating and reflected waves. it is driven by observations of the photospheric magnetic field which provide the magnetic field at the inner boundary for the model. we use data-products from different sources like, gong and hmi along with adapt magnetograms to investigate how the model results vary based on the initial driving condition. observation inaccuracies in the magnetic field measurements may lead to uncertainties in the modeling of the plasma environment into which cmes propagate. these modeling efforts are validated by both in-situ and remote observations including euv observations in the low corona from stereo-a/b and sdo-aia as well as plasma parameters at l1 from the omni database.awsom results are also dependent on a few free parameters that vary depending on the phase of the solar cycle. for example, during periods of higher solar activity the poynting flux parameter at the inner boundary needs to be adjusted to match the observations well to model the solar wind background and provide correct initial conditions for cme propagation. this work is in preparation for simulating cmes launched from the sun and propagating into correct solar wind background to achieve accurate and reliable space weather modeling and prediction. | modeling and validating the solar wind using the alfven wave solar atmosphere model |
sunquakes are seismic waves observed on the sun's photosphere connected to the occurrence of some large solar flares. the exact physical mechanism that causes sunquakes is still a topic of discussion, but it is thought to be related to the release of magnetic energy during a flare. this study investigates the temporal and spatial evolution of permanent changes in the magnetic field geometry associated with sunquakes. we analyze a sample of seven acoustically active flaring events that occurred during solar cycle 24. the data used for this analysis were higher cadence (135 s) vector magnetograms acquired with the sdo/hmi instrument. we find that the highly energetic flaring events under investigation may inject enough energy into the photosphere and sub-photosphere to generate acoustic responses observed as sunquakes. by analyzing the changes in magnetic field geometry and strength associated with these events, we aim to shed light on the physical mechanisms that lead to the generation of sunquakes. this work represents an important step towards a better understanding of sunquakes and their underlying physical mechanisms. it also contributes to the development of more accurate models and simulations of these phenomena. ultimately, this could lead to a better understanding of the physics of solar flares. | the role of photospheric magnetic field variations in sunquake generation |
the sun's activity varies over timescales ranging from the very short to the very long. long-term solar activity variations impact the earth's atmosphere and climate and have implications for space mission planning and life-time estimates. this variability across different scales is driven by solar magnetic fields which originate in the deep convection zone, emerge through the surface and evolve. understanding the physical basis for long-term variability over decadal scales and longer is important for developing predictive models for sunspot cycle which is an outstanding challenge. concurrently, exploring the basis of fluctuations that lead to extreme episodes such as grand maxima in solar activity remains an important exercise with no clear indication that such episodes are predictable. we shall review here the current state of our understanding of long-term solar variability, and identify challenges that are expected to spur future developments in this field. | long-term solar variability and solar cycle predictions: current state of understanding |
charged-current scattering of νe below 100 mev from 16 o nucleus is not yet measured. this interaction is a νe detection channel for water cherenkov detectors in case of a supernova burst. furthermore, in super-kamiokande diffuse supernova neutrino background (dsnb) is being searched with the inverse beta decay process and νe -16 o interactions from atmospheric neutrinos are a background to this search. finally, atmospheric neutrinos at this energy range will be a background for the future wimp dark matter searches via coherent elastic neutrino-nucleus scattering, and νe -16 o interactions are a way to probe the atmospheric νe flux at low energies to better estimate this background. a study for the first observation of this interaction from atmospheric νe with 20 years of super-kamiokande data is currently underway, with the goal of measuring atmospheric νe flux weighted cross section below 100 mev. for this purpose, a custom event generator that can accurately simulate products of νe -16 o and νe -16 o interactions has been built, and now methods to separate signal from the backgrounds are being studied. in this talk, both the event generator and the status of the analysis will be discussed. | νe - 16 o interactions in super-kamiokande with low energy atmospheric neutrinos |
charged-current scattering of νe below 100 mev from 16o nucleus is not yet measured. this interaction is a νe detection channel for water cherenkov detectors in case of a supernova burst. furthermore, in super-kamiokande diffuse supernova neutrino background (dsnb) is being searched with the inverse beta decay process and νe interactions from atmospheric neutrinos are a background to this search. finally, atmospheric neutrinos at this energy range will be a background for the future wimp dark matter searches via coherent elastic neutrino-nucleus scattering and νe -16o interactions are a way to probe the atmospheric νe flux at low energies to better estimate this background. a study for the first observation of this interaction from atmospheric νe with 20 years of super-kamiokande data is currently underway, with the goal of measuring atmospheric νe flux weighted cross section below 100 mev. for this purpose, a custom event generator that can accurately simulate products of νe -16o and background interactions has been built. based on the output of this event generator and detector simulations, visible energy distributions and probability of gamma and neutron emissions in super-kamiokande for νe -16o and background interactions are obtained. currently statistical methods to use this information to extract number of νe -16o events in the super-kamiokande data are being studied. in this talk, both the event generator and the current status of the analysis will be discussed. i thank department of energy for their funding. | search for νe -16o interactions from low energy atmospheric neutrinos in super-kamiokande |
astronomers can get a better view of more distant supernova explosions by searching for high-energy neutrinos, according to a study conducted by researchers at uppsala university in sweden.detecting ghostly particles from collapsing starsthe icecube laboratory at the amundsen-scott south pole station in antarctica. [felipe pedreros, icecube/nsf]a core-collapse supernova represents the cataclysmic end of a massive stars life. as the star collapses, its iron core disintegrates, producing vast numbers of sub-atomic particles called neutrinos that stream outwards in unimaginable quantities somewhere around 10 billion trillion trillion trillion trillion of them. their outwards pressure blasts the star apart and detonates the supernova.there are various neutrino detectors scattered across the planet, including icecube in antarctica. it can detect the neutrinos produced by core-collapse supernovae, which typically have energies in the mega-electronvolt range. however, the way its detectors work means that supernova neutrinos from farther away than the magellanic clouds the largest satellite galaxies of the milky way are too faint for it to see.circumstellar collisions and choked jetsnora valtonen-mattila and erin osullivan (both uppsala university) think that there may be higher-energy neutrinos on offer, which would extend that range to more than a hundred million light-years. that would include supernovae in many other galaxies and local galaxy clusters.these neutrinos in the giga- to tera-electronvolt range arent produced by the supernova itself, though. instead valtonen-mattila and osullivan consider two alternative production mechanisms.before stars go supernova they tend to eject vast amounts of material in fierce stellar winds. when the supernova detonates, the debris from the explosion hits this circumstellar material. protons collide like they do in particle accelerators here on earth and should produce high-energy neutrinos in the process. however, these neutrinos have yet to be observed.a similar thing could be achieved through a jet of material that becomes trapped behind the stars outer shell. this is called a choked jet. ordinary particles may be trapped, but neutrinos are famously ghost-like and can stream through dense material with ease. in fact, a light-year of lead would only have fifty-fifty chance of stopping a neutrino.the predicted number of observable neutrinos in the northern sky based on the choked-jet model (top) for two detection methods. the bottom panel shows the cumulative number of supernovae observed by the zwicky transient facility each year. [adapted from valtonen-mattila and osullivan 2023]expanding our horizonsits these high-energy neutrinos from choked jets that offer the greatest possible extension to icecubes range, pushing it out to 277 million light-years in the northern sky and 65 million light-years in the southern sky. however, choked jets are rare, with just 14% of core-collapse supernovae thought to have one.these are important considerations, particularly in light of the ongoing upgrade to icecube-gen2, which should be completed by 2033. perhaps then well have a clearer picture of core-collapse supernovae and the neutrinos they produce.citationprospects for extending the core-collapse supernova detection horizon using high-energy neutrinos, nora valtonen-mattila and erin osullivan 2023 apj 945 98. doi:10.3847/1538-4357/acb33fthe post more supernova detections may be on the horizon with high-energy neutrinos appeared first on aas nova. | more supernova detections may be on the horizon with high-energy neutrinos |
charged-current scattering of νe below 100 mev from 16o nucleus is not yet measured. this interaction is an important background for diffuse supernova background searches (dsnb) with inverse beta decay process in water cherenkov detectors, a useful νe detection channel in case of a supernova burst, and a possible way to probe atmospheric neutrino flux at low energies that will be a background for the future wimp dark matter searches. a study for the first observation of this interaction from atmospheric νe with 20 years of super-kamiokande data is currently underway, with the goal of measuring atmospheric νe flux weighted cross section below 100 mev. for this purpose, a custom event generator that can accurately simulate products of νe - 16o and νe - 16o interactions has been built, and now methods to separate signal from the backgrounds are being studied. in this talk, both the event generator and the current status of the analysis will be discussed. | search for νe - 16o interactions from low energy atmospheric neutrinos in super-kamiokande |
hyper-kamiokande is a proposed next-generation water cherenkov detector. if a galactic supernova happens, it will deliver a high event rate ({\mathscr{o}}(105) neutrino events in total) as well as event-by-event energy information. recent supernova simulations exhibit the standing accretion shock instability (sasi) which causes oscillations in the number flux and mean energy of neutrinos. the amplitude of these oscillations is energy-dependent, so the energy information available in hyper-kamiokande could be used to improve the detection prospects of these sasi oscillations. to determine whether this can be achieved in the presence of detector effects like backgrounds and finite energy uncertainty, we have started work on a detailed simulation of hyper-kamiokande’s response to a supernova neutrino burst. | simulating fast time variations in the supernova neutrino flux in hyper-kamiokande |
the icecube neutrino observatory is designed to detect high-energy neutrinos of astrophysical origin via cherenkov radiation detectors located deep within the antarctic ice. the wavelength-shifting optical module (wom), a cherenkov light detector with scalable photosensitive area and very low noise has been proposed to replace current detectors at icecube in order to detect lower energy astrophysical neutrinos from extragalactic supernovae. the matryoshka wom, one possible style of wom, consists of one wom within a larger wom in order to increase the overall sensitivity of the detector. i will present the methods used in a series of simulations made with the reactor analysis tool (rat) to simulate the optical efficiency of a variety of possible matryoshka wom configurations. i will also present the results of these simulations demonstrating how the optical efficiency of the matryoshka wom compares with similar cherenkov radiation detectors. | simulated optical efficiency of the icecube matryoshka wom |
the liquid argon time projection chamber (lartpc) is a unique technology well suited for large scale neutrino detectors. it allows for millimeter scale 3d precision particle tracking and calorimetry with good de/dx resolution, which provides excellent efficiency of particle identification and background rejection. while studies of detector response to high energy events have begun, there has been no direct demonstration of lartpcs capabilities in producing ground breaking physics results with solar and supernova low-energy neutrinos. we aim to facilitate the development of low-energy lartpc capabilities by developing the first 1-10 mev calibration subsystems for large lartpcs. in this thesis, i will provide an introduction to neutrino physics, discuss standard detection methods including those used in lartpcs, review the low-energy lartpc calibration designs we are developing, and present the work that lays the foundation for the development of such calibration. | probing mev physics in lartpcs with radioactive calibration sources |
detection of the diffuse supernova neutrino background (dsnb) is of great importance, which will greatly help the understanding of both core-collapse physics and neutrino physics. however, after tens of years' effort of super-kamiokande (sk), dsnb is still hidden in the remaining backgrounds, dominated by decay electrons from invisible muons induced by atmospheric νμ and prompt electrons from atmospheric νe interactions. in this work we explore the underlying physics of the dominant backgrounds and propose new detection methods for dsnb observation, for both current sk and future sk-gd, which has the neutron-tagging ability. these methods, if adopted by sk, will greatly improve the detectability of dsnb. bz and fb are supported by nsf grant phy-1714479. bz is also supported by osu graduate fellowship. | distinguishing dsnb signal from atmospheric neutrino backgrounds in super-kamiokande and future sk-gd |
sbnd is a liquid argon detector being constructed along the fermilab booster neutrino beamline. as a part of the short baseline neutrino program, it will attempt to resolve the miniboone low energy excess hinting at possible oscillations into sterile neutrinos. sbnd will install a light detection system with a much higher expected light yield than previous argon neutrino experiments. this will enable scintillation light to play a key role in measuring the properties of neutrinos, and improve the sensitivity to interesting low energy physics such as supernova neutrinos or nucleon decay. a challenge for low energy measurements in large liquid argon detectors is the contribution from 39ar, which being present in atmospheric argon, provides a steady source of scintillation light. i will present studies to develop methods of reducing the impact of 39ar backgrounds while maintaining sensitivity to low energy physics signals. | scintillation light background discrimination in the sbnd experiment |
core-collapse supernovae (ccsne) are among the most energetic processes in our universe and are crucial for the understanding of the formation and chemical composition of the universe. the precise measurement of the neutrino light curve from ccsne is crucial to understanding the hydrodynamics and fundamental processes that drive ccsne. the icecube neutrino observatory has mass-independent sensitivity within the milky way and some sensitivity to the higher mass ccsne in the large and small magellanic clouds. the envisaged large-scale extension of the icecube detector, icecube-gen2, opens the possibility for new sensor design and trigger concepts that could increase the number of neutrinos detected from a ccsne burst compared to icecube. in this contribution, we study how wavelength-shifting technology can be used in icecube-gen2 to measure the fast modulations of the neutrino signal due to standing accretion shock instabilities (sasi). | prospects for the detection of the standing accretion shock instability in icecube-gen2 |
the icecube neutrino observatory is uniquely sensitive to mev neutrinos emitted during a core-collapse supernova. the supernova data acquisition system (sndaq) monitors in real-time the detector rate deviation searching for bursts of mev neutrinos. we present a new analysis stream that uses sndaq to respond to external alerts from gravitational waves detected in ligo-virgo-kagra. | realtime follow-up of external alerts with the icecube supernova data acquisition system |
the icecube observatory at the south pole has been operating in its full configuration since may 2011 with a duty cycle of about 99%. its main component consists of a cubic-kilometer array of optical sensors deployed deep in the glacial ice designed for the detection of high-energy astrophysical neutrinos. a surface array for cosmic ray air shower detection, icetop, and a denser inner subdetector, deepcore, significantly enhance the capabilities of the observatory, making it a multipurpose facility. this list of contributions to the 38th international cosmic ray conference in nagoya, japan (july 26 - august 3, 2023) summarizes the latest results from icecube covering a broad set of key questions in physics and astrophysics. the papers in this index are grouped topically to highlight icecube contributions related to high-energy neutrino and multi-messenger astrophysics, cosmic-ray physics, low-energy neutrino transients such as galactic supernovae, fundamental physics, detector calibration and event reconstruction, education and public outreach, and research and development for the icecube upgrade, a scheduled dense sensor infill complemented by calibration devices. contributions related to icecube-gen2, the future extension of icecube, are available in a separate collection. | the icecube collaboration -- contributions to the 38th international cosmic ray conference (icrc2023) |
the next galactic core-collapse supernova (ccsn) presents a once-in-a-lifetime opportunity to make astrophysical measurements using neutrinos, gravitational waves, and electromagnetic radiation. ccsne local to the milky way are extremely rare, so it is paramount that detectors are prepared to observe the signal when it arrives. the icecube neutrino observatory, a gigaton water cherenkov detector below the south pole, is sensitive to the burst of neutrinos released by a galactic ccsn at a level $>$10$\sigma$. this burst of neutrinos precedes optical emission by hours to days, enabling neutrinos to serve as an early warning for follow-up observation. icecube's detection capabilities make it a cornerstone of the global network of neutrino detectors monitoring for galactic ccsne, the supernova early warning system (snews 2.0). in this contribution, we describe icecube's sensitivity to galactic ccsne and strategies for operational readiness, including "fire drill" data challenges. we also discuss coordination with snews 2.0. | galactic core-collapse supernovae at icecube: "fire drill" data challenges and follow-up |
supernova neutrino detection in neutrino and dark matter experiments is usually implemented as a real-time trigger system based on counting neutrino interactions within a moving time window. the sensitivity reach of such experiments can be improved by taking into account the time profile of the expected signal. we propose a shape analysis of the incoming experimental data based on a log likelihood ratio variable containing the assumed signal shape. this approach also allows a combination of potential supernova signals in different detectors for a further sensitivity boost. the method is tested on the nova detectors to study their combined sensitivity to the core-collapse supernova signal, and also on kamland, borexino and sk-gd as potential detectors of presupernova neutrinos. using the shape analysis enhances the signal significance for supernova detection and prediction, as well as the sensitivity reach of the experiment. it also extends the supernova prediction time when applied to the presupernova neutrino signal detection. enhancements achieved with the shape analysis persist even in the case when the actual signal doesn't match the expected signal model. | combined detection of supernova neutrino signals |
core-collapse supernovae are expected to produce multimessenger signals. low-energy neutrinos and gravitational waves are important to study the explosion mechanism of these events. the simulations and detections of gravitational waves from these events are still challenging due to broad range of expected progenitors as well as their stochasticity. in this work, we present a possible method to combine low-energy neutrinos and gravitational waves to search for core-collapse supernovae, which is based on \cite{halim2021} (arxiv:2107.02050). we discuss how to exploit the time profile of the neutrino signals in order to improve the efficiency search. moreover, we describe the combination of neutrino data from several detectors. the goal is to produce a strategy for combining the neutrinos and gravitational waves as a multimessenger search. | core-collapse supernova search strategy: gravitational waves and low-energy neutrinos |
americium-beryllium (ambe), a well-known tagged neutron source, is commonly used for evaluating the neutron detection efficiency of detectors used in ultralow background particle physics experiments, such as reactor neutrino and diffuse supernova neutrino background experiments. in particular, ambe sources are used to calibrate neutron tagging by selecting the 4438 kev γ-ray signal, which is simultaneously emitted with a neutron signal. therefore, analyzing the neutron and γ-ray emission properties of ambe sources is crucial. in this study, we used the theoretical shape of a neutron energy spectrum, which was divided into three parts, to develop models of the energy spectrum and verify the results using experimental data. we used an ambe source to measure the energy spectra of simultaneously emitted neutrons and γ-rays and determine the emission ratio of the neutrons with and without γ-ray emission. the measured spectrum was consistent with that obtained from the simulated result, whereas the measured emission ratio was significantly different from the corresponding simulated result. here, we also discuss the feasibility of determining the neutron emission rates from the spectra divided into three parts. | analyzing the neutron and γ-ray emission properties of an americium-beryllium tagged neutron source |
the haze is an excess of microwave intensity emission surrounding the galactic centre. it is spatially correlated with the γ-ray fermi bubbles, and with the s-pass radio polarization plumes, suggesting a possible common provenance. the models proposed to explain the origin of the haze, including energetic events at the galactic centre and dark matter decay in the galactic halo, do not yet provide a clear physical interpretation. in this paper, we present a reanalysis of the haze including new observations from the multi-frequency instrument (mfi) of the q-u-i joint tenerife (quijote) experiment, at 11 and 13 ghz. we analyse the haze in intensity and polarization, characterizing its spectrum. we detect an excess of diffuse intensity signal ascribed to the haze. the spectrum at frequencies 11 ghz $\, \le \nu \le \,$ 70 ghz is a power law with spectral index βh = -2.79 ± 0.08, which is flatter than the galactic synchrotron in the same region (βs = -2.98 ± 0.04), but steeper than that obtained from previous works (βh ~ -2.5 at 23 ghz $\, \le \, \nu \le \,$ 70 ghz). we also observe an excess of polarized signal in the quijote-mfi maps in the haze area. this is a first hint detection of polarized haze, or a consequence of curvature of the synchrotron spectrum in that area. finally, we show that the spectrum of polarized structures associated with galactic centre activity is steep at low frequencies (β ~ -3.2 at 2.3 ghz ≤ ν ≤ 23 ghz), and becomes flatter above 11 ghz. | quijote scientific results - vi. the haze as seen by quijote |
most searches for dark matter primarily focus on the wimp paradigm, which predicts dark matter masses in the gev - 10 tev range. however, these relatively low energy searches continue to produce null results, possibly suggesting that dark matter is something other than wimps. gravitinos, on the other hand, can satisfy the cosmological constraints on dark matter, and decay with a lifetime orders of magnitude longer than the age of the universe, producing extremely high energy neutrinos. the icecube neutrino observatory has already had success detecting ehe extragalactic neutrinos, and is well suited to search for dark matter in this high energy regime. this analysis sets limits on the gravitino lifetime from the high energy neutrino events observed at icecube using three possible astrophysical explanations of the neutrino flux. the most conservative limit on the gravitino lifetime using the softest two-body decay mode was found to be taudm = 1027.6s. this is the first analysis developed to place a limit on the gravitino lifetime using icecube software and simulation files, and the results are comparable to theoretical limits based on the same data set. | dark matter decays from the galactic center using icecube-86 |
we report the spectroscopic classification of ztf19aamdmcs flagged by a custom filter on the antares alert-broker (https://antares.noao.edu/) as a promising stellar explosive event from the public alert stream of the ztf survey. | spectroscopic classification of the transient ztf19aamdmcs/at2020nef |
we present the results of our analysis of multiwavelength observations for the long gamma-ray burst grb 200829a. the burst redshift $z≈ 1.29± 0.04$ has been determined photometrically at the afterglow phase. in gamma rays the event is one of the brightest (in isotropic equivalent), $e_{\textrm{iso}}\gtrsim 10^{54}$ erg. the multicolor light curve of the grb 200829a afterglow is characterized by chromatic behavior and the presence of a plateau gradually transitioning into a power-law decay that can also be interpreted as a quasi-synchronous inhomogeneity (flare). we assume that the presence of a chromatic inhomogeneity in the early afterglow is consistent with the model of a structured jet. | chromatic afterglow of grb 200829a |
the second gamma-ray burst catalogue (2flgc) was announced by the fermi large area telescope (fermi-lat) collaboration. it includes 29 bursts with photon energy higher than 10 gev. gamma-ray burst (grb) afterglow observations have been adequately explained by the classic synchrotron forward-shock model, however, photon energies greater than 10 gev from these transient events are challenging, if not impossible, to characterize using this afterglow model. recently, the closure relations (crs) of the synchrotron self-compton (ssc) forward-shock model evolving in a stellar wind and homogeneous medium was presented to analyse the evolution of the spectral and temporal indexes of those bursts reported in 2flgc. in this work, we provide the crs of the same afterglow model, but evolving in an intermediate density profile (∝r-k) with 0 ≤ k ≤ 2.5, taking into account the adiabatic/radiative regime and with/without energy injection for any value of the electron spectral index. the results show that the current model accounts for a considerable subset of grbs that cannot be interpreted in either stellar-wind or homogeneous afterglow ssc model. the analysis indicates that the best-stratified scenario is most consistent with k = 0.5 for no-energy injection and k = 2.5 for energy injection. | closure relations of synchrotron self-compton in afterglow-stratified medium and fermi-lat detected gamma-ray bursts |
the transiting exoplanet survey satellite (tess; ricker et al. 2015) observed the location of the bright gamma-ray burst grb230307a during the second orbit of observation sector 62. images of the burst region, located in tess camera 4, ccd 4, were captured in the tess full-frame images (ffis). the observations of the field were made at a cadence of 200 seconds and were continuous from 3 days before the trigger to 3 days after the trigger. analysis of the tess ffis using tica (fausnaugh et al. 2020; doi:10.3847/2515-5172/abd63a <https://ui.adsabs.harvard.edu/link_gateway/2020rnaas...4..251f/doi:10.3847/2515-5172/abd63a>) data confirms the existence of the optical counterpart reported by levan et al. (gcn circular 33439), lipunov et al. (gcn circular 33441), o'conner et al. (gcn circular 33447), and im et al. (gcn circular 33449). the signal is seen as a point source brighter than 15th magnitude in a single 200s ffi. the measured position is 4:03:26.5, -75:22:45 with an estimated 1-sigma error of 4", consistent with previously-reported localizations. tica data can be found at https://archive.stsci.edu/hlsp/tica. | tess detection of the optical afterglow of grb230307a |
as the most energetic explosions in the universe, gamma-ray bursts (grbs) are commonly believed to be generated by relativistic jets. recent observational evidence suggests that the jets producing grbs are likely to have a structured nature. some studies have suggested that non-axisymmetric structured jets may be formed through internal non-uniform magnetic dissipation processes or the precession of the central engine. in this study, we analyze the potential characteristics of grb afterglows within the framework of non-axisymmetric structured jets. we simplify the profile of the asymmetric jet as a step function of the azimuth angle, dividing the entire jet into individual elements. by considering specific cases, we demonstrate that the velocity, energy, and line-of-sight direction of each jet element can greatly affect the behaviour of the overall light curve. the radiative contributions from multiple elements may lead to the appearance of multiple distinct peaks or plateaus in the light curve. furthermore, fluctuations in the rising and declining segments of each peak can be observed. these findings establish a theoretical foundation for future investigations into the structural characteristics of grbs by leveraging grb afterglow data. | characteristics of gamma-ray burst afterglows in the context of non-axisymmetric structured jets |
many high-energy astrophysical sources accelerate electrons to relativistic velocities, resulting in broadband emission from synchrotron and other radiation processes. gamma-ray burst afterglow observations provide a unique probe of electron acceleration due to the relativistic nature of their collimated outflows. the peaks in gamma-ray burst radio light curves and spectral energy distributions pin down the characteristic synchrotron frequency related to the minimum lorentz factor of the accelerated electrons, γm. we present a method that constrains the fraction of shock energy that resides in electrons, εe, and the fraction of electrons that is shocked into a power-law energy distribution, ξe. based on a large sample of radio afterglows, we show narrow distributions for εe and γm, largely independent of other physical parameters describing the micro- and macrophysics of these gamma-ray burst jets and their environment. we also put strong constraints on ξe, which is almost impossible to do with broadband modeling due to its degeneracy with other physical parameters. these results have implications for multi-wavelength modeling of gamma-ray burst afterglows and the derived physical parameters, and for simulations of electron acceleration in relativistic shocks, which is relevant for a large variety of high-energy astrophysical sources. | probing electron acceleration in gamma-ray burst afterglows |
grb 221009a is an exceptionally bright gamma-ray burst that reached earth on october 9th 2022. a fraction of the soft x-rays of the bright prompt emission were scattered by galactic dust clouds along the line of sight and produced halo rings echoing for a few days after the initial trigger. the imaging x-ray polarimetry explorer (ixpe) pointed at grb 221009a on october 11th to observe, for the first time, the 2-8 kev x-ray polarization of a grb afterglow. exceeding the primary goal, ixpe also captured the echoes of the grb prompt emission detecting two halo-rings of scattered photons. in this presentation we report the results of the ixpe observation of grb 221009a. a special thank goes to the ixpe collaboration and operation team, and in particular to the working group that supported both data analysis and interpretation. | the ixpe view of grb 221009a |
grb 221009a was the brightest gamma-ray burst (grb) detected since the start of grb observations in the 1960s. even though it saturated fermi-gbm, it presents a rich and unique data set to test theoretical models of gamma-ray production. we present spectral analysis of the pulses not affected by instrumental saturation and confront the synchrotron emission model with the observations. interestingly, the afterglow is also detected by gbm. we determine the transition from prompt emission to afterglow, and derive the lorentz factor of the outflow using multiple methods. | physical properties of the brightest gamma-ray burst based on observations by the fermi gamma-ray burst monitor |
swift-xrt has performed follow-up observations of the fermi/gbm-detected burst grb 230911a, collecting 2.8 ks of photon counting (pc) mode data between t0+215.5 ks and t0+326.2 ks. an uncatalogued x-ray source is detected consistent with the goto position (gcn circ. 34681) of the fading optical counterpart candidate, and this is therefore believed to be the afterglow. the position of this source (astrometrically enhanced by using the xrt-uvot alignment and matching uvot field sources to the usno-b1 catalogue) is ra, dec=57.5027, -29.8259 which is equivalent to: ra (j2000): 03:50:0.65 dec(j2000): -29:49:33.2 with an uncertainty of 3.4 arcsec (radius, 90% confidence). position enhancement is described by goad et al. (2007, a&a, 476, 1401) and evans et al. (2009, mnras, 397, 177). this position is 3.1 arcsec from the goto position. the results of the xrt-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021622. the results of the full analysis of the xrt observations are available at https://www.swift.ac.uk/too_grbs/00021622. this circular is an official product of the swift-xrt team. | grb 230911a: swift-xrt afterglow detection |
x-ray polarization probes the particle acceleration zone in relativistic jets as well as the geometry of the central region of active galactic nuclei. in its first year of operations, the imaging x-ray polarimetry explorer (ixpe) has completed a first-time characterization of the x-ray polarization properties of a sample of active galactic nuclei. highlights of these observations include the detection of significant x-ray polarization from the jets of three high-frequency-peaked bl lac-type blazars (mrk 501, mrk 421, and 1es 1959+650) that point to energy-stratified shocks as the most plausible sites for electron acceleration in jets. the lack of measurable polarization from low-frequency-peaked blazars, on the other hand, constrains the potential contribution from proton synchrotron radiation to the observed x-ray emission. observations of the radio and seyfert galaxies have provided constraints on the jet magnetic field and the torus geometry of the central region of active galactic nuclei. this talk will review the ixpe extragalactic program, discussing results from observations of active galactic nuclei and the gamma-ray burst grb 221009a from which x-ray polarization constraints on the prompt and afterglow emission were derived. the imaging x ray polarimetry explorer (ixpe) is a joint us and italian mission. the us contribution is supported by the national aeronautics and space administration (nasa) and led and managed by its marshall space flight center (msfc), with industry partner ball aerospace (contract nnm15aa18c). the italian contribution is supported by the italian space agency (agenzia spaziale italiana, asi) through contract asi-ohbi-2017-12-i.0, agreements asi-inaf-2017-12-h0 and asi-infn-2017.13-h0, and its space science data center (ssdc) with agreements asi- inaf-2022-14-hh.0 and asi-infn 2021-43-hh.0, and by the istituto nazionale di astrofisica (inaf) and the istituto nazionale di fisica nucleare (infn) in italy. this research used data products provided by the ixpe team (msfc, ssdc, inaf, and infn) and distributed with additional software tools by the high-energy astrophysics science archive research center (heasarc), at nasa goddard space flight center (gsfc). | active galactic nuclei revealed by x-ray polarization measurements from the imaging x-ray polarimetry explorer (ixpe) |
swift-xrt has performed follow-up observations of thefermi/gbm and ipn detected burst grb 200826a (gupta et al. gcn circ. 28288, hurley et al., gcn circ. 28291) in a series of observations tiled on the sky. the total exposure time is 7.4 ks, distributed over 4 tiles; the maximum exposure at a single sky location was 5.2 ks. the data were collected between t0+59.9 ks and t0+169.6 ks, and are entirely in photon counting (pc) mode. seven uncatalogued x-ray sources are detected, of which one ("source 3") is consistent with ztf20abwysqy, reported by ztf (ahumada et al. gcn circ. 28293 and 28295). source 3 is fading with 3-sigma significance, and is therefore likely the grb afterglow. using 1504 s of pc mode data and 1 uvot image, we find an enhanced xrt position (using the xrt-uvot alignment and matching uvot field sources to the usno-b1 catalogue): ra, dec = 6.78466, +34.02670 which is equivalent to: ra (j2000): 00h 27m 08.32s dec(j2000): +34d 01' 36.1" with an uncertainty of 3.3 arcsec (radius, 90% confidence). this position is 104 arcsec from the fermi/gbm position. the light curve can be modelled with a power-law decay with a decay index of alpha=1.3 (+0.7, -0.5). a spectrum formed from the pc mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+0.6, -0.5). the best-fitting absorption column is 2.2 (+2.5, -1.6) x 10^21 cm^-2, consistent with the galactic value of 6.0 x 10^20 cm^-2 (willingale et al. 2013). the counts to observed (unabsorbed) 0.3-10 kev flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (4.6 x 10^-11) erg cm^-2 count^-1. a summary of the pc-mode spectrum is thus: total column: 2.2 (+2.5, -1.6) x 10^21 cm^-2 galactic foreground: 6.0 x 10^20 cm^-2 excess significance: <1.6 sigma photon index: 1.8 (+0.6, -0.5) the results of the xrt-team automatic analysis of the likely afterglow are at https://www.swift.ac.uk/xrt_products/tiled_grb00093/source3.php. the results of the full analysis of the tiled xrt observations are available at https://www.swift.ac.uk/xrt_products/tiled_grb00093. the swift/uvot began observations of the optical counterpart ztf20abwysqy, 140ks after the fermi/gbm trigger (fermi gbm team, gcn circ. 28284). a source consistent with ztf20abwysqy is detected in the uvot summed exposures. ztf20abwysqy is consistent with xrt source 3, however no earlier uvot photometry is avaliable since xrt source 3 is outside of the field of view for all earlier tiles. the uvot detection is possibly explained by the underlying galaxy emission. the preliminary detection and 3-sigma upper limits using the uvot photometric system (breeveld et al. 2011, aip conf. proc. 1358, 373) for the early exposures are: filter t_start(s) t_stop(s) exp(s) mag white 140782 169578 5032 21.86 +/- 0.13 the magnitudes in the table are not corrected for the galactic extinction due to the reddening of e(b-v) = 0.07 in the direction of the burst (schlegel et al. 1998). this circular is an official product of the swift-xrt and swift-uvot teams. | grb 200826a: swift-xrt afterglow detection |
we report previously-unpublished director's discretionary time xmm-newton observations of neutron star merger gw170817 (pi: schartel). this xmm-newton observation at 162 days post-merger confirms previous chandra observations of a plateau in the x-ray light curve of gw170817's non-thermal synchrotron afterglow, before its current fading (e.g., nynka et al., 2018, alexander et al. 2018). | combined xmm-newton and chandra observations of the x-ray light curve plateau in gw170817 |
gamma-ray bursts (grbs) are bright extragalactic flashes of gamma-ray radiation and briefly the most energetic explosions in the universe. their catastrophic origin —the merger of compact objects or the collapse of massive stars— drives the formation of a newborn compact remnant (black hole or magnetar) that powers two highly relativistic jets. as these jets continue to travel outwards, they collide with the external material surrounding the dying star, producing a long-lasting afterglow that can be seen across the entire electromagnetic spectrum, from the most energetic gamma-ray emission to radio wavelengths. but how can such material be accelerated and focused into narrow beams? the internal shock model proposes that repeated collisions between material blasted out during the explosion can produce the gamma-ray flash. the competing magnetic model credits primordial large-scale ordered magnetic fields that collimate and accelerate the relativistic outflows. to distinguish between these models and ultimately determine the power source for these energetic explosions, our team studies the polarization of the light during the first minutes after the explosion (using novel instruments on fully autonomous telescopes around the globe) to directly probe the magnetic field properties in these extragalactic jets. this technology allowed the detection of highly polarized optical light in grb 120308a and confirmed the presence of mildly magnetized jets with large-scale primordial magnetic fields in a reduced sample of grbs (e.g. grb 090102, grb 110205a, grb 101112a, grb 160625b). here we discuss the observations of the most energetic and first grb detected at very high tev energies, grb 190114c, which opens a new frontier in grb magnetic field studies suggesting that some jets can be launched highly magnetized and that the collapse and destruction of these magnetic fields at very early times may have powered the explosion itself. additionally, our most recent polarimetric observations of the jet of grb 141220a indicate that, when the jetted ejected material is decelerated by the surrounding environment, the magnetic field amplification mechanisms at the front shock —needed to generate the observed synchrotron emission— produce small magnetic domains. these measurements validate theoretical expectations and contrast with previous observations that suggest large magnetic domains in collisionless shocks (i.e. grb 091208b). | the role of the magnetic fields in grb outflows |
observations of the afterglows of grbs are used to infer properties of the jet and local environment through model fitting. short grbs were first discovered in the 1960s, but only recently were confirmed to come from relativistic jets produced by the merger of compact objects. however, the angular structure of the jet is difficult to determine. previous modeling typically assumes a "top-hat" jet with a constant energy out to sharp cutoff at some angle. however, the afterglow associated with the ns-ns merger gw170817 provided clear evidence for a structured, centrally peaked jet, at least in that case. we take a structured jet, produced by a hydrodynamic simulation and which provided a good fit for gw170817, and are using it to fit other short grbs. here, we present our efforts to fit grb 130603b. this grb has a complex, well-observed afterglow with detections in x-ray, optical, and radio, and evidence for a jet break. we compare fitting with a structured jet to fitting with top-hat jet. our goal is to see if all short grbs can be fit with same structured jet, or, if not, what range of jet shapes and energies may be needed. | finding best fit models of afterglows of gamma ray bursts |
more than 10000 gamma-ray bursts (grbs) have been detected since discovery. long-term observations of about 850 grb afterglow in optic since 1998 have shown that a core-collapse supernova (sn) accompanies about 50 nearby grb sources. we have collected about two dozen sne' multicolor light curves associated with grbs. the sample is based on published data, obtained during observations of grb-sn cases by ground-based observatories all around the world including our own observations. a description of the procedure for the extraction of the sn's light curve, its analysis, and phenomenological classification of sns are presented. we also discuss the current status and problems of investigations of sn associated with grb. | the diversity of light curves of supernovae associated with gamma-ray bursts |
in this paper, the temporal evolution of three-dimensional relativistic current sheets in poynting-dominated plasma is studied for the first time. over the past few decades, a lot of efforts have been conducted on studying the evolution of current sheets in two-dimensional space, and concluded that sufficiently long current sheets always evolve into the so-called plasmoid chain, which provides a fast reconnection rate independent of its resistivity. however, it is suspected that plasmoid chain can exist only in the case of two-dimensional approximation, and would show transition to turbulence in three-dimensional space. we performed three-dimensional numerical simulation of relativistic current sheet using resistive relativistic magnetohydrodynamic approximation. the results showed that the three-dimensional current sheets evolve not into plasmoid chain but turbulence. the resulting reconnection rate is 0.004, which is much smaller than that of plasmoid chain. the energy conversion from magnetic field to kinetic energy of turbulence is just 0.01 per cent, which is much smaller than typical non-relativistic cases. using the energy principle, we also showed that the plasmoid is always unstable for a displacement in the opposite direction to its acceleration, probably interchange-type instability, and this always results in seeds of turbulence behind the plasmoids. finally, the temperature distribution along the sheet is discussed, and it is found that the sheet is less active than plasmoid chain. our finding can be applied for many high-energy astrophysical phenomena, and can provide a basic model of the general current sheet in poynting-dominated plasma. | evolution of three-dimensional relativistic current sheets and development of self-generated turbulence |
simulations generally show that non-self-gravitating clouds have a lognormal column density ($\sigma$) probability distribution function (pdf), while self-gravitating clouds with active star formation develop a distinct power-law tail at high column density. although the growth of the power law can be attributed to gravitational contraction leading to the formation of condensed cores, it is often debated if an observed lognormal shape is a direct consequence of supersonic turbulence alone, or even if it is really observed in molecular clouds. in this paper we run three-dimensional magnetohydrodynamic simulations including ambipolar diffusion with different initial conditions to see the effect of strong magnetic fields and nonlinear initial velocity perturbations on the evolution of the column density pdfs. our simulations show that column density pdfs of clouds with supercritical mass-to-flux ratio, with either linear perturbations or nonlinear turbulence, quickly develop a power-law tail such that $dn/d \log \sigma \propto \sigma^{-\alpha}$ with index $\alpha \simeq 2$. interestingly, clouds with subcritical mass-to-flux ratio also proceed directly to a power-law pdf, but with a much steeper index $\alpha \simeq 4$. this is a result of gravitationally-driven ambipolar diffusion. however, for nonlinear perturbations with a turbulent spectrum ($v_{k}^{2} \propto k^{-4}$), the column density pdfs of subcritical clouds do retain a lognormal shape for a major part of the cloud evolution, and only develop a distinct power-law tail with index $\alpha \simeq 2$ at greater column density when supercritical pockets are formed. | the effect of magnetic fields and ambipolar diffusion on the column density probability distribution function in molecular clouds |
we previously proposed that betelgeuse might have been spun up by accreting a companion of about 1 m⊙. here we explore in more detail the possible systematics of such a merger and a larger range of accreted masses. we use the stellar evolutionary code mesa to add angular momentum to a primary star in core helium burning, core carbon burning, or shell carbon burning. our models provide a reasonable "natural" explanation for why betelgeuse has a large, but sub-keplerian equatorial velocity. they eject sufficient mass and angular momentum in rotationally induced mass loss to reproduce the observed ratio of the equatorial velocity to escape velocity of betelgeuse, ≈0.23, within a factor of 3 nearly independent of the primary mass, the secondary mass, and the epoch at which merger occurs. our models suggest that merger of a primary of somewhat less than 15 m⊙ with secondaries from 1 to 10 m⊙ during core helium burning or core carbon burning could yield the equatorial rotational velocity of ∼15 km s-1 attributed to betelgeuse. for accreting models, a wave of angular momentum is halted at the composition boundary at the edge of the helium core. the inner core is thus not affected by the accretion of the companion in these simulations. accretion has relatively little effect on the production of magnetic fields in the inner core. our results do not prove, but do not negate, that betelgeuse might have ingested a companion of several m⊙. | the betelgeuse project. iii. merger characteristics |
fragmentation of highly differentially rotating massive stars that undergo collapse has been suggested as a possible channel for binary black hole formation. such a scenario could explain the formation of the new population of massive black holes detected by the ligo/virgo gravitational wave laser interferometers. we probe that scenario by performing general relativistic magnetohydrodynamic simulations of differentially rotating massive stars supported by thermal radiation pressure plus a gas pressure perturbation. the stars are initially threaded by a dynamically weak, poloidal magnetic field confined to the stellar interior. we find that magnetic braking and turbulent viscous damping via magnetic winding and the magnetorotational instability in the bulk of the star redistribute angular momentum, damp differential rotation and induce the formation of a massive and nearly uniformly rotating inner core surrounded by a keplerian envelope. the core+disk configuration evolves on a secular timescale and remains in quasistationary equilibrium until the termination of our simulations. our results suggest that the high degree of differential rotation required for m =2 seed density perturbations to trigger gas fragmentation and binary black hole formation is likely to be suppressed during the normal lifetime of the star prior to evolving to the point of dynamical instability to collapse. other cataclysmic events, such as stellar mergers leading to collapse, may therefore be necessary to reestablish sufficient differential rotation and density perturbations to drive nonaxisymmetric modes leading to binary black hole formation. | magnetic braking and damping of differential rotation in massive stars |
neutron stars (nss) play essential roles in modern astrophysics. the magnetic fields and spin periods of newborn (zero-age) nss have a large impact on the further evolution of nss, which are, however, poorly explored in observations due to the difficulty of finding newborn nss. in this work, we aim to infer the magnetic fields and spin periods (b i and p i) of zero-age nss from the observed properties of the ns population. we select nonaccreting nss whose evolution is solely determined by magnetic dipole radiation. we find that both b i and p i can be described by lognormal distribution, and the fitting sensitively depends on our parameters. | back to the starting point: on the simulation of the initial magnetic fields and spin periods of nonaccreting pulsars |
firehose-like instabilities (fis) are cited in multiple astrophysical applications. of particular interest are the kinetic manifestations in weakly collisional or even collisionless plasmas, where these instabilities are expected to contribute to the evolution of macroscopic parameters. relatively recent studies have initiated a realistic description of fis, as induced by the interplay of both species, electrons and protons, dominant in the solar wind plasma. this work complements the current knowledge with new insights from linear theory and the first disclosures from 2d-pic simulations, identifying the fastest growing modes near the instability thresholds and their long-run consequences on the anisotropic distributions. thus, unlike previous setups, these conditions are favorable to those aperiodic branches that propagate obliquely to the uniform magnetic field, with (maximum) growth rates higher than periodic, quasi-parallel modes. theoretical predictions are, in general, confirmed by the simulations. the aperiodic electron fi (a-efi) remains unaffected by the proton anisotropy, and saturates rapidly at low-level fluctuations. regarding the fi at proton scales, we see a stronger competition between the periodic and aperiodic branches. for the parameters chosen in our analysis, the aperiodic proton fi (a-pfi) is excited before than the periodic proton fi (p-pfi), with the latter reaching a significantly higher fluctuation power. however, both branches are significantly enhanced by the presence of anisotropic electrons. the interplay between efis and pfis also produces a more pronounced proton isotropization. | mixing the solar wind proton and electron scales. theory and 2d-pic simulations of firehose instability |
ngc 300 ulx1 is a pulsating ultraluminous x-ray source (pulx) with the longest spin period of $p\simeq 31.6\, \rm s$ and a high spin-up rate of $\dot{p}\simeq -5.56\times 10^{-7}\, \rm s\, s^{-1}$ that is ever seen in the confirmed pulxs. in this paper, the inferred magnetic field of ngc 300 ulx1 is $\sim 3.0\times 10^{14}\, \rm g$ using the recent observed parameters after its first detection of pulsations. according to the evolved simulation of the magnetic field and the spin period, it will become a recycled pulsar or a millisecond pulsar under the conditions of the companion mass and the accretion rate limitation. we suggest that ngc 300 ulx1 is an accreting magnetar accounting for its super eddington luminosity. we also propose that there might be other accreting magnetars in the confirmed pulxs. such pulxs will be helpful for understanding the magnetar evolution and the millisecond pulsar formation whose magnetic field is stronger than $\sim 10^{9}\, \rm g$. | study on the magnetic field strength of ngc 300 ulx1 |
context. magnetic fields are predicted to have a crucial impact on the structure, evolution, and chemistry of protoplanetary disks. however, a direct detection of the magnetic field toward these objects has yet to be achieved.aims: in order to characterize the magnetic fields of protoplanetary disks, we investigate the impact of the zeeman effect on the (polarized) radiative transfer of emission from paramagnetic molecules excited in protoplanetary disks.methods: while the effects of the zeeman effect are commonly studied in the circular polarization of spectral lines, we also performed a comprehensive modeling of the zeeman-induced broadening of spectral lines and their linear polarization. we developed simplified radiative transfer models adapted to protoplanetary disks, which we compare to full three-dimensional polarized radiative transfer simulations.results: we find that the radiative transfer of circular polarization is heavily affected by the expected polarity change of the magnetic field between opposite sides of the disk. in contrast, zeeman broadening and linear polarization are relatively unaffected by this sign change due to their quadratic dependence on the magnetic field. we can match our simplified radiative transfer models to full polarization modeling with high fidelity, which in turn allows us to prescribe straightforward methods to extract magnetic field information from zeeman broadening and linear polarization observations.conclusions: we find that zeeman broadening and linear polarization observations are highly advantageous methods to characterize protoplanetary disk magnetic fields as they are both sensitive probes of the magnetic field and are marginally affected by any sign change of the disk magnetic field. applying our results to existing circular polarization observations of protoplanetary disk specral lines suggests that the current upper limits on the toroidal magnetic field strengths have to be raised. | three-dimensional magnetic field imaging of protoplanetary disks using zeeman broadening and linear polarization observations |
aims: we study the acceleration of the stellar winds of rapidly rotating low mass stars and the transition between the slow magnetic rotator and fast magnetic rotator regimes. we aim to understand the properties of stellar winds in the fast magnetic rotator regime and the effects of magneto-centrifugal forces on wind speeds and mass loss rates.methods: we extend a solar wind model to 1d magnetohydrodynamic simulations of the winds of rotating stars. we test two assumptions for how to scale the wind temperature to other stars and assume the mass loss rate scales as dot{m_star ∝ r_star2 ω_star1.33 m_star-3.36}, in the unsaturated regime, as estimated from observed rotational evolution.results: for 1.0 m⊙ stars, the winds can be accelerated to several thousand km s-1, and the effects of magneto-centrifugal forces are much weaker for lower mass stars. we find that the different assumptions for how to scale the wind temperature to other stars lead to significantly different mass loss rates for the rapid rotators. if we assume a constant temperature, the mass loss rates of solar mass stars do not saturate at rapid rotation, which we show to be inconsistent with observed rotational evolution. if we assume the wind temperatures scale positively with rotation, the mass loss rates are only influenced significantly at rotation rates above 75 ω⊙. we suggest that models with increasing wind speed for more rapid rotators are preferable to those that assume a constant wind speed. if this conclusion is confirmed by more sophisticated wind modelling. it might provide an interesting observational constraint on the properties of stellar winds. all of the codes and output data used in this paper can be downloaded from https://zenodo.org/record/160052#.v_y6drwkvc1 or obtained by contacting the author. | on the fast magnetic rotator regime of stellar winds |
gravitational instability (gi) features in several aspects of protostellar disc evolution, most notably in angular momentum transport, fragmentation, and the outbursts exemplified by fu ori and ex lupi systems. the outer regions of protostellar discs may also be coupled to magnetic fields, which could then modify the development of gi. to understand the basic elements of their interaction, we perform local 2d ideal and resistive magnetohydrodynamics simulations with an imposed toroidal field. in the regime of moderate plasma beta, we find that the system supports a hot gravitoturbulent state, characterized by considerable magnetic energy and stress and a surprisingly large toomre parameter q ≳ 10. this result has potential implications for disc structure, vertical thickness, ionization, etc. our simulations also reveal the existence of long-lived and dense `magnetic islands' or plasmoids. lastly, we find that the presence of a magnetic field has little impact on the fragmentation criterion of the disc. though our focus is on protostellar discs, some of our results may be relevant for the outer radii of agn. | gravitoturbulence in magnetized protostellar discs |
theory of fossil magnetic field is based on the observations, analytical estimations and numerical simulations of magnetic flux evolution during star formation in the magnetized cores of molecular clouds. basic goals, main features of the theory and manifestations of mhd effects in young stellar objects are discussed. | theory of fossil magnetic field |
combining numerical simulations and analytical modelling, we investigate whether close binary systems form by the effect of magnetic braking. using magnetohydrodynamics simulations, we calculate the cloud evolution with a sink, for which we do not resolve the binary system or binary orbital motion to realize long-term time integration. then, we analytically estimate the binary separation using the accreted mass and angular momentum obtained from the simulation. in unmagnetized clouds, wide binary systems with separations of >100 au form, in which the binary separation continues to increase during the main accretion phase. in contrast, close binary systems with separations of <100 au can form in magnetized clouds. since the efficiency of magnetic braking strongly depends on both the strength and configuration of the magnetic field, they also affect the formation conditions of a close binary. in addition, the protostellar outflow has a negative impact on close binary formation, especially when the rotation axis of the prestellar cloud is aligned with the global magnetic field. the outflow interrupts the accretion of gas with small angular momentum, which is expelled from the cloud, while gas with large angular momentum preferentially falls from the side of the outflow on to the binary system and widens the binary separation. this study shows that a cloud with a magnetic field that is not parallel to the rotation axis is a favourable environment for the formation of close binary systems. | impact of magnetic braking on high-mass close binary formation |
we numerically model evolution of magnetic fields inside a neutron star under the influence of ambipolar diffusion in the weak-coupling mode in the one-fluid mhd approximation. our simulations are 3d and performed in spherical coordinates. our model covers the neutron star core and includes crust where the magnetic field decay is due to ohmic decay. we discover an instability of poloidal magnetic field under the influence of ambipolar diffusion. this instability develops in the neutron star core and grows on a time-scale of 0.2 dimensionless times, reaching saturation by 2 dimensionless times. the instability leads to formation of azimuthal magnetic field with azimuthal wavenumber m = 14 (at the moment of saturation) which keeps merging and reaches m = 4 by 16 dimensionless times. over the course of our simulations (16 dimensionless times) the surface dipolar magnetic field decays, reaching 20 per cent of its original value and keeps decaying. the decay time-scale for the total magnetic energy is six dimensionless times. the ambipolar diffusion induces electric currents in the crust where these currents dissipate efficiently. strong electric currents in the crust lead to heating, which could correspond to luminosities of ≈1029 erg s-1 during hundreds of myrs for an initial magnetic field of 1014 g. ambipolar diffusion leads to formation of small-scale magnetic fields at the neutron star surface. | three-dimensional numerical simulations of ambipolar diffusion in ns cores in the one-fluid approximation: instability of poloidal magnetic field |
context. the stellar winds of the massive stars in high-mass microquasars are thought to be inhomogeneous. the interaction of these inhomogeneities, or clumps, with the jets of these objects may be a major factor in gamma-ray production.aims: our goal is to characterize a typical scenario of clump-jet interaction, and calculate the contribution of these interactions to the gamma-ray emission from these systems.methods: we use axisymmetric, relativistic hydrodynamical simulations to model the emitting flow in a typical clump-jet interaction. using the simulation results we perform a numerical calculation of the high-energy emission from one of these interactions. the radiative calculations are performed for relativistic electrons locally accelerated at the jet shock, and the synchrotron and inverse compton radiation spectra are computed for different stages of the shocked clump evolution. we also explore different parameter values, such as viewing angle and magnetic field strength. the results derived from one clump-jet interaction are generalized phenomenologically to multiple interactions under different wind models, estimating the clump-jet interaction rates, and the resulting luminosities in the gev range.results: if particles are efficiently accelerated in clump-jet interactions, the apparent gamma-ray luminosity through inverse compton scattering with the stellar photons can be significant even for rather strong magnetic fields and thus efficient synchrotron cooling. moreover, despite the standing nature or slow motion of the jet shocks for most of the interaction stage, doppler boosting in the postshock flow is relevant even for mildly relativistic jets.conclusions: for clump-to-average wind density contrasts greater than or equal to ten, clump-jet interactions could be bright enough to match the observed gev luminosity in cyg x-1 and cyg x-3 when a jet is present in these sources, with required non-thermal-to-total available power fractions greater than 0.01 and 0.1, respectively. | gamma rays from clumpy wind-jet interactions in high-mass microquasars |
we present a systematic study of magnetized neutron star head-on collisions. we investigate the resulting magnetic field geometries as the two neutron stars merge. furthermore, we analyze the luminosity produced in these collisions and monitor the evolution of the magnetic fields from the time of merger until the subsequent production of a black hole. at the time of black hole formation, the luminosity peaks and rings down following the decay of the electromagnetic fields. a comparison is presented for three different cases: one where the initial magnetic field in both neutron stars is aligned, one where they are anti-aligned, and one where they initially have unequal magnetic field strength. we identify regions and set limits so that pair creation and magnetic reconnection would occur in this scenario and further discuss limits and differences in the radiated energy. this study should be regarded as a toy model of the case where the remnant of a binary neutron star merger undergoes a prompt collapse to a black hole with a negligible surrounding disk. we note that the generated electromagnetic pulses resemble the fast radio burst phenomenology. we consider implications of the high-mass mergers leading to a fast prompt collapse to a black hole and the expected flux to be observed at a distance similar to the binary neutron star gravitational wave detection gw190425. | a toy model for the electromagnetic output of neutron-star merger prompt collapse to a black hole: magnetized neutron-star collisions |
we used our newly developed magnetohydrodynamic (mhd) code to perform 2.5d simulations of a fast-mode mhd wave interacting with coronal holes (chs) of varying alfvén speed that result from assuming different ch densities. we find that this interaction leads to effects like reflection, transmission, stationary fronts at the ch boundary, and the formation of a density depletion that moves in the opposite direction to the incoming wave. we compare these effects with regard to the different ch densities and present a comprehensive analysis of morphology and kinematics of the associated secondary waves. we find that the density value inside the ch influences the phase speed and the amplitude values of density and magnetic field for all different secondary waves. moreover, we observe a correlation between the ch density and the peak values of the stationary fronts at the ch boundary. the findings of reflection and transmission on the one hand and the formation of stationary fronts caused by the interaction of mhd waves with chs on the other hand strongly support the theory that large-scale disturbances in the corona are fast-mode mhd waves. | numerical simulation of coronal waves interacting with coronal holes. ii. dependence on alfvén speed inside the coronal hole |
in nearby star-forming clouds, amplification and dissipation of the magnetic field are known to play crucial roles in the star-formation process. the star-forming environment varies from place to place and era to era in galaxies. in this study, amplification and dissipation of magnetic fields in star-forming clouds are investigated under different environments using magnetohydrodynamics (mhd) simulations. we consider various star-forming environments in combination with the metallicity and the ionization strength, and prepare prestellar clouds having two different mass-to-flux ratios. we calculate the cloud collapse until protostar formation using ideal and non-ideal (inclusion and exclusion of ohmic dissipation and ambipolar diffusion) mhd calculations to investigate the evolution of the magnetic field. we perform 288 runs in total and show the diversity of the density range within which the magnetic field effectively dissipates, depending on the environment. in addition, the dominant dissipation process (ohmic dissipation or ambipolar diffusion) is shown to strongly depend on the star-forming environment. especially, for the primordial case, magnetic field rarely dissipates without ionization source, while it efficiently dissipates when very weak ionization sources exist in the surrounding environment. the results of this study help to clarify star formation in various environments. | evolution of magnetic fields in collapsing star-forming clouds under different environments |
the magnetic fields observed in ap stars, white dwarfs, and neutron stars are known to be stable for long times. however, the physical conditions inside the stellar interiors that allow these states are still a matter of research. it has been formally demonstrated that both purely toroidal and purely poloidal magnetic fields develop instabilities at some point in the star. on the other hand, numerical simulations have proved the stability of roughly axisymmetric magnetic field configurations inside stably stratified stars. these configurations consist of mutually stabilizing toroidal and poloidal components in a twisted torus shape. previous studies have proposed rough upper and lower bounds on the ratio of the magnetic energy in the toroidal and poloidal components of the magnetic field. with the purpose of mapping out the parameter space under which such configurations remain stable, we used the pencil code to perform 3d magnetohydrodynamic simulations of the evolution of the magnetic field in non-rotating, non-degenerate stars in which viscosity is the only dissipation mechanism, both for stars with a uniform (barotropic) and radially increasing (stably stratified) specific entropy. furthermore, we considered different conditions regarding the degree of stable stratification and the magnetic energy in each component, roughly confirming the previously suggested stability boundaries for the magnetic field. | stability of axially symmetric magnetic fields in stars |
we have performed a full time and luminosity-resolved spectral analysis of the high-mass x-ray binary 4u 1538-522 using the available rxte, integral, and suzaku data, examining both phase-averaged and pulse-phase-constrained data sets and focusing on the behaviour of the cyclotron resonance scattering feature (crsf). no statistically significant trend between the energy of the crsf and luminosity is observed in the combined data set. however, the crsf energy appears to have increased by ∼1.5 kev in the ∼8.5 yr between the rxte and suzaku measurements, with monte carlo simulations finding the suzaku measurement 4.6σ above the rxte points. interestingly, the increased suzaku crsf energy is much more significant and robust in the pulse-phase-constrained spectra from the peak of the main pulse, suggesting a change that is limited to a single magnetic pole. the seven years of rxte measurements do not show any strongly significant evolution with time on their own. we discuss the significance of the crsf's behaviour with respect to luminosity and time in the context of historical observations of this source as well as recent observational and theoretical work concerning the neutron star accretion column, and suggest some mechanisms by which the observed change over time could occur. | evidence for an evolving cyclotron line energy in 4u 1538-522 |
we study the evolution of the field on the surface of proto-neutron stars in the immediate aftermath of stellar core collapse by analyzing the results of self-consistent, axisymmetric simulations of the cores of rapidly rotating high-mass stars. to this end, we compare the field topology and the angular spectra of the poloidal and toroidal field components over a time of about one seconds for cores. both components are characterized by a complex geometry with high power at intermediate angular scales. the structure is mostly the result of the accretion of magnetic flux embedded in the matter falling through the turbulent post-shock layer onto the pns. our results may help to guide further studies of the long-term magneto-thermal evolution of proto-neutron stars. we find that the accretion of stellar progenitor layers endowed with low or null magnetization bury the magnetic field on the pns surface very effectively. | evolution of the surface magnetic field of rotating proto-neutron stars |
context. magnetic fields play a very important role in the evolution of galaxies through their direct impact on star formation and stellar feedback-induced turbulence. however, their co-evolution with these processes has still not been thoroughly investigated, and the possible effect of the initial conditions is largely unknown.aims: this letter presents the first results from a series of high-resolution numerical models, aimed at deciphering the effect of the initial conditions and of stellar feedback on the evolution of the galactic magnetic field in isolated milky way-like galaxies.methods: the models start with an ordered magnetic field of varying strength, either poloidal or toroidal, and are evolved with and without supernova feedback. they include a dark matter halo, a stellar and a gaseous disk, as well as the appropriate cooling and heating processes for the interstellar medium.results: independently of the initial conditions, the galaxies develop a turbulent velocity field and a random magnetic field component in under 15 myr. supernova feedback is extremely efficient in building a random magnetic field component up to large galactic heights. however, a random magnetic field emerges even in runs without feedback, which points to an inherent instability of the ordered component.conclusions: supernova feedback greatly affects the velocity field of the galaxy up to large galactic heights, and helps restructure the magnetic field up to 10 kpc above the disk, independently of the initial magnetic field morphology. on the other hand, the initial morphology of the magnetic field can accelerate the development of a random component at large heights. these effects have important implications for the study of the magnetic field evolution in galaxy simulations. | magnetic fields in massive spirals: the role of feedback and initial conditions |
context. recently, compact black hole x-ray binaries xte j 1118+480 and a0620-00 have been reported to be experiencing a fast orbital period decay, which is two orders of magnitude higher than expected with gravitational wave radiation. magnetic braking of an ap/bp star has been suggested to account for the period change when the surface magnetic field of the companion star bs ≳ 104 g. however, our calculation indicates that anomalous magnetic braking cannot significantly contribute to the large orbital period decay rates observed in these two sources even if bs ≳ 104 g.aims: observations have provided evidence that circumbinary disks around two compact black hole x-ray binaries may exist. our analysis shows that, for some reasonable parameters, tidal torque between the circumbinary disk and the binary can efficiently extract the orbital angular momentum from the binary, and result in a large orbital period change rate.methods: based on the circumbinary disk model, we simulate the evolution of xte j 1118+480 via a stellar evolution code.results: our computations are approximatively in agreement with the observed data (the masses of two components, donor star radius, orbital period, and orbital period derivative).conclusions: the mass transfer rate and circumbinary disk mass are obviously far greater than the inferred values from observations. therefore, it seems that the circumbinary disk is unlikely to be the main cause of the rapid orbital decay observed in some compact black hole x-ray binaries. | orbital period decay of compact black hole x-ray binaries: the influence of circumbinary disks? |
close-in gas giants are expected to have a strong magnetic field of ∼10-100 g. magnetic fields in extrasolar giant planets are detectable by future radio observations in ≳10 mhz and the spectropolarimetry of atomic lines. in contrast, the elusive interiors of exoplanets remain largely unknown. here we consider the possibility of inferring the existence of the innermost cores of extrasolar giant planets through the detection of planetary magnetic fields. we simulated the long-term thermal evolution of close-in giant planets with masses of 0.2-10 mjup to estimate their magnetic field strengths. a young, massive gas giant tends to have a strong magnetic field. the magnetic field strength of a hot jupiter is insensitive to its core mass, whereas the core strongly affects the emergence of a planetary dynamo in a hot saturn. no dynamo-driven magnetic field is generated in a hot saturn with no core or a small one until ∼10-100 myr if metallization of hydrogen occurs at ≳1-1.5 mbar. the magnetic field strength of an evolved gas giant after ∼100 myr is almost independent of the stellar incident flux. detecting the magnetic field of a young, hot saturn as a good indicator of its core may be challenging because of the weakness of radio signals and the shielding effect of plasma in earth's ionosphere. hot jupiters with ≳0.4 mjup can be promising candidates for future ground-based radio observations. | the linkage between the core mass and the magnetic field of an extrasolar giant planet from future radio observations |
we investigate the temporal evolution of an axisymmetric magnetosphere around a rapidly rotating stellar-mass black hole by applying a two-dimensional particle-in-cell simulation scheme. adopting homogeneous pair production and assuming that the mass accretion rate is much less than the eddington limit, we find that the black hole's rotational energy is preferentially extracted from the middle latitudes and that this outward energy flux exhibits an enhancement that lasts approximately 160 dynamical timescales. it is demonstrated that the magnetohydrodynamic approximations cannot be justified in such a magnetically dominated magnetosphere because ohm's law completely breaks down and the charge-separated electron-positron plasmas are highly nonneutral. an implication is given regarding the collimation of relativistic jets. | two-dimensional particle-in-cell simulations of axisymmetric black hole magnetospheres |
when magnetohydrodynamic turbulence evolves in the presence of a large-scale mean magnetic field, an anisotropy develops relative to that preferred direction. the well-known tendency is to develop stronger gradients perpendicular to the magnetic field, relative to the direction along the field. this anisotropy of the spectrum is deeply connected with the anisotropy of estimated timescales for dynamical processes and requires reconsideration of basic issues such as scale locality and spectral transfer. here, analysis of high-resolution three-dimensional simulations of unforced magnetohydrodynamic turbulence permits quantitative assessment of the behavior of theoretically relevant timescales in fourier wavevector space. we discuss the distribution of nonlinear times, alfvén times, and estimated spectral transfer rates. attention is called to the potential significance of special regions of the spectrum, such as the two-dimensional limit and the "critical balance" region. a formulation of estimated spectral transfer in terms of a suppression factor supports a conclusion that the quasi-two-dimensional fluctuations (characterized by strong nonlinearities) are not a singular limit, but may be in general expected to make important contributions. | a detailed examination of anisotropy and timescales in three-dimensional incompressible magnetohydrodynamic turbulence |
we apply theoretical spin-down models of magnetospheric evolution and magnetic field decay to simulate the possible evolution of psr j0250+5854, which is the slowest-spinning radio pulsar detected to date. considering the alignment of inclination angle in a 3d magnetosphere, it is possible that psr j0250+5854 has a high magnetic field comparable with magnetars or/and high magnetic field pulsars, if a small inclination angle is considered. our calculations show that similar long-period pulsars tend to have a relatively low period derivative in this case. in another case of magnetic field decay, calculations also show a possible connection between psr j0250+5854 and high dipole-magnetic field magnetars. the evolutionary path indicates a relatively high spin-down rate for similar long-period pulsars. | rotational evolution of the slowest radio pulsar, psr j0250+5854 |
recent timing observation reported that the radio pulsar psr j1734-3333 with a rotating period p = 1.17 s is slowing down with a period derivative dot{p}=2.28× 10^{-12} s s^{-1}. its derived braking index n = 0.9 ± 0.2 is the lowest value among young radio pulsars with the measured braking indices. in this letter, we attempt to investigate the influence of the braking torque caused by the interaction between the fall-back disc and the strong magnetic field of the pulsar on the spin evolution of psr j1734-3333. analytical result show that this braking torque is obviously far more than that by magnetic dipole radiation for pulsars with spin period of >0.1 s, and play an important role during the spin-down of the pulsars. our simulated results indicate that, for some typical neutron star parameters, the braking index and the period derivative approximately in agreement with the measured value of psr j1734-3333 if the material inflow rate in the fall-back disc is 2 × 1017 g s- 1. in addition, our scenario can account for the measured braking indices of four young pulsars. however, our predicted x-ray luminosity are one to two order of magnitude higher than the observation. we proposed that this discrepancy may originate from the instability of fall-back disc. | low braking index of psr j1734-3333: an interaction between fall-back disc and magnetic field? |
to understand the essential physics needed to reproduce magnetic reconnection events in 2.5-d particle-in-cell (pic) simulations, we revisit the geospace environmental modeling (gem) setup. we set up a 2-d harris current sheet (that also specifies the initial conditions) to evolve the reconnection of antiparallel magnetic fields. in contrast to the gem setup, we use a much smaller initial perturbation to trigger the reconnection and evolve it more self-consistently. from pic simulation data with high-quality particle statistics, we study a symmetric reconnection site, including separatrix layers, as well as the inflow and the outflow regions. the velocity distribution functions (vdfs) of electrons have a fine structure and vary strongly depending on their location within the reconnection setup. the goal is to start cataloging multidimensional fine-structured electron velocity distributions showing different reconnection processes in the earth's magnetotail under various conditions. this will enable a direct comparison with observations from, e.g., the nasa magnetospheric multiscale (mms) mission, to identify reconnection-related events. we find regions with strong non-gyrotropy also near the separatrix layer and provide a refined criterion to identify an electron diffusion region in the magnetotail. the good statistical significance of this work for relatively small analysis areas reveals the gradual changes within the fine structure of electron vdfs depending on their sampling site. | catalog of fine-structured electron velocity distribution functions - part 1: antiparallel magnetic-field reconnection (geospace environmental modeling case) |
context. x-ray observations of protostellar jets show evidence of strong shocks heating the plasma up to temperatures of a few million degrees. in some cases, the shocked features appear to be stationary. they are interpreted as shock diamonds.aims: we investigate the physics that guides the formation of x-ray emitting stationary shocks in protostellar jets; the role of the magnetic field in determining the location, stability, and detectability in x-rays of these shocks; and the physical properties of the shocked plasma.methods: we performed a set of 2.5-dimensional magnetohydrodynamic numerical simulations that modelled supersonic jets ramming into a magnetized medium and explored different configurations of the magnetic field. the model takes into account the most relevant physical effects, namely thermal conduction and radiative losses. we compared the model results with observations, via the emission measure and the x-ray luminosity synthesized from the simulations.results: our model explains the formation of x-ray emitting stationary shocks in a natural way. the magnetic field collimates the plasma at the base of the jet and forms a magnetic nozzle there. after an initial transient, the nozzle leads to the formation of a shock diamond at its exit which is stationary over the time covered by the simulations ( 40-60 yr; comparable with timescales of the observations). the shock generates a point-like x-ray source located close to the base of the jet with luminosity comparable with that inferred from x-ray observations of protostellar jets. for the range of parameters explored, the evolution of the post-shock plasma is dominated by the radiative cooling, whereas the thermal conduction slightly affects the structure of the shock. a movie is available at http://www.aanda.org | formation of x-ray emitting stationary shocks in magnetized protostellar jets |
we investigate the accretion-induced spin-up of the black hole via numerical simulations. our method is based on general relativistic magnetohydrodynamics of the slowly rotating flows in the kerr metric, where possibly transonic shock fronts may form. we account for the changing black hole mass and spin during accretion that enforces dynamical evolution of the spacetime metric. we first study nonmagnetized flows with shocks, and we also include magnetic field endowed in the gas. the aim of this study is to verify whether the high-mass black holes may be produced with large spins, even though at birth the collapsars might have contained slowly or moderately spinning cores. in this way, we put constraints on the content of angular momentum in the collapsing massive stars. our studies are also showing that shock fronts and magnetic fields may halt accretion and limit the black hole spin-up in the exploding supernovae. | accretion-induced black hole spin-up revised by numerical general relativistic mhd |
the magnetic inclination angle χ, namely the angle between the spin and magnetic axes of a neutron star, plays a vital role in its observational characteristics. however, there are few systematic investigations of its long-term evolution, especially for accreting nss in binary systems. applying the model of biryukov & abolmasov and the binary evolution code mesa, we simultaneously simulate the evolution of the accretion rate, spin period, magnetic field, and magnetic inclination angle of accreting nss in intermediate/low x-ray binaries. we show that the evolution of χ depends not only on the initial parameters of the binary systems, but also on the mass transfer history and the efficiency of pulsar loss. based on the calculated results we present the characteristic distribution of χ for various types of systems including ultracompact x-ray binaries, binary millisecond pulsars, and ultraluminous x-ray sources, and discuss their possible observational implications. | magnetic inclination evolution of accreting neutron stars in intermediate/low-mass x-ray binaries |
in this paper we present the first set of 3d magnetohydrodynamic (mhd) simulations performed with the riemann geomesh code. we study the dynamics of the magnetically channeled winds of magnetic massive stars in full three dimensions using a code that is uniquely suited to spherical problems. specifically, we perform isothermal simulations of a smooth wind on a rotating star with a tilted, initially dipolar field. we compare the mass-loss, angular momentum loss, and magnetospheric dynamics of a template star (with the properties that are reminiscent of the o4 supergiant ζ pup) over a range of rotation rates, magnetic field strengths, and magnetic tilt angles. the simulations are run up to a quasi-steady state and the results are observed to be consistent with the existing literature, showing the episodic centrifugal breakout events of the mass outflow, confined by the magnetic field loops that form the closed magnetosphere of the star. the catalogued results provide perspective on how angular-momentum loss varies for different configurations of rotation rate, magnetic field strength, and large magnetic tilt angles. in agreement with previous 2d mhd studies, we find that high magnetic confinement reduces the overall mass-loss rate, and higher rotation increases the mass-loss rate. this and future studies will be used to estimate the angular-momentum evolution, spin-down time, and mass-loss evolution of magnetic massive stars as a function of magnetic field strength, rotation rate, and dipole tilt. | modelling magnetically channeled winds in 3d - i. isothermal simulations of a magnetic o supergiant |
we investigate dispersive and kinetic effects on the evolution of a two-dimensional kinked alfvén wave packet by comparing results from mhd, hall-mhd and hybrid simulations of a low-$\beta$ plasma. we find that the hall term determines the overall evolution of the wave packet over a characteristic time $\tau^*=\tau_a\ell/d_i$ in both fluid and hybrid models. dispersion of the wave packet leads to the conversion of the wave energy into internal plasma energy. when kinetic protons are considered, the proton internal energy increase has contributions from both plasma compressions and phase space mixing. the latter occurs in the direction parallel to the guiding mean magnetic field, due to protons resonating at the alfvén speed with a compressible mode forced by the wave packet. implications of our results for switchbacks observations and solar wind energetics are discussed. | dispersive and kinetic effects on kinked alfvén wave packets: a comparative study with fluid and hybrid models |
thermal performance maximization is a necessity in various systems that are dealing with any scale of heat flux. this requirement has been intensified due to introduction of more complex and highly crammed products, making heat dissipation a daunting task. addressing this challenge, a curved porous star-shaped enclosure with a rounded cavity and occupied by hybrid nanoparticles of fe3o4-al2o3 scattered uniformly in 1-hexanol has been simulated by galerkin finite element method. the temperature difference between inner cavity and outer wavy surface stirred the heat flux within the bounded domain. the determinants of thermal evolution are classified by porosity alteration, radiation intensity, magnetic field and natural convection strength in the form of corresponding dimensionless numbers namely epsilon, rd, ha and ra. the results revealed that 0.01, 99.99 and 0.57 are the optimum values for ha, ra and rd while regarding the porosity, the best output was recognized at ε of 0.1, with radiation having no sizeable impact on the nu and flow field. due to contradictory influence of the studies factors, an optimization by rsm and taguchi incorporation led to the detection of optimum nu and introduction of an expression for average nu based on the investigated determinants. | radiation and convection heat transfer optimization with mhd analysis of a hybrid nanofluid within a wavy porous enclosure |
exploding granules constitute the strongest horizontal flows on the quiet sun and contribute to the structure of the surface horizontal velocity fields which build the large-scale organization of the discrete magnetic field. in this work we explore exploding granule expansion through the observations of the ground-based themis telescope, iris, sdo, and the hinode space-borne instruments, and finally with the magnetohydrodynamics simulation. we evaluate the detection and the expansion of exploding granules at several wavelengths and at various spatial and temporal resolutions. to analyze the different temporal sequences, two methods of image segmentation are applied to select the granules. the first allows us to follow individually the exploding granules observed simultaneously by themis, iris, and sdo. the second uses long time independent sequences from themis, iris, sdo, hinode, and a simulation. in the first method (called manual) the segmentation isolates the cell of the granules (bright granules and intergranular parts), while in the second method (called statistical) only the bright part of the granules are isolated. the results obtained with simultaneous or distinct temporal observations using the two methods of segmentation are in good agreement. the granule area evolves linearly with an expansion velocity that decreases with the radius. a rapid decrease in the velocity expansion in the first two minutes is observed. the detection and measurement of the dynamics of the explosive granules can be performed from ground- and space-based instruments. our work reveals the usefulness of sdo data, with low spatial resolution, to study the dynamics of the exploding granules all over the solar surface. | evolution of exploding granules from coordinated observations by themis, iris, sdo/hmi, and hinode, and a simulation |
we use 2.5d magnetohydrodynamic simulations to investigate the spectral signatures of the non-linear disruption of a tearing unstable current sheet via the generation of multiple secondary current sheets and magnetic islands. during the non-linear phase of tearing mode evolution, there develops a regime in which the magnetic energy density shows a spectrum with a power law close to b(k)2 ∼ k-0.8. such an energy spectrum is found in correspondence of the neutral line, within the diffusion region of the primary current sheet, where energy is conveyed towards smaller scales via a 'recursive' process of fast tearing-type instabilities. far from the neutral line, we find that magnetic energy spectra evolve towards slopes compatible with the 'standard' kolmogorov spectrum. starting from a self-similar description of the non-linear stage at the neutral line, we provide a model that predicts a reconnecting magnetic field energy spectrum scaling as k-4/5, in good agreement with numerical results. an extension of the predicted power law to generic current sheet profiles is also given and possible implications for turbulence phenomenology are discussed. these results provide a step forward to understand the 'recursive' generation of magnetic islands (plasmoids), which has been proposed as a possible explanation for the energy release during flares, but which, more in general, can have an impact on the subsequent turbulent evolution of unstable sheets that naturally form in the high lundquist number and collisionless plasmas found in most of the astrophysical environments. | spectral signatures of recursive magnetic field reconnection |
context. the recent mass measurements of two binary millisecond pulsars, psr j1614-2230 and psr j0751+1807 with a mass m = 1.97 ± 0.04 m⊙ and m = 1.26 ± 0.14 m⊙, respectively, indicate a wide range of masses for such objects and possibly also a broad spectrum of masses of neutron stars born in core-collapse supernovae.aims: starting from the zero-age main sequence binary stage, we aim at inferring the birth masses of psr j1614-2230 and psr j0751+1807 by taking the differences in the evolutionary stages preceding their formation into account.methods: using simulations for the evolution of binary stars, we reconstruct the evolutionary tracks leading to the formation of psr j1614-2230 and psr j0751+1807. we analyse in detail the spin evolution due to the accretion of matter from a disk in the intermediate-mass/low-mass x-ray binary. we consider two equations of state of dense matter, one for purely nucleonic matter and the other one including a high-density softening due to the appearance of hyperons. stationary and axisymmetric stellar configurations in general relativity are used, together with a recent magnetic torque model and observationally-motivated laws for the decay of magnetic field.results: the estimated birth mass of the neutron stars psr j0751+1807 and psr j1614-2230 could be as low as 1.0 m⊙ and as high as 1.9 m⊙, respectively. these values depend weakly on the equation of state and the assumed model for the magnetic field and its accretion-induced decay.conclusions: the masses of progenitor neutron stars of recycled pulsars span a broad interval from 1.0 m⊙ to 1.9 m⊙. including the effect of a slow roche-lobe detachment phase, which could be relevant for psr j0751+1807, would make the lower mass limit even lower. a realistic theory for core-collapse supernovæ should account for this wide range of mass. | progenitor neutron stars of the lightest and heaviest millisecond pulsars |
context. recent spectropolarimetric surveys of main-sequence intermediate-mass stars have exhibited a dichotomy in the distribution of the observed magnetic field between the kg dipoles of ap/bp stars and the sub-gauss magnetism of vega and sirius.aims: we would like to test whether this dichotomy is linked to the stability versus instability of large-scale magnetic configurations in differentially rotating radiative zones.methods: we computed the axisymmetric magnetic field obtained from the evolution of a dipolar field threading a differentially rotating shell. a full parameter study including various density profiles and initial and boundary conditions was performed with a 2d numerical code. we then focused on the ratio between the toroidal and poloidal components of the magnetic field and discuss the stability of the configurations dominated by the toroidal component using local stability criteria and insights from recent 3d numerical simulations.results: the numerical results and a simple model show that the ratio between the toroidal and the poloidal magnetic fields is highest after an alfvén crossing time of the initial poloidal field. for high density contrasts, this ratio converges towards an asymptotic value that can thus be extrapolated to realistic stellar cases. we then consider the stability of the magnetic configurations to non-axisymmetric perturbations and find that configurations dominated by the toroidal component are likely to be unstable if the shear strength is significantly higher than the poloidal alfvén frequency. an expression for the critical poloidal field below which magnetic fields are likely to be unstable is found and is compared to the lower bound of ap/bp magnetic fields. | evolution of a magnetic field in a differentially rotating radiative zone |
the regulation of galactic-scale star formation rates (sfrs) is a basic problem for theories of galaxy formation and evolution: which processes are responsible for making observed star formation rates so inefficient compared to maximal rates of gas content divided by dynamical timescale? here we study the effect of magnetic fields of different strengths on the evolution of giant molecular clouds (gmcs) within a kiloparsec patch of a disk galaxy and resolving scales down to ≃ 0.5 pc. including an empirically motivated prescription for star formation from dense gas ({{n}h}\gt {{10}5} c{{m}-3}) at an efficiency of 2% per local free-fall time, we derive the amount of suppression of star formation by magnetic fields compared to the nonmagnetized case. we find gmc fragmentation, dense clump formation, and sfr can be significantly affected by the inclusion of magnetic fields, especially in our strongest investigated b-field case of 80 μg. however, our chosen kiloparsec-scale region, extracted from a global galaxy simulation, happens to contain a starbursting cloud complex that is only modestly affected by these magnetic fields and likely requires internal star formation feedback to regulate its sfr. | magnetic fields and galactic star formation rates |
aims: we conduct simulations of the inner regions of protoplanetary disks (ppds) to investigate the effects of protostellar magnetic fields on their long-term evolution. we use an inner boundary model that incorporates the influence of a stellar magnetic field. the position of the inner disk is dependent on the mass accretion rate as well as the magnetic field strength. we use this model to study the response of a magnetically truncated inner disk to an episodic accretion event. additionally, we vary the protostellar magnetic field strength and investigate the consequences of the magnetic field on the long-term behavior of ppds.methods: we use the fully implicit 1+1d tapir code which solves the axisymmetric hydrodynamic equations self-consistently. our model allows us to investigate disk dynamics close to the star and to conduct long-term evolution simulations simultaneously. we assume a hydrostatic vertical configuration described via an energy equation which accounts for the radiative transport in the vertical direction in the optically thick limit and the equation of state. moreover, our model includes the radial radiation transport in the stationary diffusion limit and takes protostellar irradiation into account.results: we include stellar magnetic torques, the influence of a pressure gradient, and a variable inner disk radius in the tapir code to describe the innermost disk region in a more self-consistent manner. we can show that this approach alters the disk dynamics considerably compared to a simplified diffusive evolution equation, especially during outbursts. during a single outburst, the angular velocity deviates significantly from the keplerian velocity because of the influence of stellar magnetic torques. the disk pressure gradient switches sign several times and the inner disk radius is pushed towards the star, approaching < 1.2 r⋆. additionally, by varying the stellar magnetic field strength, we can demonstrate several previously unseen effects. the number, duration, and the accreted disk mass of an outburst as well as the disk mass at the end of the disk phase (after several million years) depend on the stellar field strength. furthermore, we can define a range of stellar magnetic field strengths, in which outbursts are completely suppressed. the robustness of this result is confirmed by varying different disk parameters.conclusions: the influences of a prescribed stellar magnetic field, local pressure gradients, and a variable inner disk radius result in a more consistent description of the gas dynamics in the innermost regions of ppds. combining magnetic torques acting on the innermost disk regions with the long-term evolution of ppds yields previously unseen results, whereby the whole disk structure is affected over its entire lifetime. additionally, we want to emphasize that a combination of our 1+1d model with more sophisticated multi-dimensional codes could improve the understanding of ppds even further. | time-dependent, long-term hydrodynamic simulations of the inner protoplanetary disk. i. the importance of stellar magnetic torques |
we analyze the forces that control the dynamic evolution of a flux rope eruption in a three-dimensional radiative magnetohydrodynamic simulation. the confined eruption of the flux rope gives rise to a c8.5 flare. the flux rope rises slowly with an almost constant velocity of a few kilometers per second in the early stage when the gravity and lorentz force are nearly counterbalanced. after the flux rope rises to the height at which the decay index of the external poloidal field satisfies the torus instability criterion, the significantly enhanced lorentz force breaks the force balance and drives the rapid acceleration of the flux rope. fast magnetic reconnection is immediately induced within the current sheet under the erupting flux rope, which provides strong positive feedback to the eruption. the eruption is eventually confined due to the tension force from the strong external toroidal field. our results suggest that the gravity of plasma plays an important role in sustaining the quasi-static evolution of the preeruptive flux rope. the lorentz force, which is contributed from both the ideal magnetohydrodynamic instability and magnetic reconnection, dominates the dynamic evolution during the eruption process. | radiative magnetohydrodynamic simulation of the confined eruption of a magnetic flux rope: unveiling the driving and constraining forces |
we study the evolution of abelian electromagnetic as well as non-abelian gauge fields, in the presence of space-time oscillations. in the non-abelian case, we consider linear approximation, to analyse the time evolution of the field modes. in both abelian and non-abelian, the mode equations, show the presence of the same parametric resonant spatial modes. the large growth of resonant modes induces large fluctuations in physical observables including those that break the $cp-$symmetry. we also evolve small random fluctuations of fields, using numerical simulations in $2+1$ dimensions. these simulations help study non-linear effects $vs$ the gauge coupling, in the non-abelian case. our results show that there is an increase in energy density with the coupling, at late times. these results suggest that gravitational waves may excite non-abelian gauge fields more efficiently than electromagnetic fields. also, gravitational waves in the early universe and from the merger of neutron stars, black holes etc. may enhance $cp-$violation and generate an imbalance in chiral charge distributions, magnetic fields etc. | parametric resonance in abelian and non-abelian gauge fields via space-time oscillations |
we study the photospheric evolution of an exploding granule observed in the quiet sun at high spatial (∼0"3) and temporal (31.5 s) resolution by the imaging magnetograph sunrise/imax in 2009 june. these observations show that the exploding granule is cospatial to a magnetic flux emergence event occurring at mesogranular scale (up to ∼12 mm2 area). using a modified version of the sir code for inverting the imax spectropolarimetric measurements, we obtain information about the magnetic configuration of this photospheric feature. in particular, we find evidence of highly inclined emerging fields in the structure, carrying a magnetic flux content up to ∼4 × 1018 mx. the balance between gas and magnetic pressure in the region of flux emergence, compared with a very quiet region of the sun, indicates that the additional pressure carried by the emerging flux increases the total pressure by about 5% and appears to allow the granulation to be modified, as predicted by numerical simulations. the overall characteristics suggest that a multipolar structure emerges into the photosphere, resembling an almost horizontal flux sheet. this seems to be associated with exploding granules. finally, we discuss the origin of such flux emergence events. | on the magnetic nature of an exploding granule as revealed by sunrise/imax |
ultraluminous x-ray sources are usually believed to be black holes with mass about 102-3 m⊙. however, the recent discovery of nustar j095551+6940.8 in m82 by bachetti et al. shows that it holds the spin period p = 1.37 s and period derivative \dot{p}≈ -2× 10^{-10} s s^{-1}, which provides a strong evidence that some ultraluminous x-ray sources could be neutron stars. we obtain that the source may be an evolved magnetar according to our simulation by employing the model of accretion induced the polar magnetic field decay and standard spin-up torque of an accreting neutron star. the results show that nustar j095551+6940.8 is still in the spin-up process, and the polar magnetic field decays to about 4.5 × 1012 g after accreting ∼10-2.5 m⊙, while the strong magnetic field exists in the out-polar region, which could be responsible for the observed low field magnetar. the ultra luminosity of the source can be explained by the beaming effect and two kinds of accretion-radial random accretion and disc accretion. since the birth rate of magnetars is about ten per cent of the normal neutron stars, we guess that several ultraluminous x-ray sources should share the similar properties to that of nustar j095551+6940.8. | the magnetic field evolution of ulx nustar j095551+6940.8 in m82 - a legacy of accreting magnetar |
we investigate the dependence of the gamma-ray burst (grb) jet structure and its evolution on the properties of the accreting torus in the central engine. our models numerically evolve the accretion disk around a kerr black hole using three-dimensional general relativistic magnetohydrodynamic simulations. we use two different analytical hydrodynamical models of the accretion disk, based on the fishbone-moncrief and chakrabarti solutions, as our initial states for the structure of the collapsar disk and the remnant after a binary neutron star (bns) merger, respectively. we impose poloidal magnetic fields of two different geometries upon the initial stable solutions. we study the formation and evolution of the magnetically arrested disk state and its effect on the properties of the emitted jet. the jets produced in our models are structured and have a relatively hollow core and reach higher lorentz factors at an angle ≳9° from the axis. the jet in our short grb model has an opening angle of up to ~25° while our long grb engine produces a narrower jet, of up to ~11°. we also study the time variability of the jets and provide an estimate of the minimum variability timescale in our models. the application of our models to the grb jets in the bns postmerger system and to the ultrarelativistic jets launched from collapsing stars are briefly discussed. | modeling the gamma-ray burst jet properties with 3d general relativistic simulations of magnetically arrested accretion flows |
we carry out three-dimensional and two-dimensional particle-in-cell simulations of the expansion of a magnetized plasma that initially uniformly fills a half-space and contains a semicylindrical region of heated electrons elongated along the surface of the plasma boundary. this geometry is related, for instance, to ablation of a plane target by a femtosecond laser beam under quasi-cylindrical focusing. we find that a decay of the inhomogeneous plasma-vacuum discontinuity is strongly affected by an external magnetic field parallel to its boundary. we observe various transient phenomena, including an anisotropic scattering of electrons and an accompanying weibel instability, and reveal various spatial structures of the arising magnetic field and current, including multiple flying-apart filaments of a z-pinch type and slowly evolving current sheets with different orientations. the magnitude of the self-generated magnetic field can be of the order of, or significantly exceed that of, the external one. such phenomena are expected in the laser and cosmic plasmas, including the explosive processes in the planetary magnetospheres and stellar coronal arches. | multiscale magnetic field structures in an expanding elongated plasma cloud with hot electrons subject to an external magnetic field |
context. in the absence of an initial seed, the biermann battery term of a non-ideal induction equation acts as a source that generates weak magnetic fields. these fields are then amplified via a dynamo mechanism. the kelvin-helmholtz instability is a fluid phenomenon that takes place in many astrophysical scenarios and can trigger the action of the biermann battery and dynamo processes.aims: we aim to investigate the effect of the ionisation degree of the plasma and the interaction between the charged and neutral species on the generation and amplification of magnetic fields during the different stages of the instability.methods: we use the two-fluid model implemented in the numerical code mancha-2f. we perform 2d simulations starting from a configuration with no initial magnetic field and which is unstable due to a velocity shear. we vary the ionisation degree of the plasma and we analyse the role that the different collisional terms included in the equations of the model play on the evolution of the instability and the generation of magnetic field.results: we find that when no collisional coupling is considered between the two fluids, the effect of the biermann battery mechanism does not depend on the ionisation degree. however, when elastic collisions are taken into account, the generation of magnetic field is increased as the ionisation degree is reduced. this behaviour is slightly enhanced if the process of charge-exchange is also considered. we also find a dependence on the total density of the plasma related to the dependence on the coupling degree between the two fluids. as the total density is increased, the results from the two-fluid model converge to the predictions of single-fluid models.conclusions: the charged-neutral interaction in a partially ionised plasmas has a non-negligible effect on the biermann battery mechanism and it effectively enhances the generation of a magnetic field. in addition, single-fluid models, which assume a very strong coupling between the two species, may overestimate the contribution of this interaction in comparison with two-fluid models. movies associated to figs. 2 and a.2 are available at https://www.aanda.org | simulations of the biermann battery mechanism in two-fluid partially ionised plasmas |
a pulsating ultraluminous x-ray source (pulx) is a new kind of pulsar (psr) whose characteristics are different from all known neutron stars. the magnetic field of pulx is suspected to be the main reason to support its supper eddington luminosity of pulx. ngc 7793 p13, which is the second confirmed pulx, can be easily studied due to its nearby position and isolation from other sources in its host galaxy. in this paper, we calculate its magnetic field to be ~1.0 × 1012 g based on the continued observations from 2016 to 2020. the magnetic field evolution of ngc 7793 p13 is analyzed, which shows that the source has spent about 104 yr for the field decaying from the simulated initial strength 4.0 × 1014 g to the present value. in case of an assumed constant accretion and the limitation of the companion mass, it will be a recycled psr whose magnetic field is ~109 g and spin period is a few hundred milliseconds. we estimate the field strength of the other confirmed pulxs and find main range is 1013-1014 g. their positions of the magnetic field and spin period are around or below the magnetars. this is because these pulxs are in the binary systems and are with the spin-up rate that are 2-3 orders higher than the normal binary pulsars. we suggest that pulxs are the accreting magnetars whose multi-pole strong magnetic field can support the supper eddington luminosity. they would be helpful for studying the evolution of the magnetars, the formation of the binary psrs above the eddington spin-up line, and the millisecond psrs with the magnetic field stronger than ~109 g. | research on the magnetic field of ngc 7793 p13 and other confirmed pulsating ultraluminous x-ray sources |
galaxy mergers are expected to play a central role for the evolution of galaxies and may have a strong effect on their magnetic fields. we present the first grid-based 3d magnetohydrodynamical simulations investigating the evolution of magnetic fields during merger events. for this purpose, we employed a simplified model considering the merger event of magnetized gaseous disks in the absence of stellar feedback and without a stellar or dark matter component. we show that our model naturally leads to the production of two peaks in the evolution of the average magnetic field strength within 5 kpc, within 25 kpc, and on scales in between 5 and 25 kpc. the latter is consistent with the peak in the magnetic field strength previously reported in a merger sequence of observed galaxies. we show that the peak on the galactic scale and in the outer regions is most likely due to geometrical effects, as the core of one galaxy enters the outskirts of the other one. in addition, the magnetic field within the central ~5 kpc is physically enhanced, which reflects the enhancement in density that is due to efficient angular momentum transport. we conclude that high-resolution observations of the central regions will be particularly relevant for probing the evolution of magnetic field structures during merger events. | magnetic fields during galaxy mergers |
context. recent sunrise/imax observations have revealed supersonic magnetic flows.aims: our aim is to determine the origin of these flows by using realistic magnetohydrodynamics simulations.methods: we simulated cancellation and emergence of magnetic flux through the solar photosphere. our first numerical experiment started with a magnetic field of both polarities. to simulate emergence into a region with pre-existing field, we introduced a large-scale horizontally uniform sheet of a horizontal field. we followed the subsequent evolution and created synthetic polarimetric observations, including known instrumental effects of the sunrise/imax and hinode/sp instruments. we compared the simulated and observed spectropolarimetric signals.results: strongly blue- and redshifted stokes v signals are produced in locations where strong line-of-sight velocities coincide with the strong line-of-sight component of the magnetic field. the size and strength of simulated events is smaller than observed, and they are mostly associated with downflows, contrary to observations. in a few cases where they appear above a granule, single blue-lobed stokes v are produced by strong gradients in magnetic field and velocity. no change of magnetic field sign is detected along the line of sight in these instances. more high-speed magnetised flows occurred when an emergence was simulated than when no horizontal field was added.conclusions: the simulations indicate that the observed events result from magnetic flux emergences in which reconnection may take place, but does not seem to be necessary. the movies are available in electronic form at http://www.aanda.org | simulated magnetic flows in the solar photosphere |
context. conservation properties of magnetic helicity and energy in the quasi-ideal and low-β solar corona make these two quantities relevant for the study of solar active regions and eruptions.aims: based on a decomposition of the magnetic field into potential and nonpotential components, magnetic energy and relative helicity can both also be decomposed into two quantities: potential and free energies, and volume-threading and current-carrying helicities. in this study, we perform a coupled analysis of their behaviors in a set of parametric 3d magnetohydrodynamic (mhd) simulations of solar-like eruptions.methods: we present the general formulations for the time-varying components of energy and helicity in resistive mhd. we calculated them numerically with a specific gauge, and compared their behaviors in the numerical simulations, which differ from one another by their imposed boundary-driving motions. thus, we investigated the impact of different active regions surface flows on the development of the energy and helicity-related quantities.results: despite general similarities in their overall behaviors, helicities and energies display different evolutions that cannot be explained in a unique framework. while the energy fluxes are similar in all simulations, the physical mechanisms that govern the evolution of the helicities are markedly distinct from one simulation to another: the evolution of volume-threading helicity can be governed by boundary fluxes or helicity transfer, depending on the simulation.conclusions: the eruption takes place for the same value of the ratio of the current-carrying helicity to the total helicity in all simulations. however, our study highlights that this threshold can be reached in different ways, with different helicity-related processes dominating for different photospheric flows. this means that the details of the pre-eruptive dynamics do not influence the eruption-onset helicity-related threshold. nevertheless, the helicity-flux dynamics may be more or less efficient in changing the time required to reach the onset of the eruption. | energy and helicity fluxes in line-tied eruptive simulations |
context. while non-potential (free) magnetic energy is a necessary element of any active phenomenon in the solar corona, its role as a marker of the trigger of the eruptive process remains elusive. meanwhile, recent analyses of numerical simulations of solar active events have shown that quantities based on relative magnetic helicity could highlight the eruptive nature of solar magnetic systems.aims: based on the unique decomposition of the magnetic field into potential and non-potential components, magnetic energy and helicity can also both be uniquely decomposed into two quantities. using two 3d magnetohydrodynamics parametric simulations of a configuration that can produce coronal jets, we compare the dynamics of the magnetic energies and of the relative magnetic helicities.methods: both simulations share the same initial setup and line-tied bottom-boundary driving profile. however, they differ by the duration of the forcing. in one simulation, the system is driven sufficiently so that a point of no return is passed and the system induces the generation of a helical jet. the generation of the jet is, however, markedly delayed after the end of the driving phase; a relatively long phase of lower-intensity reconnection takes place before the jet is eventually induced. in the other reference simulation, the system is driven during a shorter time, and no jet is produced.results: as expected, we observe that the jet-producing simulation contains a higher value of non-potential energy and non-potential helicity compared to the non-eruptive system. focussing on the phase between the end of the driving-phase and the jet generation, we note that magnetic energies remain relatively constant, while magnetic helicities have a noticeable evolution. during this post-driving phase, the ratio of the non-potential to total magnetic energy very slightly decreases while the helicity eruptivity index, which is the ratio of the non-potential helicity to the total relative magnetic helicity, significantly increases. the jet is generated when the system is at the highest value of this helicity eruptivity index. this proxy critically decreases during the jet-generation phase. the free energy also decreases but does not present any peak when the jet is being generated.conclusions: our study further strengthens the importance of helicities, and in particular of the helicity eruptivity index, to understand the trigger mechanism of solar eruptive events. | comparison of magnetic energy and helicity in coronal jet simulations |
using two-dimensional simulations, we numerically explore the dependences of kelvin-helmholtz (kh) instability upon various physical parameters, including viscosity, the width of the sheared layer, flow speed, and magnetic field strength. in most cases, a multi-vortex phase exists between the initial growth phase and the final single-vortex phase. the parametric study shows that the evolutionary properties, such as phase duration and vortex dynamics, are generally sensitive to these parameters, except in certain regimes. an interesting result is that for supersonic flows, the phase durations and saturation of velocity growth approach constant values asymptotically as the sonic mach number increases. we confirm that the linear coupling between magnetic field and kh modes is negligible if the magnetic field is weak enough. the morphological behavior suggests that the multi-vortex coalescence might be driven by the underlying wave-wave interaction. based on these results, we present a preliminary discussion of several events observed in the solar corona. the numerical models need to be further improved to perform a practical diagnostic of the coronal plasma properties. | numerical simulations of kelvin-helmholtz instability: a two-dimensional parametric study |
using two-dimensional hybrid expanding box simulations we study the competition between the continuously driven parallel proton temperature anisotropy and fire hose instabilities in collisionless homogeneous plasmas. for a quasi-radial ambient magnetic field the expansion drives tp\vert >tp\bot and the system becomes eventually unstable with respect to the dominant parallel fire hose instability. this instability is generally unable to counteract the induced anisotropization and the system typically becomes unstable with respect to the oblique fire hose instability later on. the oblique instability efficiently reduces the anisotropy and the system rapidly stabilizes, while a significant part of the generated electromagnetic fluctuations is damped to protons. as long as the magnetic field is in the quasi-radial direction, this evolution repeats itself and the electromagnetic fluctuations accumulate. for a sufficiently oblique magnetic field the expansion drives tp\bot >tp\vert and brings the system to the stable region with respect to the fire hose instabilities. | proton fire hose instabilities in the expanding solar wind |
we investigate the role of ambipolar diffusion (ad) in collisions between magnetized giant molecular clouds (gmcs), which may be an important mechanism for triggering star cluster formation. three-dimensional simulations of gmc collisions are performed using a version of the enzo magnetohydrodynamics code that has been extended to include ad. the resistivities are calculated using the 31-species chemical model of wu et al. (2015). we find that in the weak-field, 10 μ {{g}} case, ad has only a modest effect on the dynamical evolution during the collision. however, for the stronger-field, 30 μ {{g}} case involving near-critical clouds, ad results in the formation of dense cores in regions where collapse is otherwise inhibited. the overall efficiency of formation of cores with {n}{{h}}≥slant {10}6 {{cm}}-3 in these simulations is increases from about 0.2% to 2% once ad is included, comparable to observed values in star-forming gmcs. the gas around these cores typically has relatively slow infall at speeds that are a modest fraction of the free-fall speed. | gmc collisions as triggers of star formation. iv. the role of ambipolar diffusion |
kinetic plasma simulations are nowadays commonly used to study a wealth of nonlinear behaviours and properties in laboratory and space plasmas. in particular, in high-energy physics and astrophysics, the plasma usually evolves in ultra-strong electromagnetic fields produced by intense laser beams for the former or by rotating compact objects such as neutron stars and black holes for the latter. in these ultra-strong electromagnetic fields, the gyro-period is several orders of magnitude smaller than the time scale on which we desire to investigate the plasma evolution. some approximations are required such as, for instance, artificially decreasing the electromagnetic field strength, which is certainly not satisfactory. the main flaw of this downscaling is that it cannot reproduce particle acceleration to ultra-relativistic speeds with a lorentz factor above $γ ≈ 10^3$-$10^4$. in this paper, we design a new algorithm able to catch particle motion and acceleration to a lorentz factor of up to $10^{15}$ or even higher by using lorentz boosts to special frames where the electric and magnetic field are parallel. assuming that these fields are locally uniform in space and constant in time, we solve analytically the equation of motion in a tiny region smaller than the length scale of the spatial and temporal gradient of the field. this analytical integration of the orbit severely reduces the constraint on the time step, allowing us to use large time steps, avoiding resolving the ultra-high gyro-frequency. we performed simulations in ultra-strong spatially and time-dependent electromagnetic fields, showing that our particle pusher is able to follow accurately the exact analytical solution for very long times. this property is crucial to properly capture for instance lepton electrodynamics in electromagnetic waves produced by fast rotating neutron stars. we conclude with a simple implementation of our new pusher into a one-dimensional relativistic electromagnetic particle-in-cell code, testing it against plasma oscillations, two-stream instabilities and strongly magnetized relativistic shocks. | a relativistic particle pusher for ultra-strong electromagnetic fields |
magnetohydrodynamics simulation of active region noaa 11515 is performed to examine the initiation of the m5.6 flaring event that starts around 10:43 ut on 2 july 2012. the simulation is conducted using an extrapolated non-force-free magnetic field generated from the photospheric vector magnetogram of the active region as the initial magnetic field. the magnetic field shows the presence of a three-dimensional (3d) magnetic null with the corresponding dome overlying a filament and a low-lying magnetic flux rope, observed in 304 å and 131 å respectively. the simulated dynamics, triggered by the initial lorentz force, lead to the bifurcations of the flux rope, which is similar to the observed bifurcation in the 131 å brightenings. additionally, the rope exhibits a rise and reconnects at the 3d null. these reconnections convert field lines of the rope into the anchored outer spine of the 3d null—explaining the occurrence of a nearby confined c-class flare. further, the results show that the field lines of the flux rope reach the vicinity of the filament and become non-parallel to the field lines of the filament. this initiates the reconnections between the rope and the field lines of the filament—activating the filament for the eruption. this interesting interaction of the flux rope and filament seems to contribute to the onset of the m-class flare. | magnetohydrodynamics evolution of three-dimensional magnetic null in noaa active region 11515 initiated using non-force-free field extrapolation |
turbulent dynamo theories have faced difficulties in obtaining evolution of large-scale magnetic fields on short dynamical time-scales due to the constraint imposed by magnetic helicity balance. this has critical implications for understanding the large-scale magnetic field evolution in astrophysical systems like the sun, stars, and galaxies. direct numerical simulations (dns) in the past with isotropically forced helical turbulence have shown that large-scale dynamo saturation time-scales are dependent on the magnetic reynolds number (rm). in this work, we have carried out periodic box dns of helically forced turbulence leading to a large-scale dynamo with two kinds of forcing function, an isotropic one based on that used in pencil-code and an anisotropic one based on galloway-proctor flows. we show that when the turbulence is forced anisotropically, the non-linear (saturation) behaviour of the large-scale dynamo is only weakly dependent on rm. in fact, the magnetic helicity evolution on small and large scales in the anisotropic case is distinctly different from that in the isotropic case. this result possibly holds promise for the alleviation of important issues like catastrophic quenching. | saturation of large-scale dynamo in anisotropically forced turbulence |
we examine the temporary evolution of axisymmetric magnetospheres around rapidly rotating black holes (bhs), by applying our two-dimensional particle-in-cell simulation code. assuming a stellar-mass bh, we find that the created pairs fail to screen the electric field along the magnetic field, provided that the mass accretion rate is much small compared to the eddington limit. magnetic islands are created by reconnection near the equator and migrate toward the event horizon, expelling magnetic flux tubes from the bh vicinity during a large fraction of time. when the magnetic islands stick to the horizon due to redshift and virtually vanish, a strong magnetic field penetrates the horizon, enabling efficient extraction of energy from the bh. during this flaring phase, a bh gap appears around the inner light surface with a strong meridional return current toward the equator within the ergosphere. if the mass accretion rate is 0.025% of the eddington limit, the bh's spin-down luminosity becomes 16-19 times greater than its analytical estimate during the flares, although its long-term average is only 6% of it. we demonstrate that the extracted energy flux concentrates along the magnetic field lines threading the horizon in the middle latitudes. it is implied that this meridional concentration of the poynting flux may result in the formation of limb-brightened jets from low-accreting bh systems. | two-dimensional particle-in-cell simulations of axisymmetric black hole magnetospheres: angular dependence of the blandford-znajek flux |
theories for the origins of the statistical properties of binary stellar systems are reviewed. the observed properties of binary stellar systems are briefly summarized, followed by the mechanisms by which binary systems may be formed. the role that accretion is thought to play in determining the final properties of binaries is discussed, and we assess how magnetic fields may affect both binary formation, and their evolution due to accretion. this is followed by a discussion of what has been learnt about the origins of the statistical properties of binary systems from hydrodynamical simulations of stellar group and cluster formation. | origins of the statistical properties of binary systems |
context. the vertical component of the magnetic field was found to reach a constant value at the boundary between penumbra and umbra of stable sunspots in a recent statistical study of hinode/sp data. this finding has profound implications as it can serve as a criterion to distinguish between fundamentally different magneto-convective modes operating in the sun.aims: the objective of this work is to verify the existence of a constant value for the vertical component of the magnetic field (b⊥) at the boundary between umbra and penumbra from ground-based data in the near-infrared wavelengths and to determine its value for the gregor infrared spectrograph (gris@gregor) data. this is the first statistical study on the jurčák criterion with ground-based data, and we compare it with the results from space-based data (hinode/sp and sdo/hmi).methods: eleven spectropolarimetric data sets from the gris@gregor slit-spectograph containing fully-fledged stable sunspots were selected from the gris archive. sir inversions including a polarimetric straylight correction are used to produce maps of the magnetic field vector using the fe i 15648 å and 15662 å lines. averages of b⊥ along the contours between penumbra and umbra are analyzed for the 11 data sets. in addition, contours at the resulting b⊥const are drawn onto maps and compared to intensity contours. the geometric difference between these contours, δp, is calculated for each data set.results: averaged over the 11 sunspots, we find a value of b⊥const = (1787 ± 100) gauss. the difference from the values previously derived from hinode/sp and sdo/hmi data is explained by instrumental differences and by the formation characteristics of the respective lines that were used. contours at b⊥ = b⊥const and contours calculated in intensity maps match from a visual inspection and the geometric distance δp was found to be on the order of 2 pixels. furthermore, the standard deviation between different data sets of averages along umbra-penumbra contours is smaller for b⊥ than for b∥ by a factor of 2.4.conclusions: our results provide further support to the jurčák criterion with the existence of an invariable value b⊥const at the umbra-penumbra boundary. this fundamental property of sunspots can act as a constraining parameter in the calibration of analysis techniques that calculate magnetic fields. it also serves as a requirement for numerical simulations to be realistic. furthermore, it is found that the geometric difference, δp, between intensity contours and contours at b⊥ = b⊥const acts as an index of stability for sunspots. the data from the gris instrument is publicly available in the archive at http://sdc.leibniz-kis.de. | characterization of the umbra-penumbra boundary by the vertical component of the magnetic field. analysis of ground-based data from the gregor infrared spectrograph |
we perform 2.5d particle-in-cell simulations of decaying turbulence in the presence of a guide (out-of-plane) background magnetic field. the fluctuating magnetic field initially consists of fourier modes at low wavenumbers (long wavelengths). with time, the electromagnetic energy is converted to plasma kinetic energy (bulk flow+thermal energy) at the rate per unit volume of j · e for current density j and electric field e . such decaying turbulence is well known to evolve toward a state with strongly intermittent plasma current. here we decompose the electric field into components that are irrotational, e ir, and solenoidal (divergence-free), e so. e ir is associated with charge separation, and j · e ir is a rate of energy transfer between ions and electrons with little net change in plasma kinetic energy. therefore, the net rate of conversion of electromagnetic energy to plasma kinetic energy is strongly dominated by j · e so, and for a strong guide magnetic field, this mainly involves the component e so,∥ parallel to the total magnetic field b . we examine various indicators of the spatial distribution of the energy transfer rate j ∥ · e so,∥, which relates to magnetic reconnection, the best of which are (1) the ratio of the out-of-plane electric field to the in-plane magnetic field, (2) the out-of-plane component of the nonideal electric field, and (3) the magnitude of the estimate of current helicity | role of parallel solenoidal electric field on energy conversion in 2.5d decaying turbulence with a guide magnetic field |
at present, j1819-1458 is the only rotating radio transient (rrat) detected in x-rays. we have studied the long-term evolution of this source in the fallback disc model. the model can reproduce the period, period derivative, and x-ray luminosity of j1819-1458 simultaneously in the accretion phase at ages ∼2 × 105 yr. we obtained reasonable model curves with a magnetic dipole field strength b0 ∼ 5 × 1011 g on the pole of the neutron star, which is much weaker than the field inferred from the dipole-torque formula. with this b0 and the measured period, we find j1819-1458 below and close to the radio pulsar death line. our results are not sensitive to initial period, and the source properties can be produced with a large range of disc masses. our simulations indicate that j1819-1458 is evolving towards the properties of dim isolated neutron stars at later phases of evolution. this implies a close evolutionary link between rrats and dim isolated neutron stars. for other rrats with measured period derivatives and unknown x-ray luminosities, we have estimated the lower limits on the b0 values in the fallback disc model. these limits allow a dipole field distribution for rrats that could fill the b0 gap between the estimated b0 ranges of dim thermal isolated neutron stars and central compact objects in the same model. | long-term evolution of rrat j1819-1458 |
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