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the frequency dependence of the longitudinal group speeds of trapped sausage waves plays an important role in determining impulsively generated wave trains, which have often been invoked to account for quasi-periodic signals in coronal loops. we examine how the group speeds ({v}{gr}) depend on angular frequency (ω) for sausage modes in pressureless coronal tubes with continuous transverse density distributions by solving the dispersion relation pertinent to the case where the density inhomogeneity of arbitrary form occurs in a transition layer of arbitrary thickness. we find that in addition to the transverse lengthscale l and density contrast {ρ }{{i}}/{ρ }{{e}}, the group speed behavior also depends on the detailed form of the density inhomogeneity. for parabolic profiles, {v}{gr} always decreases with ω first before increasing again, as happens for the much studied top-hat profiles. for linear profiles, however, the behavior of the ω -{v}{gr} curves is more complex. when {ρ }{{i}}/{ρ }{{e}}≲ 6, the curves become monotonical for large values of l. on the other hand, for higher density contrasts, a local maximum {v}{gr}\maxexists in addition to a local minimum {v}{gr}\minwhen coronal tubes are diffuse. with time-dependent computations, we show that the different behavior of group speed curves, the characteristic speeds {v}{gr}\minand {v}{gr}\maxin particular, is reflected in the temporal evolution and morlet spectra of impulsively generated wave trains. we conclude that the observed quasi-periodic wave trains not only can be employed to probe such key parameters as density contrasts and profile steepness, but also have the potential to discriminate between the unknown forms of the transverse density distribution. | impulsively generated sausage waves in coronal tubes with transversally continuous structuring |
we study the buoyant rise of magnetic flux tubes embedded in an adiabatic stratification using two-and three-dimensional, magnetohydrodynamic simulations. we analyze the dependence of the tube evolution on the field line twist and on the curvature of the tube axis in different diffusion regimes. to be able to achieve a comparatively high spatial resolution we use the flash code, which has a built-in adaptive mesh refinement (amr) capability. our 3d experiments reach reynolds numbers that permit a reasonable comparison of the results with those of previous 2d simulations. when the experiments are run without amr, hence with a comparatively large diffusivity, the amount of longitudinal magnetic flux retained inside the tube increases with the curvature of the tube axis. however, when a low-diffusion regime is reached by using the amr algorithms, the magnetic twist is able to prevent the splitting of the magnetic loop into vortex tubes and the loop curvature does not play any significant role. we detect the generation of vorticity in the main body of the tube of opposite sign on the opposite sides of the apex. this is a consequence of the inhomogeneity of the azimuthal component of the field on the flux surfaces. the lift force associated with this global vorticity makes the flanks of the tube move away from their initial vertical plane in an antisymmetric fashion. the trajectories have an oscillatory motion superimposed, due to the shedding of vortex rolls to the wake, which creates a von karman street. | multi-parametric study of rising 3d buoyant flux tubes in an adiabatic stratification using amr |
recently, the black hole x-ray binary (bhxb) nova muscae 1991 has been reported to be experiencing an extremely rapid orbital decay. so far, three bhxbs have anomalously high orbital-period derivatives, which cannot be interpreted by the standard stellar evolution theory. in this work, we investigate whether the resonant interaction between the binary and a surrounding circumbinary (cb) disk could produce the observed orbital-period derivatives. analytical calculations indicate that the observed orbital-period derivatives of xte j1118+480 and a0620-00 can originate from the tidal torque between the binary and a cb disk with a mass of 10-9 m ⊙, which is approximately in agreement with the dust disk mass detected in these two sources. however, nova muscae 1991 was probably surrounded by a heavy cb disk with a mass of 10-7 m ⊙. based on the cb disk model and the anomalous magnetic braking theory, we simulate the evolution of the three bhxbs with intermediate-mass donor stars by using the mesa code. our simulated results are approximately consistent with the observed donor-star masses, orbital periods, and orbital-period derivatives. however, the calculated effective temperatures of the donor stars are higher than indicated by the observed spectral types of two sources. | fast orbital shrinkage of black hole x-ray binaries driven by circumbinary disks |
we investigate the behavior of quantum speed limit (qsl) time for a typical non-markovian system, a central spin coupled to a spin star configuration. we connect the qsl time with an external control and show that the effectiveness of the external magnetic field, as well as the coupling strength, is related to the fundamental bounds that affect the maximum speed at which a quantum system can evolve in its state space. we also demonstrate that a spin bath with larger size may shorten the qsl time, while the upper state population plays an important role for the acceleration of quantum evolution in the memory surrounding. | quantum speed limit in a qubit-spin-bath system |
a significant fraction of white dwarfs possess a magnetic field with strengths ranging from a few kg up to about 1000 mg. however, the incidence of magnetism varies when the white dwarf population is broken down into different spectral types providing clues on the formation of magnetic fields in white dwarfs. several scenarios for the origin of magnetic fields have been proposed from a fossil field origin to dynamo generation at various stages of evolution. offset dipoles are often assumed sufficient to model the field structure, however time-resolved spectropolarimetric observations have revealed more complex structures such as magnetic spots or multipoles. surface mapping of these field structures combined with measured rotation rates help distinguish scenarios involving single star evolution from other scenarios involving binary interactions. i describe key observational properties of magnetic white dwarfs such as age, mass, and field strength, and confront proposed formation scenarios with these properties. | clues to the origin and properties of magnetic white dwarfs |
isolated magnetic white dwarfs have field strengths ranging from 103g to 109g, and constitute an interesting class of objects. the origin of the magnetic field is still the subject of a hot debate. whether these fields are fossil, hence the remnants of original weak magnetic fields amplified during the course of the evolution of the progenitor of white dwarfs, or on the contrary, are the result of binary interactions or, finally, other physical mechanisms that could produce such large magnetic fields during the evolution of the white dwarf itself, remains to be elucidated. in this work, we review the current status and paradigms of magnetic fields in white dwarfs, from both the theoretical and observational points of view. | magnetic white dwarfs: observations, theory and future prospects |
in 2003, the magnetar xte j1810-197 started an outburst that lasted until early 2007. in the following 11 yr, the source stayed in a quiescent/low-activity phase. xte j1810-197 is one of the closest magnetars, hence its x-ray properties can be studied in detail even in quiescence and an extended monitoring has been carried out to study its long-term timing and spectral evolution. here, we report the results of new x-ray observations, taken between 2017 september and 2018 april, with xmm-newton, chandra, and nicer. we derived a phase-connected timing solution yielding a frequency derivative of -9.26(6) × 10-14 hz s-1. this value is consistent with that measured between 2009 and 2011, indicating that the pulsar spin-down rate remained quite stable during the long quiescent period. a spectral analysis of all the x-ray observations taken between 2009 and 2018 does not reveal significant spectral and/or flux variability. the spectrum of xte j1810-197 can be described by the sum of two thermal components with temperatures of 0.15 and 0.3 kev, plus a power-law component with photon index 0.6. we also found evidence for an absorption line at ∼1.2 kev and width of 0.1 kev. due to the long exposure time of the summed xmm-newton observations, we could also carry out a phase-resolved spectral analysis for this source in quiescence. this showed that the flux modulation can be mainly ascribed to the warmer of the two thermal components, whose flux varies by ∼45 per cent along the pulse phase. | the 11 yr of low activity of the magnetar xte j1810-197 |
we present analytic calculations of the electromagnetic torques acting on a magnetic neutron star rotating in vacuum, including near-zone torques associated with the inertia of dipole and quadrupole magnetic fields. we incorporate these torques into the rotational dynamics of a rigid-body neutron star, and show that the effects of the inertial torque can be understood as a modification of the moment of inertia tensor of the star. we apply our rotational dynamics equation to the crab pulsar, including intrinsic distortions of the star and various electromagnetic torques, to investigate the possibility that the counter-alignment of the magnetic inclination angle, as suggested by recent observations, could be explained by pulsar precession. we find that if the effective principal axis of the pulsar is nearly aligned with either the magnetic dipole axis or the rotation axis, then precession may account for the observed counter-alignment over decade time-scales. over the spindown time-scale of the pulsar, the magnetic inclination angle always decreases. | electromagnetic torques, precession and evolution of magnetic inclination of pulsars |
we investigate the density-shear instability in hall-magnetohydrodynamics (hall-mhd) via numerical simulation of the full non-linear problem in the context of magnetar activity. we confirm the development of the instability of a plane-parallel magnetic field with an appropriate intensity and electron density profile, in accordance with analytic theory. we find that the instability also appears for a monotonically decreasing electron number density and magnetic field, a plane-parallel analogue of an azimuthal or meridional magnetic field in the crust of a magnetar. the growth rate of the instability depends on the hall properties of the field (magnetic field intensity, electron number density and the corresponding scaleheights), while being insensitive to weak resistivity. since the hall effect is the driving process for the evolution of the crustal magnetic field of magnetars, we argue that this instability is critical for systems containing strong meridional or azimuthal fields. we find that this process mediates the formation of localized structures with much stronger magnetic field than the average, which can lead to magnetar activity and accelerate the dissipation of the field and consequently the production of ohmic heating. assuming a 5 × 1014 g magnetic field at the base of crust, we anticipate that magnetic field as strong as 1015 g will easily develop in regions of typical size of a few hundred metres, containing magnetic energy of 1043 erg, sufficient to power magnetar bursts. these active regions are more likely to appear in the magnetic equator where the tangential magnetic field is stronger. | magnetar activity via the density-shear instability in hall-mhd |
we have analysed xmm-newton and chandra observations of the transient magnetar xte j1810-197 spanning more than 11 yr, from the initial phases of the 2003 outburst to the current quiescent level. we investigated the evolution of the pulsar spin period and we found evidence for two distinct regimes: during the outburst decay, dot{ν } was highly variable in the range -(2-4.5) × 10-13 hz s-1, while during quiescence the spin-down rate was more stable at an average value of -1 × 10-13 hz s-1. only during ∼3000 d (from mjd 54165 to mjd 56908) in the quiescent stage it was possible to find a phase-connected timing solution, with dot{ν }=-4.9× 10^{-14} hz s-1, and a positive second frequency derivative, ddot{ν }=1.8× 10^{-22} hz s-2. these results are in agreement with the behaviour expected if the outburst of xte j1810-197 was due to a strong magnetospheric twist. | the variable spin-down rate of the transient magnetar xte j1810-197 |
the configuration of the magnetic field in the interior of a neutron star is mostly unknown from observations. theoretical models of the interior magnetic field geometry tend to be oversimplified to avoid mathematical complexity and tend to be based on axisymmetric barotropic fluid systems. these static magnetic equilibrium configurations have been shown to be unstable on a short time-scale against an infinitesimal perturbation. given this instability, it is relevant to consider how more realistic neutron star physics affects the outcome. in particular, it makes sense to ask if elasticity, which provides an additional restoring force on the perturbations, may stabilize the system. it is well known that the matter in the neutron star crust forms an ionic crystal. the interactions between the crystallized nuclei can generate shear stress against any applied strain. to incorporate the effect of the crust on the dynamical evolution of the perturbed equilibrium structure, we study the effect of elasticity on the instability of an axisymmetric magnetic star. in particular, we determine the critical shear modulus required to prevent magnetic instability and consider the corresponding astrophysical consequences. | does elasticity stabilize a magnetic neutron star? |
the evolution of a large-scale poloidal magnetic field in an accretion disc is an important problem because it determines the launching of winds and the feasibility of the magnetorotational instability to generate turbulence or channel flows. recent studies, both semi-analytical calculations and numerical simulations, have highlighted the crucial role non-ideal magnetohydrodynamic effects (ohmic resistivity, hall drift, and ambipolar diffusion), relevant in the protoplanetary disc context, might play in magnetic flux evolution in the disc. we investigate the flux transport in discs through the use of two 1d semi-analytic models in the vertical direction, exploring regimes where different physical source terms and effects dominate. the governing equations for both models are derived by performing an asymptotic expansion in the limit of a thin disc, with the different regimes isolated through setting the relative order of the leading terms between variables. flux transport rates and vertical structure profiles are calculated for a range of diffusivities and disc magnetizations. we found that ohmic and ambipolar diffusivities drive radially outward flux transport with an outwardly inclined field. a wind outflow drives inward flux transport, which is significantly enhanced in the presence of hall drift in the positive polarity case, η _ h (\boldsymbol{b}_ z \cdot \boldsymbol{ω }) > 0, an effect which has only been briefly noted before. coupled only with outward inclination, the hall effect reduces the flux transport given by a background ohmic and/or ambipolar diffusivity, but drives no flux transport when it is the only non-ideal effect present. | local semi-analytic models of magnetic flux transport in protoplanetary discs |
the dependence dp/dt(p) is analyzed for three groups of pulsars with different period values: p > 2 s, 0.1 s < p < 2 s, and p < 0.1 s. under the assumption of a slow decay of the magnetic field of neutron stars during their evolution, as well as the slow change in the angle between its magnetic moment and the rotation axis, we carried out the comparison of observed dependencies dp/dt(p) and the predictions of various models of slowing down the rotation of the pulsar. it is shown that long-period pulsars are braked by losses of the angular momentum by the accelerated particles (the pulsar wind). the mean value of the wind power lp = 2.3 × 1030 erg/s is needed to explain the observed statistical dependence. for pulsars with intermediate values of the period, it is necessary to take into account the joint action of magnetic dipole radiation and the pulsar wind. pulsars with p < 0.1 s are divided into three non-overlapping groups: p < 10 ms, p = 10-100 ms at dp/dt < 10-16 and p = 10-100 ms for dp/dt > 10-16. in the first of them, the magnetic dipole braking and the pulsar wind work together with the predominance of the first mechanism. the slowing down of rotation in the second group is caused by a pulsar wind, in the third one by magnetic dipole radiation. | on braking mechanisms in radio pulsars |
the extreme properties of neutron stars provide unique opportunities to put constraints on new particles and interactions. in this paper, we point out a few interesting ideas that place constraints on light millicharged fermions, with masses below around an ev, from neutron star astrophysics. the model-independent bounds are obtained leveraging the fact that light millicharged fermions may be pair produced copiously via non-perturbative processes in the extreme electromagnetic environments of a neutron star, like a magnetar. the limits are derived based on the requirement that conventional magnetar physics not be catastrophically affected by this non-perturbative production. it will be seen that magnetar energetics, magnetic field evolution and spin-down rates may all be influenced to various degrees by the presence of the millicharged particles. | novel astrophysical probes of light millicharged fermions through schwinger pair production |
we study the evolution of newborn neutron stars in high-mass x-ray binaries interacting with a wind-fed super-eddington disk. the inner disk is regularized to a radiation-dominated quasi-spherical configuration for which we calculate the inner radius of the disk, the total luminosity of the system and the torque acting on the neutron star accordingly, following the evolution of the system through the ejector and early propeller stages. we find that the systems with b ≳ 1013 g pass through a short (∼20 yr) ejector stage appearing as supernova impostors followed by a propeller stage lasting ∼103 yr. in the super-critical propeller stage the system is still bright (l ∼ 1040 erg s-1) due to the spindown power and therefore appears as an ultra-luminous x-ray source (ulx). the system evolves into pulsating ulx (pulx) when the neutron star spins down to a period (p ∼ 1 s) allowing for accretion onto its surface to commence. systems with lower magnetic fields, b ∼ 1011 g, pass through a long (105 yr) super-critical propeller stage with luminosities similar to those of the ultra-luminous super-soft sources (uls), l ≲ 1040 erg s-1. the equilibrium periods of these systems in the accretion stage is about 10 ms, which is much smaller than the typical period range of pulx observed to date. such systems could have a larger population, but their pulsations would be elusive due to the smaller size of the magnetosphere. our results suggest that the uls and some nonpulsating ulx are rapidly spinning and highly magnetized young neutron stars at the super-critical propeller stage. | ultra-luminous x-ray sources as super-critical propellers |
force on proton vortices in superfluid and superconducting matter of neutron stars is calculated at vanishing stellar temperature. both longitudinal (dissipative) and transverse (lorentz-type) components of the force are derived in a coherent way and compared in detail with the corresponding expressions available in the literature. this allows us to resolve a controversy about the form of the lorentz-type force component acting on proton vortices. the calculated force is a key ingredient in magnetohydrodynamics of superconducting neutron stars and is important for modelling the evolution of stellar magnetic field. | force on proton vortices in superfluid neutron stars |
we conducted near-infrared (jhk_s) imaging polarimetry toward the infrared dark cloud (irdc) m 17 swex, including almost all of the irdc filaments as well as its outskirts, with the polarimeter sirpol on the irsf 1.4 m telescope. we revealed the magnetic fields of m 17 swex with our polarization-detected sources that were selected by some criteria based on their near-ir colors and the column densities toward them, which were derived from the herschel data. the selected sources indicate not only that the ordered magnetic field is perpendicular to the cloud elongation as a whole, but also that at both ends of the elongated cloud the magnetic field appears to be bent toward its central part, i.e., a large-scale hourglass-shaped magnetic field perpendicular to the cloud elongation. in addition to this general trend, the elongations of the filamentary subregions within the dense parts of the cloud appear to be mostly perpendicular to their local magnetic fields, while the magnetic fields of the outskirts appear to follow the thin filaments that protrude from the dense parts. the magnetic strengths were estimated to be ∼70-300 μg in the subregions, of which the lengths and average number densities are ∼3-9 pc and ∼2-7 × 103 cm-3, respectively, by the davis-chandrasekhar-fermi method with the angular dispersion of our polarization data and the velocity dispersion derived from the c18o (j = 1-0) data obtained by the nobeyama 45 m telescope. these field configurations and our magnetic stability analysis of the subregions imply that the magnetic field has controlled the formation/evolution of the m 17 swex cloud. | near-infrared imaging polarimetry toward m 17 swex |
we report on pulse profile decomposition analysis of bright transient x-ray pulsar 1a 0535+262 using broadband insight-hxmt observations during a giant outburst of the source in 2020. we show that the observed pulse profile shape can be described in terms of a combination of two symmetric single-pole contributions for a wide range of energies and luminosities for a fixed geometry defining the basic geometry of the pulsar. this corresponds to a slightly distorted dipole magnetic field, i.e., one pole has to be offset by ~12° from the antipodal position of the other pole. we reconstruct the intrinsic beam patterns of the pulsar assuming that the geometry is recovered from the decomposition analysis, and we find evidence for a transition between "pencil" and "fan" beams in energy ranges above the cyclotron line energy, which can be interpreted as a transition from sub- to supercritical accretion regimes associated with the onset of an accretion column. at lower energies, however, the beam pattern appears to be more complex and contains a substantial "fan" beam and an additional "pencil" beam component at all luminosities. the latter is not related to the accretion rate and is stronger in the fading phase of the outburst. we finally discuss the results in the context of other observational and theoretical findings earlier reported for the source in the literature. | beam pattern evolution of accreting x-ray pulsar 1a 0535+262 during its 2020 giant outburst |
an integral field unit (ifu) based on image slicers has been added to the gregor infrared spectrograph (gris). this upgrade to the instrument makes possible 2d spectropolarimetry in the near-infrared by simultaneously recording the full stokes profiles of spectral lines (in a given spectral interval) at all the points in the field of view (fov). it provides high-cadence spectropolarimetric observations at the instrument’s high spatial resolution and high polarization sensitivity at the gregor solar telescope. the ifu is ideal for observing the polarized spectrum of fast-evolving solar features at high spatial and spectral resolutions. the high observing cadence opens the possibility of time-series observations. the analysis of observations to this level of accuracy is essential for understanding the complex dynamics and interactions of solar plasma and magnetic fields. the image slicer of the ifu has eight slices of width 100μm, covering a total fov of 6′′×3′′. it was designed and built within the framework of the european projects solarnet and grest, as a prototype for future instruments of the european solar telescope (est) and was integrated into gris. after two commissioning campaigns in 2017 and 2018, the ifu was finally installed at the end of september 2018 and offered to all observers who use the telescope. | first light of the integral field unit of gris on the gregor solar telescope |
the high-mass x-ray binary ls i + 61°303 is also cataloged as a gamma-ray binary as a result of frequent outbursts at tev photon energies. the system has released two soft-gamma flares in the past, suggesting a magnetar interpretation for the compact primary. this inference has recently gained significant traction following the discovery of transient radio pulses, detected in some orbital phases from the system, as the measured rotation and tentative spin-down rates imply a polar magnetic field strength of bp≳ 1014 g if the star is decelerating via magnetic dipole braking. in this paper, we scrutinize magnetic field estimates for the primary in ls i + 61°303 by analyzing the compatibility of available data with the system's accretion dynamics, spin evolution, age limits, gamma-ray emissions, and radio pulsar activation. we find that the neutron star's age and spin evolution are theoretically difficult to reconcile unless a strong propeller torque is in operation. this torque could be responsible for the bulk of even the maximum allowed spin-down, potentially weakening the inferred magnetic field by more than an order of magnitude. | does the gamma-ray binary ls i + 61°303 harbor a magnetar? |
in this paper, we report 23 magnetic field measurements of the b3iv star hd 23478: 12 obtained from high-resolution stokes v spectra using the espadons (canada-france-hawaii telescope) and narval (télescope bernard lyot) spectropolarimeters, and 11 from medium-resolution stokes v spectra obtained with the dimapol spectropolarimeter (dominion astronomical observatory). hd 23478 was one of two rapidly rotating stars identified as potential `centrifugal magnetosphere' hosts based on ir observations from the apache point observatory galactic evolution experiment survey. we derive basic physical properties of this star including its mass (m=6.1^{+0.8}_{-0.7} m_{⊙), effective temperature (teff = 20 ± 2 kk), radius (r=2.7^{+1.6}_{-0.9} r_{⊙}), and age (τ_age=3^{+37}_{-1} myr). we repeatedly detect weakly variable zeeman signatures in metal, he, and h lines in all our observations corresponding to a longitudinal magnetic field of <bz> ≈ -2.0 kg. the rotational period is inferred from hipparcos photometry (prot = 1.0498(4) d). under the assumption of the oblique rotator model, our observations yield a surface dipole magnetic field of strength bd ≥ 9.5 kg that is approximately aligned with the stellar rotation axis. we confirm the presence of strong and broad hα emission and gauge the volume of this star's centrifugal magnetosphere to be consistent with those of other hα emitting centrifugal magnetosphere stars based on the large inferred alfvén to kepler radius ratio. | confirming hd 23478 as a new magnetic b star hosting an hα-bright centrifugal magnetosphere |
turbulence in space plasmas usually exhibits two regimes separated by a spectral break that divides the so called inertial and kinetic ranges. large scale magnetic fluctuations are dominated by non-linear mhd wave-wave interactions following a -5/3 or -2 slope power-law spectrum. after the break, at scales in which kinetic effects take place, the magnetic spectrum follows a steeper power-law $k^{-\alpha}$ shape given by a spectral index $\alpha > 5/3$. despite its ubiquitousness, the possible effects of a turbulent background spectrum in the quasilinear relaxation of solar wind temperatures are usually not considered. in this work, a quasilinear kinetic theory is used to study the evolution of the proton temperatures in an initially turbulent collisionless plasma composed by cold electrons and bi-maxwellian protons, in which electromagnetic waves propagate along a background magnetic field. four wave spectrum shapes are compared with different levels of wave intensity. we show that a sufficient turbulent magnetic power can drive stable protons to transverse heating, resulting in an increase in the temperature anisotropy and the reduction of the parallel proton beta. thus, stable proton velocity distribution can evolve in such a way as to develop kinetic instabilities. this may explain why the constituents of the solar wind can be observed far from thermodynamic equilibrium and near the instability thresholds. | the effect of background turbulence on the relaxation of ion temperature anisotropy in space plasmas |
aims: massive b-type stars with strong magnetic fields and fast rotation are very rare and pose a mystery for theories of star formation and magnetic field evolution. only two such stars, called σ ori e analogues, were known until recently. a team involved in apogee, one of the sloan digital sky survey iii programs, announced the discovery of two additional rigidly rotating magnetosphere stars, hd 23478 and hd 345439. the magnetic fields in these newly discovered σ ori e analogues have not been investigated so far.methods: in the framework of our eso large programme and one normal eso programme, we carried out low-resolution fors 2 spectropolarimetric observations of hd 23478 and hd 345439.results: in the measurements of hydrogen lines, we discover a rather strong longitudinal magnetic field of up to 1.5 kg in hd 23478 and up to 1.3 kg using the entire spectrum. the analysis of hd 345439 using four subsequent spectropolarimetric subexposures does not reveal a magnetic field at a significance level of 3σ. on the other hand, individual subexposures indicate that hd 345439 may host a strong magnetic field that rapidly varies over 88 min. the fast rotation of hd 345439 is also indicated by the behaviour of several metallic and he i lines in the low-resolution fors 2 spectra that show profile variations already on this short time-scale. based on observations obtained in the framework of the eso prgs. 191.d-0255(e) and 094.d-0355(b).appendix a is available in electronic form at http://www.aanda.org | b fields in ob stars (bob): fors 2 spectropolarimetric follow-up of the two rare rigidly rotating magnetosphere stars hd 23478 and hd 345439 |
the sun and sun-like stars lose angular momentum to their magnetized stellar winds. this braking torque is coupled to the stellar magnetic field, such that changes in the strength and/or geometry of the field modifies the efficiency of this process. since the space age, we have been able to directly measure solar wind properties using in situ spacecraft. furthermore, indirect proxies such as sunspot number, geomagnetic indices, and cosmogenic radionuclides, constrain the variation of solar wind properties on centennial and millennial timescales. we use near-earth measurements of the solar wind plasma and magnetic field to calculate the torque on the sun throughout the space age. then, reconstructions of the solar open magnetic flux are used to estimate the time-varying braking torque during the last nine millennia. we assume a relationship for the solar mass-loss rate based on observations during the space age which, due to the weak dependence of the torque on mass-loss rate, does not strongly affect our predicted torque. the average torque during the last nine millennia is found to be 2.2 × 1030 erg, which is comparable to the average value from the last two decades. our data set includes grand minima (such as the maunder minimum), and maxima in solar activity, where the torque varies from ∼1 to 5 × 1030 erg (averaged on decadal timescales), respectively. we find no evidence for any secular variation of the torque on timescales of less than 9000 yr. | solar angular momentum loss over the past several millennia |
recent exoplanet studies have revealed that the orbital planes of planets are not always aligned with one another or with the equatorial plane of the central star. the misalignment has been ascribed to gravitational scattering by giant planets and/or companion stars1-3 or to fly-bys in stellar cluster environments4. alternatively, the misalignment could be natal: that is, such planets were born in a warped protostellar disk5,6. warped disk structures have been reported in some transition disks and protoplanetary disks7,8, but not in the earlier stages of protostar evolution, although such a possibility is suggested by outflow morphology9,10. here we report millimetre-wavelength dust continuum observations of the young embedded protostar iras 04368+2557 in the protostellar core l1527 at a distance11 of 137 parsecs; the protostar's disk is almost edge-on12-16. the inner and outer parts of the disk have slightly different orbital planes, connected at 40 to 60 astronomical units from the star, but the disk has point symmetry with respect to the position of the protostar. we interpret it as a warped disk that is rotationally supported. because there is no evidence for a companion source17,18, the warped structure must be due to either anisotropic accretion of gas with different rotational axes, or misalignment of the rotation axis of the disk with the magnetic field direction. | a warped disk around an infant protostar |
by applying magnetohydrodynamic simulations, we investigate the launching of jets driven by a disk magnetic field generated by a mean-field disk dynamo. extending our earlier studies, we explore the bipolar evolution of the disk α 2ω-dynamo and the outflow. we confirm that a negative dynamo-α leads to a dipolar field geometry, whereas positive values generate quadrupolar fields. the latter remain mainly confined to the disk and cannot launch outflows. we investigate a parameter range for the dynamo-α ranging from a critical value below which field generation is negligible, {α }0,{crit}=-0.0005, to α 0 = -1.0. for weak | {α }0| ≤slant 0.07, two magnetic loop structures with opposite polarity may arise, which leads to reconnection and disturbs the field evolution and accretion-ejection process. for a strong dynamo-α, a higher poloidal magnetic energy is reached, roughly scaling with {e}mag}∼ | {α }0| , which also leads to higher accretion and ejection rates. the terminal jet speed is governed by the available magnetic energy and increases with the dynamo-α. we find jet velocities on the order of the inner disk keplerian velocity. for a strong dynamo-α, oscillating dynamo modes may occur that can lead to a pulsed ejection. this is triggered by an oscillating mode in the toroidal field component. the oscillation period is comparable to the keplerian timescale in the launching region, thus too short to be associated with the knots in observed jets. we find a hemispherically asymmetric evolution for the jet and counter-jet in the mass flux and field structure. | bipolar jets launched by a mean-field accretion disk dynamo |
fluctuations in the flow velocity and magnetic fields are ubiquitous in the solar system. these fluctuations are turbulent, in the sense that they are disordered and span a broad range of scales in both space and time. the study of solar wind turbulence is motivated by a number of factors all keys to the understanding of the solar wind origin and thermodynamics. the solar wind spectral properties are far from uniformity and evolve with the increasing distance from the sun. most of the available spectra of solar wind turbulence were computed at 1 astronomical unit, while accurate spectra on wide frequency ranges at larger distances are still few. in this paper we consider solar wind spectra derived from the data recorded by the voyager 2 mission during 1979 at about 5 au from the sun. voyager 2 data are an incomplete time series with a voids/signal ratio that typically increases as the spacecraft moves away from the sun (45% missing data in 1979), making the analysis challenging. in order to estimate the uncertainty of the spectral slopes, different methods are tested on synthetic turbulence signals with the same gap distribution as v2 data. spectra of all variables show a power law scaling with exponents between -2.1 and -1.1, depending on frequency subranges. probability density functions (pdfs) and correlations indicate that the flow has a significant intermittency. | turbulence in the solar wind: spectra from voyager 2 data at 5 au |
we employ high-resolution spectroscopy and spectropolarimetry to derive the physical properties and magnetic characteristics of the multiple system hd 164492c, located in the young open cluster m20. the spectrum reveals evidence of three components: a broad-lined early b star (hd 164492c1), a narrow-lined early b star (hd 164492c2) and a late b star (hd 164492c3). components c2 and c3 exhibit significant (>100 km s-1) bulk radial velocity variations with a period of 12.5351(7) d that we attribute to eccentric binary motion around a common centre-of-mass. component c1 exhibits no detectable radial velocity variations. using constraints derived from modelling the orbit of the c2+c3 binary and from synthesis of the combined spectrum, we determine the approximate physical characteristics of the components. we conclude that a coherent evolutionary solution consistent with the published age of m20 implies a distance to m20 of 0.9 ± 0.2 kpc, corresponding to the smallest published values. we confirm the detection of a strong magnetic field in the combined spectrum. the field is clearly associated with the broad-lined c1 component of the system. repeated measurement of the longitudinal magnetic field allows the derivation of the rotation period of the magnetic star, prot = 1.369 86(6) d. we derive the star's magnetic geometry, finding i=63± 6°, β =33± 6° and a dipole polar strength b_d=7.9^{+1.2}_{-1.0} kg. strong emission - varying according to the magnetic period - is detected in the hα profile. this is consistent with the presence of a centrifugal magnetosphere surrounding the rapidly rotating magnetic c1 component. | hd 164492c: a rapidly rotating, hα-bright, magnetic early b star associated with a 12.5 d spectroscopic binary |
young neutron stars (nss) have magnetic fields in the range 1012-1015 g, believed to be generated by dynamo action at birth. we argue that such a dynamo is actually too inefficient to explain the strongest of these fields. dynamo action in the mature star is also unlikely. instead we propose a promising new precession-driven dynamo and examine its basic properties, as well as arguing for a revised mean-field approach to ns dynamos. the precession-driven dynamo could also play a role in field generation in main-sequence stars. | generating neutron-star magnetic fields: three dynamo phases |
ram-pressure stripping is a crucial evolutionary driver for cluster galaxies and jellyfish galaxies characterized by very extended tails of stripped gas, and they are the most striking examples of it in action. recently, those extended tails are found to show ongoing star formation, raising the question of how the stripped, cold gas can survive long enough to form new stars outside the stellar disk. in this study, we summarize the most recent results achieved within the gasp collaboration to provide a holistic explanation for this phenomenon. we focus on two textbook examples of jellyfish galaxies, jo206 and jw100, for which, via multi-wavelength observations from radio to x-ray and numerical simulations, we have explored the different gas phases (neutral, molecular, diffuse-ionized, and hot). based on additional multi-phase gas studies, we now propose a scenario of stripped tail evolution including all phases that are driven by a magnetic draping sheath, where the intracluster turbulent magnetized plasma condenses onto the galaxy disk and tail and produces a magnetized interface that protects the stripped galaxy tail gas from evaporation. in such a scenario, the accreted environmental plasma can cool down and eventually join the tail gas, hence providing additional gas to form stars. the implications of our findings can shed light on the more general scenario of draping, condensation, and cooling of hot gas surrounding cold clouds that is fundamental in many astrophysical phenomena. | role of magnetic fields in ram pressure stripped galaxies |
this paper reports on the x-ray emission evolution of the ultraluminous galactic x-ray pulsar swift j0243.6+6124 during the giant outburst from 2017 october to 2018 january as observed by the maxi gsc all-sky survey. the 2-30 kev light curve and the energy spectra confirm the source luminosity lx assuming an isotropic emission reached 2.5 × 1039 erg s-1, 10 times higher than the eddington limit for a 1.4 m⊙ neutron star. when the source was luminous with lx ≳ 0. 9 × 1038 erg s-1, it generally exhibited a negative correlation on a hardness-intensity diagram. however, two hardness ratios, a soft color (=4-10 kev/2-4 kev) and a hard color (=10-20 kev/4-10 kev), showed somewhat different behavior across a characteristic luminosity of lc ≃ 5 × 1038 erg s-1. the soft color changed more than the hard color when lx < lc, whereas the opposite was observed above lc. the spectral change above lc was represented by a broad enhanced feature at ∼6 kev on top of the canonical cutoff power-law continuum. the pulse profiles, derived daily, made the transition from a single-peak to a double-peak as the source brightened across lc. these spectral and pulse-shape properties can be interpreted by a scenario in which the accretion columns on the neutron-star surface, producing the comptonized x-ray emission, gradually became taller as lx increases. the broad 6 kev enhancement could be a result of cyclotron-resonance absorption at ∼10 kev, corresponding to a surface magnetic field bs ≃ 1.1 × 1012 g. the spin-frequency derivatives calculated with the fermi gbm data showed a smooth positive correlation with lx up to the outburst peak, and its linear coefficient is comparable to those of typical be binary pulsars whose bs are (1-8) × 1012 g. these results suggest that the bs of swift j0243.6+6124 is a few times 1012 g. | x-ray emission evolution of the galactic ultraluminous x-ray pulsar swift j0243.6+6124 during the 2017-2018 outburst observed by the maxi gsc |
context. massive binary stars play a crucial role in many astrophysical fields. investigating the statistical properties of massive binary stars is essential to trace the formation of massive stars and constrain the evolution of stellar populations. however, no consensus has been achieved on the statistical properties of massive binary stars, mainly due to the lack of a large and homogeneous sample of spectroscopic observations.aims: we study the intrinsic binary fraction fbin and distributions of mass ratio f(q) and orbital period f(p) of early-type stars (comprised of o-, b-, and a-type stars) and investigate their dependences on effective temperature teff, stellar metallicity [m/h], and the projection velocity vsini, based on the homogeneous spectroscopic sample from the large sky area multi-object fiber spectroscopic telescope (lamost) data release eight (dr8).methods: we collected 886 early-type stars, each with more than six observations from the lamost dr8, and divided the sample into subgroups based on their derived effective temperature (teff), metallicity ([m/h]), and projected rotational velocity (vsini). radial velocity measurements were archived from a prior study. a set of monte carlo simulations, following distributions of f(p)∝pπ and f(q)∝qγ were applied to the observed binary fraction to correct for any observational biases. the uncertainties of the derived results induced by the sample size and observation frequency are examined systematically.results: we found that fbin increases with increasing teff. for stars in groups of b8-a, b4-b7, o-b3, the binary fractions are fbin = 48% ± 10%, 60%±10%, and 76%±10%, respectively. the binary fraction is positively correlated with metallicity for spectra in the sample, with derived values of fbin = 44% ± 10%, 60%±10%, and 72%±10% for spectra with metallicity ranges of [m/h] < −0.55, −0.55 ≤ [m/h] < −0.1, to [m/h] ≥ −0.1. over all the vsini values we considered, the fbin have constant values of ∼50%. it seems that the binary population is relatively evenly distributed over a wide range of vsini values, while the whole sample shows that most of the stars are concentrated at low values of vsini (probably from strong wind and magnetic braking of single massive stars) and at high values of vsini (likely from the merging of binary stars). stellar evolution and binary interaction may be partly responsible for this. in the case of samples with more than six observations, we derived π = −0.9 ± 0.35, −0.9 ± 0.35, and −0.9 ± 0.35, and γ = −1.9 ± 0.9, −1.1 ± 0.9, and −2 ± 0.9 for stars of types o-b3, b4-b7, and b8-a, respectively. there are no correlations found between π(γ) and teff, nor for π(γ) and [m/h]. the uncertainties of the distribution decrease toward a larger sample size with higher observational cadence. full table 2 is only available at the cds via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/j/a+a/667/a44 | the statistical properties of early-type stars from lamost dr8 |
we study the instability of magnetic fields in a neutron star core driven by the parity violating part of the electron-nucleon interaction in the standard model. assuming a seed field of the order 1012 g, that is a common value for pulsars, one obtains its amplification due to such a novel mechanism by about five orders of magnitude, up to 1017 g, at time scales ~ (103-105) yr. this effect is suggested to be a possible explanation of the origin of the strongest magnetic fields observed in magnetars. the growth of a seed magnetic field energy density is stipulated by the corresponding growth of the magnetic helicity density due to the presence of the anomalous electric current in the maxwell equation. such an anomaly is the sum of the two competitive effects: (i) the chiral magnetic effect driven by the difference of chemical potentials for the right and left handed massless electrons and (ii) constant chiral electroweak electron-nucleon interaction term, which has the polarization origin and depends on the constant neutron density in a neutron star core. the remarkable issue for the decisive role of the magnetic helicity evolution in the suggested mechanism is the arbitrariness of an initial magnetic helicity including the case of non-helical fields from the beginning. the tendency of the magnetic helicity density to the maximal helicity case at large evolution times provides the growth of a seed magnetic field to the strongest magnetic fields in astrophysics. | generation of the magnetic helicity in a neutron star driven by the electroweak electron-nucleon interaction |
almost 50 years after radio pulsars were discovered in 1967, our understanding of these objects remains incomplete. on the one hand, within a few years it became clear that neutron star rotation gives rise to the extremely stable sequence of radio pulses, that the kinetic energy of rotation provides the reservoir of energy, and that electromagnetic fields are the braking mechanism. on the other hand, no consensus regarding the mechanism of coherent radio emission or the conversion of electromagnetic energy to particle energy yet exists. in this review, we report on three aspects of pulsar structure that have seen recent progress: the self-consistent theory of the magnetosphere of an oblique magnetic rotator; the location, geometry, and optics of radio emission; and evolution of the angle between spin and magnetic axes. these allow us to take the next step in understanding the physical nature of the pulsar activity. | radio pulsars |
context. observations from the solar dynamics observatory (sdo) have the potential for allowing the helioseismic study of the formation of hundreds of active regions, which would enable us to perform statistical analyses.aims: our goal is to collate a uniform data set of emerging active regions observed by the sdo/hmi instrument suitable for helioseismic analysis, where each active region is centred on a 60° × 60° area and can be observed up to seven days before emergence.methods: we restricted the sample to active regions that were visible in the continuum and emerged into quiet sun largely avoiding pre-existing magnetic regions. as a reference data set we paired a control region (cr), with the same latitude and distance from central meridian, with each emerging active region (ear). the control regions do not have any strong emerging flux within 10° of the centre of the map. each region was tracked at the carrington rotation rate as it crossed the solar disk, within approximately 65° from the central meridian and up to seven days before, and seven days after, emergence. the mapped and tracked data, consisting of line-of-sight velocity, line-of-sight magnetic field, and intensity as observed by sdo/hmi, are stored in datacubes that are 410 min in duration and spaced 320 min apart. we call this data set, which is currently comprised of 105 emerging active regions observed between may 2010 and november 2012, the sdo helioseismic emerging active region (sdo/hear) survey.results: to demonstrate the utility of a data set of a large number of emerging active regions, we measure the relative east-west velocity of the leading and trailing polarities from the line-of-sight magnetogram maps during the first day after emergence. the latitudinally averaged line-of-sight magnetic field of all the ears shows that, on average, the leading (trailing) polarity moves in a prograde (retrograde) direction with a speed of 121 ± 22 m s-1 (-70 ± 13 m s-1) relative to the carrington rotation rate in the first day. however, relative to the differential rotation of the surface plasma, the east-west velocity is symmetric, with a mean of 95 ± 13 m s-1.conclusions: the sdo/hear data set will not only be useful for helioseismic studies, but will also be useful to study other features such as the surface magnetic field evolution of a large sample of ears. we intend to extend this survey forwards in time to include more ears observed by sdo/hmi. | sdo/hmi survey of emerging active regions for helioseismology |
in the present study, the magnetic field scaling on density, | b| \propto {ρ }κ , was revealed in a single starless core for the first time. the κ index of 0.78 ± 0.10 was obtained toward the starless dense core fest 1-457 based on the analysis of the radial distribution of the polarization angle dispersion of background stars measured at the near-infrared wavelengths. the result prefers κ = 2/3 for the case of isotropic contraction, and the difference of the observed value from κ = 1/2 is 2.8 sigma. the distribution of the ratio of mass-to-magnetic flux was evaluated. fest 1-457 was found to be magnetically supercritical near the center (λ ≈ 2), whereas nearly critical or slightly subcritical at the core boundary (λ ≈ 0.98). ambipolar diffusion-regulated star formation models for the case of moderate magnetic field strength may explain the physical status of fest 1-457. the mass-to-flux ratio distribution for typical dense cores (critical bonnor-ebert sphere with central λ = 2 and κ = 1/2-2/3) was calculated, and found to be magnetically critical/subcritical at the core edge, which indicates that typical dense cores are embedded in and evolve from magnetically critical/subcritical diffuse surrounding medium. | distortion of magnetic fields in a starless core. iv. magnetic field scaling on density and mass-to-flux ratio distribution in fest 1-457 |
context. massive stars with solar metallicity lose important amounts of rotational angular momentum through their winds. when a magnetic field is present at the surface of a star, efficient angular momentum losses can still be achieved even when the mass-loss rate is very modest, at lower metallicities, or for lower-initial-mass stars. in a close binary system, the effect of wind magnetic braking also interacts with the influence of tides, resulting in a complex evolution of rotation.aims: we study the interactions between the process of wind magnetic braking and tides in close binary systems.methods: we discuss the evolution of a 10 m⊙ star in a close binary system with a 7 m⊙ companion using the geneva stellar evolution code. the initial orbital period is 1.2 days. the 10 m⊙ star has a surface magnetic field of 1 kg. various initial rotations are considered. we use two different approaches for the internal angular momentum transport. in one of them, angular momentum is transported by shear and meridional currents. in the other, a strong internal magnetic field imposes nearly perfect solid-body rotation. the evolution of the primary is computed until the first mass-transfer episode occurs. the cases of different values for the magnetic fields and for various orbital periods and mass ratios are briefly discussed.results: we show that, independently of the initial rotation rate of the primary and the efficiency of the internal angular momentum transport, the surface rotation of the primary will converge, in a time that is short with respect to the main-sequence lifetime, towards a slowly evolving velocity that is different from the synchronization velocity. this "equilibrium angular velocity" is always inferior to the angular orbital velocity. in a given close binary system at this equilibrium stage, the difference between the spin and the orbital angular velocities becomes larger when the mass losses and/or the surface magnetic field increase. the treatment of the internal angular momentum transport has a strong impact on the evolutionary tracks in the hertzsprung-russell diagram as well as on the changes of the surface abundances resulting from rotational mixing. our modelling suggests that the presence of an undetected close companion might explain rapidly rotating stars with strong surface magnetic fields, having ages well above the magnetic braking timescale. our models predict that the rotation of most stars of this type increases as a function of time, except for a first initial phase in spin-down systems. the measure of their surface abundances, together, when possible, with their mass-luminosity ratio, provide interesting constraints on the transport efficiencies of angular momentum and chemical species.conclusions: close binaries, when studied at phases predating any mass transfer, are key objects to probe the physics of rotation and magnetic fields in stars. | close binary evolution. ii. impact of tides, wind magnetic braking, and internal angular momentum transport |
magnetic fields in wolf-rayet (wr) stars are not well explored, although there is indirect evidence, e.g. from spectral variability and x-ray emission, that magnetic fields should be present in these stars. being in an advanced stage of their evolution, wr stars have lost their hydrogen envelope, but their dense winds make the stellar core almost unobservable. to substantiate the expectations on the presence of magnetic fields in the most-evolved massive stars, we selected two wr stars, wr 46 and wr 55, for the search of the presence of magnetic fields using fors 2 spectropolarimetric observations. we achieve a formally definite detection of a variable mean longitudinal magnetic field of the order of a few hundred gauss in wr 55. the field detection in this star, which is associated with the ring nebula rcw 78 and the molecular environment, is of exceptional importance for our understanding of star formation. no field detection at a significance level of 3σ was achieved for wr 46, but the variability of the measured field strengths can be rather well phased with the rotation period of 15.5 h previously suggested by fuse(far ultraviolet spectroscopic explorer) observations. | the search for magnetic fields in two wolf-rayet stars and the discovery of a variable magnetic field in wr 55 |
the spin axis of a rotationally deformed planet is forced to precess about its orbital angular momentum vector, due to the tidal gravity of its host star, if these directions are misaligned. this induces internal fluid motions inside the planet that are subject to a hydrodynamic instability. we study the turbulent damping of precessional fluid motions, as a result of this instability, in the simplest local computational model of a giant planet (or star), with and without a weak internal magnetic field. our aim is to determine the outcome of this instability, and its importance in driving tidal evolution of the spin-orbit angle in precessing planets (and stars). we find that this instability produces turbulent dissipation that is sufficiently strong that it could drive significant tidal evolution of the spin-orbit angle for hot jupiters with orbital periods shorter than about 10-18 d. if this mechanism acts in isolation, this evolution would be towards alignment or anti-alignment, depending on the initial angle, but the ultimate evolution (if other tidal mechanisms also contribute) is expected to be towards alignment. the turbulent dissipation is proportional to the cube of the precession frequency, so it leads to much slower damping of stellar spin-orbit angles, implying that this instability is unlikely to drive evolution of the spin-orbit angle in stars (either in planetary or close binary systems). we also find that the instability-driven flow can act as a system-scale dynamo, which may play a role in producing magnetic fields in short-period planets. | on turbulence driven by axial precession and tidal evolution of the spin-orbit angle of close-in giant planets |
we investigate the evolution of fast magnetoacoustic pulses in randomly structured plasmas, in the context of large-scale propagating waves in the solar atmosphere. we perform one-dimensional numerical simulations of fast wave pulses propagating perpendicular to a constant magnetic field in a low-β plasma with a random density profile across the field. both linear and nonlinear regimes are considered. we study how the evolution of the pulse amplitude and width depends on their initial values and the parameters of the random structuring. acting as a dispersive medium, a randomly structured plasma causes amplitude attenuation and width broadening of the fast wave pulses. after the passage of the main pulse, secondary propagating and standing fast waves appear. width evolution of both linear and nonlinear pulses can be well approximated by linear functions; however, narrow pulses may have zero or negative broadening. this arises because narrow pulses are prone to splitting, while broad pulses usually deviate less from their initial gaussian shape and form ripple structures on top of the main pulse. linear pulses decay at an almost constant rate, while nonlinear pulses decay exponentially. a pulse interacts most efficiently with a random medium with a correlation length of about half of the initial pulse width. this detailed model of fast wave pulses propagating in highly structured media substantiates the interpretation of eit waves as fast magnetoacoustic waves. evolution of a fast pulse provides us with a novel method to diagnose the sub-resolution filamentation of the solar atmosphere. | evolution of fast magnetoacoustic pulses in randomly structured coronal plasmas |
measurements of solar wind turbulence reveal the ubiquity of discontinuities. in this study we investigate how the discontinuities, especially rotational discontinuities (rds), are formed in mhd turbulence. in a simulation of the decaying compressive three-dimensional (3d) mhd turbulence with an imposed uniform background magnetic field, we detect rds with sharp field rotations and little variations of magnetic field intensity, as well as mass density. at the same time, in the de hoffman-teller frame, the plasma velocity is nearly in agreement with the alfvén speed, and is field-aligned on both sides of the discontinuity. we take one of the identified rds to analyze its 3d structure and temporal evolution in detail. by checking the magnetic field and plasma parameters, we find that the identified rd evolves from the steepening of the alfvén wave with moderate amplitude, and that steepening is caused by the nonuniformity of the alfvén speed in the ambient turbulence. | the formation of rotational discontinuities in compressive three-dimensional mhd turbulence |
the modification of the rotating vector model in the case of magnetars are calculated. magnetars may have twisted magnetic field compared with normal pulsars. the polarization position angle of magnetars will change in the case of a twisted magnetic field. for a twisted dipole field, we found that the position angle will change both vertically and horizontally. during the untwisting process of the magnetar magnetosphere, the modifications of the position angle will evolve with time monotonously. this may explain the evolution of the position angle in magnetar psr j1622-4950 and xte j1810-197. the relation between the emission point and the line of sight will also change. we suggest every magnetospheric models of magnetars also calculate the corresponding changes of position angle in their models. order of magnitude estimation formula for doing this is given. this opens the possibility to extract the magnetic field geometry of magnetars from their radio polarization observations. | rotating vector model for magnetars |
the kelvin-helmholtz (kh) instability of a shear layer with an initially-uniform magnetic field in the direction of flow is studied in the framework of 2d incompressible magnetohydrodynamics with finite resistivity and viscosity using direct numerical simulations. the shear layer evolves freely, with no external forcing, and thus broadens in time as turbulent stresses transport momentum across it. as with kh-unstable flows in hydrodynamics, the instability here features a conjugate stable mode for every unstable mode in the absence of dissipation. stable modes are shown to transport momentum up its gradient, shrinking the layer width whenever they exceed unstable modes in amplitude. in simulations with weak magnetic fields, the linear instability is minimally affected by the magnetic field, but enhanced small-scale fluctuations relative to the hydrodynamic case are observed. these enhanced fluctuations coincide with increased energy dissipation and faster layer broadening, with these features more pronounced in simulations with stronger fields. these trends result from the magnetic field reducing the effects of stable modes relative to the transfer of energy to small scales. as field strength increases, stable modes become less excited and thus transport less momentum against its gradient. furthermore, the energy that would otherwise transfer back to the driving shear due to stable modes is instead allowed to cascade to small scales, where it is lost to dissipation. approximations of the turbulent state in terms of a reduced set of modes are explored. while the reynolds stress is well-described using just two modes per wavenumber at large scales, the maxwell stress is not. | the impact of magnetic fields on momentum transport and saturation of shear-flow instability by stable modes |
we present results from a circular polarimetric survey of candidate detached magnetic white dwarf - m dwarf binaries obtained using the nordic optical telescope, la palma. we obtained phase resolved spectropolarimetry and imaging polarimetry of seven systems, five of which show clearly variable circular polarisation. the data indicate that these targets have white dwarfs with magnetic field strengths >80 mg. our study reveals that cyclotron emission can dominate the optical luminosity at wavelengths corresponding to the cyclotron emission harmonics, even in systems where the white dwarfs are only wind-accreting. this implies that a very significant fraction of the stellar wind of the companion star is captured by the magnetic white dwarf reducing the magnetic braking in pre-cataclysmic variables (cvs). furthermore, the polarimetric confirmation of several detached, wind-accreting magnetic systems provides observational constraints on the models of magnetic cv evolution and white dwarf magnetic field generation. we also find that the white dwarf magnetic field configuration in at least two of these systems appears to be very complex. | circular polarimetry of suspect wind-accreting magnetic pre-polars |
in the work we present the results of two deep broadband observations of the poorly studied x-ray pulsar igr j19294+1816 obtained with the nustar observatory. the source was observed during type i outburst and in the quiescent state. in the bright state a cyclotron absorption line in the energy spectrum was discovered at ecyc = 42.8 ± 0.7 kev. spectral and timing analysis prove the ongoing accretion also during the quiescent state of the source. based on the long-term flux evolution, particularly on the transition of the source to the bright quiescent state with luminosity around 1035 erg s-1, we conclude that igr j19294+1816 switched to the accretion from the "cold" accretion disk between type i outbursts. we also report the updated orbital period of the system. | study of the x-ray pulsar igr j19294+1816 with nustar: detection of cyclotron line and transition to accretion from the cold disk |
we study the recently observed jellyfish galaxies (jfgs), which are found to have their gas content ram pressure stripped away in galaxy clusters. these galaxies are observed to have an enhanced star formation rate of about 0.2 dex compared with a control sample of the same stellar mass in their discs. we model the increase in the star formation efficiency as a function of intracluster medium pressure and parametrize the cold gas content of the galaxies as a function of cluster-centric distance. we show that regarding the external pressure as a positive feedback results in agreement with the observed distribution of enhanced star formation in the jfgs if clouds are shielded from evaporation by magnetic fields. our results predict that satellites with halo mass < 10^{11} m_⊙ moving with mach numbers m≈ 2, and inclination angles below 60 deg, are more likely to be detected as jfgs. | explaining the enhanced star formation rate of jellyfish galaxies in galaxy clusters |
x-ray observations play a crucial role in understanding the emission mechanism and relevant physical phenomena of magnetars. we report on x-ray observations made in 2016 of a young magnetar, sgr 1900+14, which is famous for a giant flare in 1998 august. simultaneous observations were conducted with xmm-newton and nustar on 2016 october 20 with 23 and 123 ks exposures, respectively. the nustar hard x-ray coverage enabled us to detect the source up to 70 kev. the 1-10 kev and 15-60 kev fluxes were 3.11(3)× 10^{-12} {erg s^{-1} cm^{-2}} and 6.8(3)× 10^{-12} {erg s^{-1} cm^{-2}}, respectively. the 1-70 kev spectra were fitted well by a blackbody plus power-law model with a surface temperature of kt=0.52(2) kev, a photon index of the hard power-law of γ = 1.21(6), and a column density of n_{ h}=1.96(11)× 10^{22} cm^{-2}. compared with previous observations with suzaku in 2006 and 2009, the 1-10 kev flux showed a decrease by 25%-40%, while the spectral shape did not show any significant change with differences of kt and nh being within 10% of each other. through timing analysis, we found that the rotation period of sgr 1900+14 on 2016 october 20 was 5.22669(3) s. the long-term evolution of the rotation period shows a monotonic decrease in the spin-down rate \dot{p} lasting for more than 15 yr. we also found characteristic behavior of the hard-tail power-law component of sgr 1900+14. the energy-dependent pulse profiles vary in morphology with a boundary of 10 kev. the phase-resolved spectra show the differences between photon indices (γ = 1.02-1.44) as a function of the pulse phase. furthermore, the photon index is positively correlated with the x-ray flux of the hard power-law component, which could not be resolved by the previous hard x-ray observations. | temporal and spectral x-ray properties of magnetar sgr 1900+14 derived from observations with nustar and xmm-newton |
ambipolar diffusion likely plays a pivotal role in the formation and evolution of dense cores in weakly ionized molecular clouds. linear analyses show that the evolutionary times and fragmentation scales are significantly greater than the hydrodynamic (jeans) values even for clouds with mildly supercritical mass-to-flux ratios. we use values of fragmentation scales and growth times that correspond to typical ionization fractions within a molecular cloud, and apply these in the context of the observed estimated lifetime of prestellar cores and the observed number of such embedded cores forming in a parent clump. by varying a single parameter - the mass-to-flux ratio - over the range of observationally measured densities, we fit the range of estimated prestellar core lifetimes (∼0.1 to a few myr) identified with herschel as well as the number of embedded cores formed in a parent clump measured in perseus with the submillimeter array. our model suggests that the prestellar cores are formed with a transcritical mass-to-flux ratio and higher densities correspond to somewhat higher mass-to-flux ratios, but the normalized mass-to-flux ratio μ remains in the range 1 ≲ μ ≲ 2. our best-fit model exhibits b ∝ n0.43 for prestellar cores because of the partial flux-freezing caused by ambipolar diffusion. | variation of the core lifetime and fragmentation scale in molecular clouds as an indication of ambipolar diffusion |
implosive formation of current sheets is a fundamental plasma process. previous studies focused on the early time evolution, while here our primary aim is to explore the longer-term evolution, which may be critical for determining the efficiency of energy release. to address this problem, we investigate two closely related problems, namely: (i) 1d, pinched anti-parallel magnetic fields and (ii) 2d, null point containing fields which are locally imbalanced ("null-collapse" or "x-point collapse"). within the framework of resistive mhd, we simulate the full nonlinear evolution through three distinct phases: the initial implosion, its eventual halting mechanism, and subsequent evolution post-halting. in a parameter study, we find that the scaling with resistivity of current sheet properties at the halting time is in good agreement—in both geometries—with that inferred from a known 1d similarity solution. we find that the halting of the implosions occurs rapidly after reaching the diffusion scale by sudden ohmic heating of the dense plasma within the current sheet, which provides a pressure gradient sufficient to oppose further collapse and decelerate the converging flow. this back-pressure grows to exceed that required for force balance and so the post-implosion evolution is characterised by the consequences of the current sheet "bouncing" outwards. these are: (i) the launching of propagating fast mhd waves (shocks) outwards and (ii) the width-wise expansion of the current sheet itself. the expansion is only observed to stall in the 2d case, where the pressurisation is relieved by outflow in the reconnection jets. in the 2d case, we quantify the maximum amount of current sheet expansion as it scales with resistivity and analyse the structure of the reconnection region, which forms post-expansion, replete with petschek-type slow shocks and fast termination shocks. | resistively-limited current sheet implosions in planar anti-parallel (1d) and null-point containing (2d) magnetic field geometries |
accreting white dwarfs in binary systems known as cataclysmic variables (cvs) have in recent years been shown to produce radio flares during outbursts, qualitatively similar to those observed from neutron star and black hole x-ray binaries, but their ubiquity and energetic significance for the accretion flow has remained uncertain. we present new radio observations of the cv ss cyg with arcminute microkelvin imager large array, which show for the second time late-ouburst radio flaring, in 2016 april. this flaring occurs during the optical flux decay phase, about 10 d after the well-established early-time radio flaring. we infer that both the early- and late-outburst flares are a common feature of the radio outbursts of ss cyg, albeit of variable amplitudes, and probably of all dwarf novae. we furthermore present new analysis of the physical conditions in the best-sampled late-outburst flare, from 2016 february, which showed clear optical depth evolution. from this we can infer that the synchrotron-emitting plasma was expanding at about 1 per cent of the speed of light, and at peak had a magnetic field of order 1 g and total energy content ≥1033 erg. while this result is independent of the geometry of the synchrotron-emitting region, the most likely origin is in a jet carrying away a significant amount of the available accretion power. | late-outburst radio flaring in ss cyg and evidence for a powerful kinetic output channel in cataclysmic variables |
we present the discovery of a magnetic field on the white dwarf component in the detached post-common envelope binary (pceb) cc cet. magnetic white dwarfs in detached pcebs are extremely rare, in contrast to the high incidence of magnetism in single white dwarfs and cataclysmic variables. we find zeeman-split absorption lines in both ultraviolet hubble space telescope (hst) spectra and archival optical spectra of cc cet. model fits to the lines return a mean magnetic field strength of ⟨|b|⟩ ≈ 600-700 kg. differences in the best-fitting magnetic field strength between two separate hst observations and the high $v\, \sin \, i$ of the lines indicate that the white dwarf is rotating with a period ~0.5 h, and that the magnetic field is not axisymmetric about the spin axis. the magnetic field strength and rotation period are consistent with those observed among the intermediate polar class of cataclysmic variable, and we compute stellar evolution models that predict cc cet will evolve into an intermediate polar in 7-17 gyr. among the small number of known pcebs containing a confirmed magnetic white dwarf, cc cet is the hottest (and thus youngest), with the weakest field strength, and cannot have formed via the recently proposed crystallization/spin-up scenario. in addition to the magnetic field measurements, we update the atmospheric parameters of the cc cet white dwarf via model spectra fits to the hst data and provide a refined orbital period and ephemeris from tess photometry. | discovery of a young pre-intermediate polar |
the last few decades have seen significant progress in our understanding of the occurrence of magnetic fields in stars with radiative envelopes, in particular in massive stars and in intermediate mass stars at early evolutionary stages. this book provides a comprehensive review of the most recent achievements in the measurements of stellar magnetic fields in o, b, and a stars. these include the archetypes of stellar magnetism, the chemically peculiar ap and bp stars, o- and early b-type stars with their magnetospheres, accreting herbig ae-be stars, wolf-rayet stars, and high-mass x-ray binaries, among others. it provides an overview of the underlying physics for the interpretation of the data and identifies the requirements, both observational and theoretical, for improving our understanding of the origin of the magnetic fields in early-type stars. it aims to educate scientists working on stars, who are not yet experts in magnetic field studies, assuming that the reader is already familiar with basic terms and concepts of stellar astrophysics. | magnetic fields in o, b, and a stars |
any observable repercussion of electromagnetic properties of neutrinos will provide a perspicuous signature of new physics. this includes the phenomenon of neutrino spin flip in the propinquity of an external magnetic field. in this work, we study the inklings of spin flip in a neutron star with a radially varying magnetic field and matter density, known as a magnetar, which is also a source of profuse production of neutrinos during its initial stage of thermal evolution. we find that a precise measurement of neutrino flux emerging from a neutron star would reeducate discrimination between dirac and majorana neutrinos as the flux reduction due to spin flip oscillations are different for both types of neutrinos. further, the reduction of flux is more preeminent near the surface as compared to the core of a neutron star. | can neutron star discriminate between dirac and majorana neutrinos? |
we present the analysis of archival xmm-newton european photon imaging camera (epic) x-ray observations of the symbiotic star r aquarii. we used the extended source analysis software package to disclose diffuse soft x-ray emission extending up to 2.'2 (≈0.27 pc) from this binary system. the depth of these xmm-newton epic observations reveals in unprecedented detail the spatial distribution of this diffuse emission, with a bipolar morphology spatially correlated with the optical nebula. the extended x-ray emission shares the same dominant soft x-ray-emitting temperature as the clumps in the jet-like feature resolved by chandra in the vicinity of the binary system. the harder component in the jet might suggest that the gas cools down; however, the possible presence of nonthermal emission produced by the presence of a magnetic field collimating the mass ejection cannot be discarded. we propose that the ongoing precessing jet creates bipolar cavities filled with x-ray-emitting hot gas that feeds the more extended x-ray bubble as they get disrupted. these epic observations demonstrate that the jet feedback mechanism produced by an accreting disk around an evolved, low-mass star can blow hot bubbles, similar to those produced by jets arising from the nuclei of active galaxies. | an xmm-newton epic x-ray view of the symbiotic star r aquarii |
in order to predict the spins of stellar remnants we need to understand the evolution of the internal rotation of stars, and to identify at which stage the rotation of the contracting cores of evolved stars decouples from their expanding envelopes. the donor stars of mass transferring binaries lose almost their entire envelope and may thus offer a direct view on their core rotation. after the mass transfer event they contract and fade rapidly, although they are well observable when caught in the short-lived b-star phase. the b-type primary of the galactic binary system lb-1, which was originally suggested to contain a massive black hole, is nicely explained as a stripped star accompanied by a fainter be star. the narrow absorption lines in the primary's spectrum signify extremely slow rotation, atypical of b-type main-sequence stars. here we investigate the evolution of mass donors in generic grids of detailed binary evolution models, where both stars include differential rotation, internal angular momentum transport, and spin-orbit coupling. whereas the mass gainers are typically spun-up during the mass transfer, we find that the spins of the stripped donor models depend sensitively on the employed mechanism for internal angular momentum transport. purely hydrodynamic transport cannot explain the observed slow rotation, while models including magnetic angular momentum transport are able to reproduce the observed rotation of lb-1 and similar stars, independent of the initial rotation rate. in such models the spin of the white dwarfs that emerge at the end of the evolution is independent of the mass stripping. we find evidence that the mass transfer in lb-1 was moderately non-conservative. | the spins of stripped b stars support magnetic internal angular momentum transport |
euv imaging observations from several space missions (soho/eit, trace, and sdo/aia) have revealed a presence of propagating intensity disturbances in solar coronal loops. these disturbances are typically interpreted as slow magnetoacoustic waves. however, recent spectroscopic observations with hinode/eis of active region loops revealed that the propagating intensity disturbances are associated with intermittent plasma upflows (or jets) at the footpoints which are presumably generated by magnetic reconnection. for this reason, whether these disturbances are waves or periodic flows is still being studied. this study is aimed at understanding the physical properties of observed disturbances by investigating the excitation of waves by hot plasma injections from below and the evolution of flows and wave propagation along the loop. we expand our previous studies based on isothermal 3d mhd models of an active region to a more realistic model that includes full energy equation accounting for the effects of radiative losses. computations are initialized with an equilibrium state of a model active region using potential (dipole) magnetic field, gravitationally stratified density and temperature obtained from the polytropic equation of state. we model an impulsive injection of hot plasma into the steady plasma outflow along the loops of different temperatures, warm (∼1 mk) and hot (∼6 mk). the simulations show that hot jets launched at the coronal base excite slow magnetoacoustic waves that propagate to high altitudes along the loops, while the injected hot flows decelerate rapidly with heights. our results support that propagating disturbances observed in euv are mainly the wave features. we also find that the effect of radiative cooling on the damping of slow-mode waves in 1-6 mk coronal loops is small, in agreement with the previous conclusion based on 1d mhd models. | excitation of flare-induced waves in coronal loops and the effects of radiative cooling |
a canonical description of a corotating solar wind high-speed stream in terms of velocity profile would indicate three main regions: a stream interface or corotating interaction region characterized by a rapid increase in flow speed and by compressive phenomena that are due to dynamical interaction between the fast wind flow and the slower ambient plasma; a fast wind plateau characterized by weak compressive phenomena and large-amplitude fluctuations with a dominant alfvénic character; and a rarefaction region characterized by a decreasing trend of the flow speed and wind fluctuations that are gradually reduced in amplitude and alfvénic character, followed by the slow ambient wind. interesting enough, in some cases, fluctuations are dramatically reduced, and the time window in which the severe reduction of these fluctuations takes place is remarkably short, about some minutes. the region in which the fluctuations are rapidly reduced is located at the flow velocity knee that separates the fast wind plateau from the rarefaction region. the aim of this work is to investigate the physical mechanisms that might be at the origin of this phenomenon. to do this, we searched for any tangential discontinuity that might have inhibited the diffusion of these large-amplitude fluctuations in the rarefaction region as well. we also searched for differences in the composition analysis because minor ions are good tracers of physical conditions in the source regions of the wind under the hypothesis that large differences in the source regions might be linked to the phenomenon observed in situ. we found no positive feedback from these analyses, and finally invoked a mechanism based on interchange reconnection experienced by the field lines at the base of the corona, within the region that separates the open field lines of the coronal hole, which is the source of the fast wind, from the surrounding regions that are mainly characterized by closed field lines. another possibility clearly is that the observed phenomenon might be due to the turbulent evolution of the fluctuations during the expansion of the wind. however, it is hard to believe that this mechanism would generate a short transition region such as is observed in the phenomenon we discuss. this type of study will greatly benefit from solar orbiter observations during the future nominal phase of the mission, when it will be possible to link remote and in-situ data, and from radial alignments between parker solar probe and solar orbiter. | sudden depletion of alfvénic turbulence in the rarefaction region of corotating solar wind high-speed streams at 1 au: possible solar origin? |
fingering convection is a turbulent mixing process that can occur in stellar radiative regions whenever the mean molecular weight increases with radius. in some cases, it can have a significant observable impact on stellar structure and evolution. the efficiency of mixing by fingering convection as a standalone process has been studied by brown et al., but other processes such as rotation, magnetic fields, and shear can affect it. in this paper, we present a first study of the effect of shear on fingering (thermohaline) convection in astrophysics. using direct numerical simulations, we find that a moderate amount of shear (that is not intrinsically shear unstable) always decreases the mixing efficiency of fingering convection, as a result of the tilt it imparts to the fingering structures. we propose a simple analytical extension of the brown et al. model in the presence of shear that satisfactorily explains the numerically derived turbulent compositional mixing coefficient for moderate shearing rates and can trivially be implemented in stellar evolution codes. we also measure from the numerical simulations a turbulent viscosity and find that the latter is strongly tied to the turbulent compositional mixing coefficient. observational implications and caveats of the model are discussed. | the interaction between shear and fingering (thermohaline) convection |
we describe a method of implementing the axisymmetric evolution of general-relativistic hydrodynamics and magnetohydrodynamics through modification of a multipatch grid scheme. in order to ease the computational requirements required to evolve the post-merger phase of systems involving binary compact massive objects in numerical relativity, it is often beneficial to take advantage of these system's tendency to rapidly settle into states that are nearly axisymmetric, allowing for 2d evolution of secular timescales. we implement this scheme in the spectral einstein code and show the results of application of this method to four test systems including viscosity, magnetic fields, and neutrino radiation transport. our results show that this method can be used to quickly allow already existing 3d infrastructure that makes use of local coordinate system transformations to be made to run in axisymmetric 2d with the flexible grid creation capabilities of multipatch methods. our code tests include a simple model of a binary neutron star postmerger remnant, for which we confirm the formation of a massive torus which is a promising source of post-merger ejecta. | axisymmetric hydrodynamics in numerical relativity using a multipatch method |
we investigate in this paper the structures of neutron stars under a strong magnetic field in the framework of f( t) gravity, where t denotes the scalar torsion. the tov equations in this theory of gravity have been considered and the numerical resolution of these equations has been performed within the perturbative approach taking into account the equation of state of neutron dense matter in magnetic field. we simplify the problem by considering the very strong magnetic field which considerably affects the dense matter; and for quadratic and cubic corrections to the teleparallel term, one finds that the mass of neutron stars can increase for different values of the perturbation parameter. the deviation from the teleparallel term for different values of magnetic field is found out and this feature is very appreciable in the case of cubic correction. our results are related to the hadronic particles description with very small hyperon contributions and the mass-radius evolution is consistent with the observational data. | strong magnetic field effects on neutron stars within f(t) theory of gravity |
we present high-speed optical photometric observations, spanning ∼2 yr, of the recently discovered white dwarf pulsar ar scorpii. the amplitudes of the orbital, spin, and beat modulations appear to be remarkably stable and repeatable over the time span of our observations. it has been suggested that the polarized and non-polarized emission from ar scorpii is powered by the spin-down of the white dwarf. however, we find that our new data are inconsistent with the published spin-down ephemeris. whilst our data are consistent with a constant spin period, further observations over an extended time-base are required in order to ascertain the true spin-evolution of the white dwarf. this may have implications for the various models put forward to explain the energetics and evolution of ar scorpii. | a reevaluation of the proposed spin-down of the white dwarf pulsar in ar scorpii |
magnetars are neutron stars with extremely high surface magnetic fields. they show diverse x-ray pulse profiles in the quiescent state. we perform a systematic fourier analysis of their soft x-ray pulse profiles. we find that most magnetars have a single-peaked profile and hence have low amplitudes of the second fourier harmonics (a2). on the other hand, the pulsed fraction (pf) spreads over a wide range. we compared the results with theoretical profiles assuming various surface hotspot asymmetries, viewing geometries, and beaming functions. we found that a single value of the intensity ratio r between two antipodal hotspots is unable to reproduce the observed distribution of a2 and pf for all magnetars. the inferred r is probably anticorrelated with the thermal luminosity, implying that high-luminosity magnetars tend to have two symmetric hotspots. our results are consistent with theoretical predictions, for which the existence of an evolving toroidal magnetic field breaks the symmetry of the surface temperature. | a systematic study of soft x-ray pulse profiles of magnetars in quiescence |
it is commonly believed that the magnetic field threading a neutron star provides the ultimate mechanism (on top of fluid viscosity) for enforcing long-term corotation between the slowly spun-down solid crust and the liquid core. we show that this argument fails for axisymmetric magnetic fields with closed field lines in the core, the commonly used `twisted torus' field being the most prominent example. the failure of such magnetic fields to enforce global crust-core corotation leads to the development of a persistent spin lag between the core region occupied by the closed field lines and the rest of the crust and core. we discuss the repercussions of this spin lag for the evolution of the magnetic field, suggesting that, in order for a neutron star to settle to a stable state of crust-core corotation, the bulk of the toroidal field component should be deposited into the crust soon after the neutron star's birth. | persistent crust-core spin lag in neutron stars |
we investigate properties of a solar wind-like plasma, including a secondary alpha particle population exhibiting a parallel temperature anisotropy with respect to the background magnetic field, using linear and quasi-linear predictions and by means of one-dimensional hybrid simulations. we show that anisotropic alpha particles can drive a parallel fire hose instability analogous to that generated by protons, but that, remarkably, can also be triggered when the parallel plasma beta of alpha particles is below unity. the wave activity generated by the alpha anisotropy affects the evolution of the more abundant protons, leading to their anisotropic heating. when both ion species have sufficient parallel anisotropies, both of them can drive the instability, and we observe the generation of two distinct peaks in the spectra of the fluctuations, with longer wavelengths associated to alphas and shorter ones to protons. if a non-zero relative drift is present, the unstable modes propagate preferentially in the direction of the drift associated with the unstable species. the generated waves scatter particles and reduce their temperature anisotropy to a marginally stable state, and, moreover, they significantly reduce the relative drift between the two ion populations. the coexistence of modes excited by both species leads to saturation of the plasma in distinct regions of the beta/anisotropy parameter space for protons and alpha particles, in good agreement with in situ solar wind observations. our results confirm that fire hose instabilities are likely at work in the solar wind and limit the anisotropy of different ion species in the plasma. | fire hose instability driven by alpha particle temperature anisotropy |
we analyze the latest suzaku observation of the bright neutron star (ns) low-mass x-ray binary serpens x-1 taken in 2013 october and 2014 april. the observation was taken using the burst mode and only suffered mild pile-up effects. a broad iron line is clearly detected in the x-ray spectrum. we test different models and find that the iron line is asymmetric and best interpreted by relativistic reflection. the relativistically broadened iron line is generally believed to originate from the innermost regions of the accretion disk, where strong gravity causes a series of special and general relativistic effects. the iron line profile indicates an inner radius of ∼8 r g, which gives an upper limit on the size of the ns. the asymmetric iron line has been observed in a number of previous observations, which gives several inner radius measurements at different flux states. we find that the inner radius of serpens x-1 does not evolve significantly over the range of l/l edd ∼ 0.4-0.6, and the lack of flux dependence of the inner radius implies that the accretion disk may be truncated outside of the innermost stable circular orbit by the boundary layer, rather than the stellar magnetic field. | on the evolution of the inner disk radius with flux in the neutron star low-mass x-ray binary serpens x-1 |
the long-term rotational evolution of the old, isolated pulsar, psr b0950+08, is intriguing in that its spin-down rate displays sinusoidal-like oscillations due to alternating variations, both in magnitude and sign, of the second time derivative of the pulse frequency. we show that the large internal temperature to pinning energy ratio towards the base of the crust implied by the recent high surface temperature measurement of psr b0950+08 leads to linear creep interaction between vortex lines and pinning sites to operate in this pulsar. vortex lines assume a parabolic shape due to pinning to nuclear clusters and finite tension of vortices acts as a restoring force that tends to bring a vortex back to its straight shape. the resulting low-frequency oscillations of vortex lines combined with the time variable coupling between the internal superfluid components and the external pulsar braking torque give rise to an oscillatory spin-down rate. we apply this model to psr b0950+08 observations for several external torque models. our model has potential to constrain the radial extension of the closed magnetic field region in the outer core of neutron stars from the oscillation period of the spin-down rate. | on the peculiar rotational evolution of psr b0950+08 |
assuming wind-fed accretion magnetars in long-period x-ray pulsars, we calculated the rotational evolution of neutron stars. our calculations considered the effects of magnetic field decay in magnetars. the results show that wind-fed accretion magnetars can evolve to long-period x-ray pulsars with a spin period much longer than 1000 s. the spin-down trend observed in 4u 2206+54-like sources is expected when young x-ray binary systems are on the way to their equilibrium period. detailed calculations showed that the spin-down may be affected by accretion with outflows or accretion while spinning down. due to magnetic field decay in magnetars, wind-fed accretion magnetars will have a decreasing equilibrium period for a constant mass accretion rate. for 2s 0114+65, the spin-up rate due to magnetic field decay is one order of magnitude smaller than observations. the spin-up rate of 2s 0114+65 may be attributed to the formation of a transient disc during wind accretion. the slowest x-ray pulsar ax j1910.7+0917 would be a link source between 4u 2206+54 and 2s 0114+65. | understanding the coexistence of spin-up and spin-down behaviours in long-period x-ray pulsars |
we present a study of timing properties of the accreting pulsar 2s 1417-624 observed during its 2018 outburst, based on swift/bat, fermi/gbm, insight-hxmt, and nicer observations. we report a dramatic change of the pulse profiles with luminosity. the morphology of the profile in the range 0.2-10.0 kev switches from double to triple peaks at ∼2.5× 10^{37}{d}_{10}^2 erg s^{-1}and from triple to quadruple peaks at ∼7× 10^{37}{d}_{10}^2 erg s^{-1}. the profile at high energies (25-100 kev) shows significant evolutions as well. we explain this phenomenon according to existing theoretical models. we argue that the first change is related to the transition from the sub to the supercritical accretion regime, while the second to the transition of the accretion disc from the gas-dominated to the radiation pressure-dominated state. considering the spin-up as well due to the accretion torque, this interpretation allows to estimate the magnetic field self-consistently at ∼7 × 1012 g. | timing analysis of 2s 1417-624 observed with nicer and insight-hxmt |
psr j1141-6545 is a precessing binary pulsar that has the rare potential to reveal the two-dimensional structure of a non-recycled pulsar emission cone. it has undergone ∼25° of relativistic spin precession in the ∼18 yr since its discovery. in this letter, we present a detailed bayesian analysis of the precessional evolution of the width of the total intensity profile, in order to understand the changes to the line-of-sight (los) impact angle (β) of the pulsar using four different physically motivated prior distribution models. although we cannot statistically differentiate between the models with confidence, the temporal evolution of the linear and circular polarizations strongly argue that our los crossed the magnetic pole around mjd 54,000 and that only two models remain viable. for both of these models, it appears likely that the pulsar will precess out of our los in the next 3-5 yr, assuming a simple beam geometry. marginalizing over β suggests that the pulsar is a near-orthogonal rotator and provides the first polarization-independent estimate of the scale factor ({a}}) that relates the pulsar beam opening angle (ρ) to its rotational period (p) as ρ ={a}}{p}-0.5: we find it to be >6° s0.5 at 1.4 ghz with 99% confidence. if all pulsars emit from opposite poles of a dipolar magnetic field with comparable brightness, we might expect to see evidence of an interpulse arising in psr j1141-6545, unless the emission is patchy. | relativistic spin precession in the binary psr j1141-6545 |
the tayler instability (ti) is a non-axisymmetric linear instability of an axisymmetric toroidal magnetic field in magnetohydrostatic equilibrium (mhse). in a differentially rotating radiative region of a star, the ti could drive the tayler-spruit dynamo, which generates magnetic fields that can significantly impact stellar structure and evolution. heuristic prescriptions disagree on the efficacy of the dynamo, and numerical simulations have yet to definitively agree upon its existence. the criteria for the ti to develop were derived using fully compressible magnetohydrodynamics, while numerical simulations of dynamical processes in stars frequently use an anelastic approximation. this motivates us to derive new anelastic tayler instability criteria. we find that some mhse configurations are unstable in the fully compressible case but become stable in the anelastic case. we find and characterize the unstable modes of a simple family of cylindrical mhse configurations using numerical calculations, and we discuss the implications for fully nonlinear anelastic simulations. | the tayler instability in the anelastic approximation |
bright-rimmed clouds (brcs) are formed at the periphery of h ii regions as the radiation from the central star interacts with dense gas. the ionization and resulting compression of the clouds may lead to cloud disruption causing secondary star formation depending on the stellar and gas parameters. here we use r-band polarimetry to probe the plane-of-the sky magnetic field for two nearby brcs, ic 59 and ic 63. both nebulae are illuminated by γ cas with the direction of the ionizing radiation being orientated parallel or perpendicular to the local magnetic field, allowing us to probe the importance of magnetic field pressure in the evolution of brcs. because of the proximity of the system (∼200 pc), we have acquired a substantial sample of over 500 polarization measurements for stars that form the background to the nebulae. on large scales, the magnetic field geometries of both clouds are anchored to the ambient magnetic field. for ic 63, the magnetic field is aligned parallel to the head-tail morphology of the main condensation, with a convex morphology relative to the direction of the ionizing radiation. we estimate the plane-of-the-sky magnetic field strength in ic 63 to be ∼ 90 μg. in ic 59, the projected magnetic field follows the m-shape morphology of the cloud. here, field lines present a concave shape with respect to the direction of the ionizing radiation from γ cas. comparing our observations to published theoretical models, we find good general agreement, supporting the importance of magnetic fields in brc evolution. | magnetic field structure of ic 63 and ic 59 associated with h ii region sh 185 |
we have developed a new numerical code that is able to perform 2.5d simulations of a magnetohydrodynamic (mhd) wave propagation in the corona, and its interaction with a low-density region, such as a coronal hole (ch). we show that the impact of the wave on the ch leads to different effects, such as reflection and transmission of the incoming wave, stationary features at the ch boundary, or formation of a density depletion. we present a comprehensive analysis of the morphology and kinematics of primary and secondary waves, i.e., we describe in detail the temporal evolution of density, magnetic field, plasma flow velocity, phase speed, and position of the wave amplitude. effects like reflection, refraction, and transmission of the wave strongly support the theory that large-scale disturbances in the corona are fast mhd waves and distinguish that theory from the competing pseudo-wave theory. the formation of stationary bright fronts was one of the main reasons for the development of pseudo-waves. here, we show that stationary bright fronts can be produced by interactions of an mhd wave with a ch. we find secondary waves that are traversing through the ch and we show that one part of these traversing waves leaves the ch again, while another part is being reflected at the ch boundary inside the ch. we observe a density depletion that is moving in the opposite direction of the primary wave propagation. we show that the primary wave pushes the ch boundary to the right, caused by the wave front exerting dynamic pressure on the ch. | a numerical simulation of coronal waves interacting with coronal holes. i. basic features |
spin precession equations including the spin-orbit (so), spin-spin (ss), quadrupole-monopole (qm) and magnetic dipole-magnetic dipole (dd) leading-order interactions are derived for compact binary systems in order to investigate the dd contribution in the orbit-averaged spin precession equations for binary neutron star systems neglecting the gravitational radiation-reaction effect. it is known that the magnitudes of spins are not conserved quantities due to the dd interaction. we give a simple analytical description for the pure dd interaction making the magnitudes of spins almost constant by neglecting the so, ss and qm contributions. we also demonstrate the evolutions of the relative angles of spins and magnetic dipoles with the help of numeric simulations including all contributions (so, ss, qm and dd) and introduce a dimensionless magnetic dipole parameter to characterize the strength of magnetic fields for some realistic neutron star binaries. we find that for realistic configurations the strong magnetic fields of neutron stars can modify the spin dynamics over long periods of time. | spin precession of binary neutron stars with magnetic dipole moments |
the origin and evolution of magnetic fields of neutron stars from birth have long been a source of debate. here, motivated by recent simulations of the hall cascade with magnetic helicity, we invoke a model where the large-scale magnetic field of neutron stars grows as a product of small-scale turbulence through an inverse cascade. we apply this model to a simulated population of neutron stars at birth and show how this model can account for the evolution of such objects across the $p\dot{p}$ diagram, explaining both pulsar and magnetar observations. under the assumption that small-scale turbulence is responsible for large-scale magnetic fields, we place a lower limit on the spherical harmonic degree of the energy-carrying magnetic eddies of ≈40. our results favor the presence of a highly resistive pasta layer at the base of the neutron star crust. we further discuss the implications of this paradigm on direct observables, such as the nominal age and braking index of pulsars. | confronting the neutron star population with inverse cascades |
we use archival wise and spitzer photometry to derive optical line fluxes for a sample of distant quasars at z ~6. we find evidence for exceptionally high equivalent width [oiii] emission (rest-frame ew ~400 å) similar to that inferred for star-forming galaxies at similar redshifts. the median halpha and hbeta equivalent widths are derived to be ~400å and 100~å, respectively, and are consistent with values seen among quasars in the local universe, and at z ~2. after accounting for the contribution of photoionization in the broad line regions of quasars, we suggest that the oiii emission corresponds to strong, narrow line emission likely arising from feedback due to massive star-formation in the quasar host. the high [oiii]/hbeta line ratios can uniquely be interpreted with radiative shock models, and translate to magnetic field strengths of ~8 microgauss with shock velocities of ~400km/s. our measurement implies that strong, coherent magnetic fields were present in the interstellar medium at a time when the universe was < 1 billion years old. comparing our estimated magnetic field strengths with models for the evolution of galaxy-scale fields, favors high seed field strengths exceeding 0.1 microgauss, the first observational constraint on such fields. this high value favors scenarios where seed magnetic fields were produced by turbulence in the early stages of galaxy formation. forthcoming mid-infrared spectroscopy with the james webb space telescope will help constrain the physical conditions in quasar hosts further. | insights into physical conditions and magnetic fields from high redshift quasars |
context. we explore the formation, energetics, and geometry of relativistic jets along with the variability of their central engine. we study both fast and slowly rotating black holes and address our simulations to active galaxy centers as well as gamma ray burst engines.aims: the structured jets are postulated to account for emission properties of high energy sources across the mass scale, launched from stellar mass black holes in gamma ray bursts (grbs) and from supermassive black holes in active galactic nuclei (agns). their active cores contain magnetized accretion disks and the rotation of the kerr black hole provides a mechanism for launching relativistic jets. this process works most effectively if the mode of accretion turns out to be magnetically arrested. in this mode, the modulation of jets launched from the engine is related to internal instabilities in the accretion flow that operate on smallest time and spatial scales. as these scales are related to the light-crossing time and the black hole gravitational radius, the universal model of jet-disk connection is expected to scale with the mass of the black hole.methods: we investigated the jet-disk connection by means of 3d general relativistic magneto-hydrodynamical simulations of the magnetically arrested disk accretion in kerr geometry. we also quantified the variability of the disk by means of a fourier analysis.results: we found that the system evolution is governed by the physical parameters of the engine, such as the black hole spin and disk size, as well as disk magnetization, and we applied our scenarios to typical types of sources in agn and grb classes. we found that the magnetically arrested disk (mad) scenario is applicable to agn engines and supports persistent jet emissions. it can also be applied to grbs, as it gives the variability pattern roughly consistent with observations. however, in some cases, strong magnetic fields may lead to jet quenching, and this effect is found to be important mainly for grb jets. we speculate that it may be related to the strength of magnetically driven winds from the grb engines. | magnetically arrested accretion disks launching structured jets in application to grb and agn engines |
situation with highly magnetized neutron stars in binary systems is not yet certain. on the one hand, all best studied magnetars seem to be isolated objects. on the other, there are many claims based on model-dependent analysis of spin properties or/and luminosity of neutron stars in x-ray binaries in favour of large fields. in addition, there are a few results suggesting a magnetar-like activity of neutron stars in close binary systems. most of theoretical considerations do not favour even existence, not speaking about active decay, of magnetar-scale fields in neutron stars older than ∼106 yrs. however, alternative scenarios of the field evolution exist. i provide a brief review of theoretical and observational results related to the presence of neutron stars with large magnetic field in binaries and discuss perspectives of future studies. | high magnetic field neutron stars and magnetars in binary systems |
we use the point process mapping (ppmap) algorithm to reanalyse the herschel and scuba-2 observations of the l1688 and l1689 subregions of the ophiuchus molecular cloud. ppmap delivers maps with high resolution (here 14 arcsec, corresponding to ${\sim}0.01\, {\rm pc}$ at ${\sim}140\, {\rm pc}$), by using the observations at their native resolutions. ppmap also delivers more accurate dust optical depths, by distinguishing dust of different types and at different temperatures. the filaments and pre-stellar cores almost all lie in regions with $n_{\rm h_2}\gtrsim 7\times 10^{21}\, {\rm cm}^{-2}$ (corresponding to av ≳ 7). the dust temperature, t, tends to be correlated with the dust opacity index, β, with low t and low β concentrated in the interiors of filaments. the one exception to this tendency is a section of filament in l1688 that falls - in projection - between the two b stars: s1 and hd147889; here t and β are relatively high, and there is compelling evidence that feedback from these two stars has heated and compressed the filament. filament fwhms are typically in the range $0.10$ to $0.15\, {\rm pc}$. most filaments have line-densities in the range $25$ to $65\, {\rm m_{\odot }\, pc^{-1}}$. if their only support is thermal gas pressure, and the gas is at the canonical temperature of $10\, {\rm k}$, the filaments are highly supercritical. however, there is some evidence from ammonia observations that the gas is significantly warmer than this, and we cannot rule out the possibility of additional support from turbulence and/or magnetic fields. on the basis of their spatial distribution, we argue that most of the starless cores are likely to disperse (rather than evolving to become pre-stellar). | a ppmap analysis of the filamentary structures in ophiuchus l1688 and l1689 |
the circumnuclear disc (cnd) orbiting the galaxy's central black hole is a reservoir of material that can ultimately provide energy through accretion, or form stars in the presence of the black hole, as evidenced by the stellar cluster that is presently located at the cnd's centre. in this paper, we report the results of a computational study of the dynamics of the cnd. the results lead us to question two paradigms that are prevalent in previous research on the galactic centre. the first is that the disc's inner cavity is maintained by the interaction of the central stellar cluster's strong winds with the disc's inner rim, and secondly, that the presence of unstable clumps in the disc implies that the cnd is a transient feature. our simulations show that, in the absence of a magnetic field, the interaction of the wind with the inner disc rim actually leads to a filling of the inner cavity within a few orbital time-scales, contrary to previous expectations. however, including the effects of magnetic fields stabilizes the inner disc rim against rapid inward migration. furthermore, this interaction causes instabilities that continuously create clumps that are individually unstable against tidal shearing. thus the occurrence of such unstable clumps does not necessarily mean that the disc is itself a transient phenomenon. the next steps in this investigation are to explore the effect of the magnetorotational instability on the disc evolution and to test whether the results presented here persist for longer time-scales than those considered here. | the inner cavity of the circumnuclear disc |
context. despite 50 yr of extensive binary research, we must conclude that the roche lobe overflow/mass transfer process that governs close binary evolution is still poorly understood.aims: it is the scope of the present paper to lift the edge of the veil by studying the spin-up and spin-down processes of the o-type components of wr+o binaries.methods: we critically analyzed the available observational data of rotation speeds of the o-type components in wr+o binaries. by combining a binary evolutionary code and a formalism that describes the effects of tides in massive stars with an envelope in radiative equilibrium, we computed the corresponding rotational velocities during the roche lobe overflow of the progenitor binaries.results: in all the wr+o binaries studied, we find that the o-type stars were affected by accretion of matter during roche lobe overflow (rlof) of the progenitor. this means that common envelope evolution, which excludes any accretion onto the secondary o star, has not played an important role in explaining wr+o binaries. moreover, although it is very likely that the o-type star progenitors were spun up by mass transfer, many ended the rlof (and mass transfer) phase with a rotational velocity that is significantly smaller than the critical rotation speed. this may indicate that during the mass transfer phase there is a spin-down process that is of the same order, although significantly less, than that of the spin-up process. we propose a spruit-tayler type dynamo spin-down suggested in the past to explain the rotation speeds of the mass gainers in long-period algols. | spin rates and spin evolution of o components in wr+o binaries |
in certain astrophysical systems, the commonly employed ideal magnetohydrodynamics (mhd) approximation breaks down. here, we introduce novel explicit and implicit numerical schemes of ohmic resistivity terms in the moving-mesh code arepo. we include these non-ideal terms for two mhd techniques: the powell 8-wave formalism and a constrained transport scheme, which evolves the cell-centred magnetic vector potential. we test our implementation against problems of increasing complexity, such as one- and two-dimensional diffusion problems, and the evolution of progressive and stationary alfvén waves. on these test problems, our implementation recovers the analytic solutions to second-order accuracy. as first applications, we investigate the tearing instability in magnetized plasmas and the gravitational collapse of a rotating magnetized gas cloud. in both systems, resistivity plays a key role. in the former case, it allows for the development of the tearing instability through reconnection of the magnetic field lines. in the latter, the adopted (constant) value of ohmic resistivity has an impact on both the gas distribution around the emerging protostar and the mass loading of magnetically driven outflows. our new non-ideal mhd implementation opens up the possibility to study magneto-hydrodynamical systems on a moving mesh beyond the ideal mhd approximation. | non-ideal magnetohydrodynamics on a moving mesh |
g. srinivasan et al. (1990) proposed a simple and elegant explanation for the reduction of the neutron star magnetic dipole moment during binary evolution leading to low mass x-ray binaries and eventually to millisecond pulsars: quantized vortex lines in the neutron star core superfluid will pin against the quantized flux lines of the proton superconductor. as the neutron star spins down in the wind accretion phase of binary evolution, outward motion of vortex lines will reduce the dipole magnetic moment in proportion to the rotation rate. the presence of a toroidal array of flux lines makes this mechanism inevitable and independent of the angle between the rotation and magnetic axes. the incompressibility of the flux-line array (abrikosov lattice) determines the epoch when the mechanism will be effective throughout the neutron star. flux vortex pinning will not be effective during the initial young radio pulsar phase. it will, however, be effective and reduce the dipole moment in proportion with the rotation rate during the epoch of spindown by wind accretion as proposed by srinivasan et al. the mechanism operates also in the presence of vortex creep. | flux-vortex pinning and neutron star evolution |
the mirror mode evolving in collisionless magnetised high-temperature thermally anisotropic plasmas is shown to develop an interesting macro-state. starting as a classical zero-frequency ion fluid instability it saturates quasi-linearly at very low magnetic level, while forming elongated magnetic bubbles which trap the electron component to perform an adiabatic bounce motion along the magnetic field. further evolution of the mirror mode towards a stationary state is determined by the bouncing trapped electrons which interact with the thermal level of ion sound waves and generate attractive wake potentials which give rise to the formation of electron pairs in the lowest-energy singlet state of two combined electrons. pairing preferentially takes place near the bounce-mirror points where the pairs become spatially locked with all their energy in the gyration. the resulting large anisotropy of pairs enters the mirror growth rate in the quasi-linearly stable mirror mode. it breaks the quasi-linear stability and causes further growth. pressure balance is either restored by dissipation of the pairs and their anisotropy or inflow of plasma from the environment. in the first case new pairs will continuously form until equilibrium is reached. in the final state the fraction of pairs can be estimated. this process is open to experimental verification. to our knowledge it is the only process in which high-temperature plasma pairing may occur and has an important observable macroscopic effect: breaking the quasi-linear limit and, via pressure balance, generation of localised diamagnetism. | electron pairing in mirror modes: surpassing the quasi-linear limit |
a framework is introduced for coupling the evolution of galactic magnetic fields sustained by the mean-field dynamo with the formation and evolution of galaxies in cold dark matter cosmology. estimates of the steady-state strength of the large-scale and turbulent magnetic fields from mean-field and fluctuation dynamo models are used together with galaxy properties predicted by semi-analytic models of galaxy formation for a population of spiral galaxies. we find that the field strength is mostly controlled by the evolving gas content of the galaxies. thus, because of the differences in the implementation of the star formation law, feedback from supernovae and ram-pressure stripping, each of the galaxy formation models considered predicts a distribution of field strengths with unique features. the most prominent of them is the difference in typical magnetic field strengths obtained for the satellite and central galaxy populations as well as the typical strength of the large-scale magnetic field in galaxies of different mass. | galactic magnetic fields and hierarchical galaxy formation |
we examine the magnetic field evolution occurring in a neutron star crust. beyond the elastic limit, the lattice ions are assumed to act as a plastic flow. the ohmic dissipation, hall drift, and bulk fluid velocity driven by the lorentz force are considered in our numerical simulation. a magnetically induced quadrupole deformation is observed in the crust during the evolution. generally, the ellipticity decreases as the magnetic energy decreases. in a toroidal-field-dominated model, the sign of the ellipticity changes. namely, the initial prolate shape tends to become oblate. this occurs because the toroidal component decays rapidly on a smaller time-scale than the poloidal dipole component. we find that the magnetic dipole component does not change significantly on the hall time-scale of ∼1 myr for the considered simple initial models. thus, a more complex initial model is required to study the fast decay of surface dipoles on the above-mentioned time-scale. | evolution of magnetic deformation in neutron star crust |
short-orbit gas giant planet formation/evolution mechanisms are still not well understood. one promising pathway to discriminate between mechanisms is to constrain the occurrence rate of these peculiar exoplanets at the earliest stage of the system's life. however, a major limitation when studying newly born stars is stellar activity. this cocktail of phenomena triggered by fast rotation, strong magnetic fields, and complex internal dynamics, especially present in very young stars, compromises our ability to detect exoplanets. in this paper, we investigated the limitations of such detections in the context of already acquired data solely using radial velocity data acquired with a non-stabilized spectrograph. we employed two strategies: doppler imaging and gaussian processes and could confidently detect hot jupiters with a semi-amplitude of 100 m s-1 buried in the stellar activity. we also showed the advantages of the gaussian process approach in this case. this study serves as a proof of concept to identify potential candidates for follow-up observations or even discover such planets in legacy data sets available to the community. | planets around young active solar-type stars: assessing detection capabilities from a non-stabilized spectrograph |
g183-35 is an unusual white dwarf that shows an h α line split into five components, instead of the usual three components seen in strongly magnetic white dwarfs. potential explanations for the unusual set of lines include a double degenerate system containing two magnetic white dwarfs and/or rotational modulation of a complex magnetic field structure. here, we present time-resolved spectroscopy of g183-35 obtained at the gemini observatory. these data reveal two sets of absorption lines that appear and disappear over a period of about 4 h. we also detect low-level (0.2 per cent) variability in optical photometry at the same period. we demonstrate that the spectroscopic and photometric variability can be explained by the presence of spots on the surface of the white dwarf and a change in the average field strength from about 4.6 to 6.2 mg. the observed variability is clearly due to g183-35's relatively short spin period. however, rotational modulation of a complex magnetic field by itself cannot explain the changes seen in the central h α component. an additional source of variability in the line profiles, most likely due to a chemically inhomogeneous surface composition, is also needed. we propose further observations of similar objects to test this scenario. | a magnetic white dwarf with five h α components |
observations of the faraday rotation measure, combined with the dispersion measure, can be used to infer the magnetoionic environment of a radio source. we investigate the magnetoionic environments of fast radio bursts (frbs) by deriving their estimated average magnetic field strengths along the line of sight <b∥> in their host galaxies and comparing them with those of galactic pulsars and magnetars. we find that for those frbs with rm measurements, the mean <b∥> are $1.77^{+9.01}_{-1.48}\, \rm \mu g$ and $1.74^{+14.82}_{-1.55}\, \rm \mu g$ using two different methods, which is slightly larger but not inconsistent with the distribution of galactic pulsars, $1.00^{+1.51}_{-0.60}\, \rm \mu g$ . only six galactic magnetars have estimated <b∥>. excluding psr j1745-2900 that has an anomalously high value due to its proximity with the galactic centre, the other five sources have a mean value of $1.70\, \rm \mu g$ , which is statistically consistent with the <b∥> distributions of both galactic pulsars and frbs. there is no apparent trend of evolution of magnetar <b∥> as a function of age or surface magnetic field strength. galactic pulsars and magnetars close to the galactic centre have relatively larger <b∥> values than other pulsars/magnetars. we discuss the implications of these results for the magnetoionic environments of frb 121102 within the context of magnetar model and the model invoking a supermassive black hole, and for the origin of frbs in general. | on the magnetoionic environments of fast radio bursts |
we report the orbital x-ray variability of the high-mass x-ray binary (hmxb) gx 301-2. gx 301-2 underwent a spin-up process in 2018-2020 with the period evolving from ~685-670 s. the energy-resolved pulse profiles of the pulsar at 1-60 kev varied from single-peaked and sinusoidal shapes to multipeaked ones across different orbital phases. pulse fractions evolving over the orbit had negative correlations with the x-ray flux. the broad-band x-ray energy spectrum of the pulsar can be described with a partially covered negative-positive cut-off power-law continuum model. near the periastron passage of the pulsar we found strong variation in the additional column density ($n_{\mathrm{h}_{2}}$), which correlated with variation of the flux. curves of growth for both fe kα and fe kβ lines were plotted to investigate the distribution of matter around the neutron star. we also found evidence for two cyclotron absorption lines in the phase-averaged spectra in gx 301-2, with one line of 30-42 kev and the other line varying over 48-56 kev. the centroid energies of both lines show a similar relationship with x-ray luminosity: positive correlation in the lower luminosity range, and a negative relation above a critical luminosity of $10^{37}\, \rm erg\, s^{-1}$. we estimate the surface magnetic field of the neutron star in gx 301-2 to be ~(0.5-2) × 1013 g. the two cyclotron line energies have a nearly fixed ratio of ~1.63 while having a low strength ratio (~0.05), suggesting that these two features may actually be one line. | timing and spectral variability of the high-mass x-ray pulsar gx 301-2 over orbital phases observed by insight-hxmt |
we investigate the relation between rotation periods p rot and photometric modulation amplitudes r per for ≈4000 sun-like main-sequence stars observed by kepler, using p rot and r per from mcquillan et al., effective temperature t eff from lamost dr6, and parallax data from gaia edr3. as has been suggested in previous works, we find that p rot scaled by the convective turnover time τ c, or the rossby number ro, serves as a good predictor of r per: r per plateaus at around 1% in relative flux for 0.2 ≲ ro/ro⊙ ≲ 0.4, and decays steeply with increasing ro for 0.4 ≲ ro/ro⊙ ≲ 0.8, where ro⊙ denotes ro of the sun. in the latter regime we find ${\rm{d}}\,\mathrm{ln}\,{r}_{\mathrm{per}}/{\rm{d}}\,\mathrm{ln}\,\mathrm{ro}$ ~ -4.5 to -2.5, although the value is sensitive to detection bias against weak modulation and may depend on other parameters including t eff and surface metallicity. the existing x-ray and ca ii h and k flux data also show transitions at ro/ro⊙ ~ 0.4, suggesting that all these transitions share the same physical origin. we also find that the rapid decrease of r per with increasing ro causes rotational modulation of fainter kepler stars with ro/ro⊙ ≳ 0.6 to be buried under the photometric noise. this effect sets the longest p rot detected in the mcquillan et al. sample as a function of t eff and obscures the signature of stalled spin down that has been proposed to set in around ro/ro⊙ ~ 1. | on the evolution of rotational modulation amplitude in solar-mass main-sequence stars |
we present a detailed study of the high-mass x-ray binary vela x-1, using observations performed by insight-hxmt in 2019 and 2020, concentrating on timing analysis and spectral studies including pulse phase-resolved spectroscopy. the cyclotron line energy is found to be ~21-27 and 43-50 kev for the fundamental and first harmonic, respectively. we present the evolution of spectral parameters and find that the two line centroid energy ratio e2/e1 evolved from ~2 before mjd 58900 to ~1.7 after that. the harmonic cyclotron line energy has no relation to the luminosity but the fundamental line energy shows a positive correlation with x-ray luminosity, suggesting that vela x-1 is located in the sub-critical accreting regime. in addition, the pulse phase-resolved spectroscopy in vela x-1 is performed. both the cyclotron resonant scattering feature and continuum parameters show strong variability over the pulse phase with the ratio of two line energies about 2 near the peak phases and down to ~1.6 around off-peak phases. long-term significant variations of the absorption column density and its evolution over the pulse phase may imply the existence of the clumpy wind structure near the neutron star. | variations of cyclotron resonant scattering features in vela x-1 revealed with insight-hxmt |
we present a study based on the high-resolution spectroscopy and k2 space photometry of five chemically peculiar stars in the region of the open cluster m44. the analysis of the high-precision photometric k2 data reveals that the light variations in hd 73045 and hd 76310 are rotational in nature and caused by spots or cloud-like co-rotating structures, which are non-stationary and short-lived. the time-resolved radial velocity measurements, in combination with the k2 photometry, confirm that hd 73045 does not show any periodic variability on time-scales shorter than 1.3 d, contrary to previous reports in the literature. in addition to these new rotational variables, we discovered a new heartbeat system, hd 73619, where no pulsational signatures are seen. the spectroscopic and spectropolarimetric analyses indicate that hd 73619 belongs to the peculiar am class, with either a weak or no magnetic field, considering the 200-g detection limit of our study. the least-squares deconvolution profiles for hd 76310 indicate a complex structure in its spectra, suggesting that this star is either part of a binary system or surrounded by a cloud shell. when placed in the hertzsprung-russell diagram, all studied stars are evolved from the main sequence and situated in the δ scuti instability strip. this work is relevant for further detailed studies of chemically peculiar stars, for example on inhomogeneities (including spots) in the absence of magnetic fields and the origin of the pulsational variability in heartbeat systems. | study of chemically peculiar stars - i. high-resolution spectroscopy and k2 photometry of am stars in the region of m44 |
context.σ ori e, a massive helium b-type star, shows high surface rotation and a strong surface magnetic field, potentially challenging the process of wind magnetic braking.aims: the gaia satellite provides an accurate distance to σ ori e and confirms its membership to the σ ori cluster. we account for these two key pieces of information in order to investigate whether single star models can reproduce the observed properties of σ ori e and provide new estimates for its metallicity, mass, and age.methods: we computed rotating stellar models accounting for wind magnetic braking and magnetic quenching of the mass loss. we considered two metallicities (z = 0.014, with a helium mass fraction y = 0.273 and z = 0.020 with y = 0.266), four initial masses between 8 and 9 m⊙, three initial rotations between 250 and 450 km s−1, and three initial surface equatorial magnetic field between 3 and 7 kg. differential rotation is assumed for the internal rotation in all models. we looked for models simultaneously accounting for the observed radius, position in the hr diagram, surface velocity, and braking timescale.results: we obtain that σ ori e is a very young star (age less than 1 myr) with an initial mass of around 9 m⊙, a surface equatorial magnetic field of around 7 kg, and a metallicity z (mass fraction of heavy elements) of around 0.020. no solution is obtained with the present models for a metallicity of z = 0.014. the initial rotation of the models fitting σ ori e is not highly constrained and could be anywhere in the range studied here. because of its very young age, models predict no observable changes of the surface abundances due to rotational mixing.conclusions: the simultaneous high surface rotation and high surface magnetic field of σ ori e may simply be a consequence of its young age. this young age implies that the processes responsible for producing the chemical inhomogeneities that are observed at its surface should be rapid. therefore, for explaining the properties of σ ori e, there is no necessity to invoke a merging event, although such a scenario cannot be discarded. other stars (hr 5907, hr 7355, hr 345439, hd 2347, cpd -50°3509) showing similar properties to σ ori e (fast rotation and strong surface magnetic field) may also be very young stars, although determination of the braking timescales is needed to confirm such a conclusion. | news from gaia on σ ori e: a case study for the wind magnetic braking process |
plasma relaxation in the presence of an initially braided magnetic field can lead to self-organization into relaxed states that retain non-trivial magnetic structure. these relaxed states may be in conflict with the linear force-free fields predicted by the classical taylor theory, and remain to be fully understood. here, we study how the individual field line helicities evolve during such a relaxation, and show that they provide new insights into the relaxation process. the line helicities are computed for numerical resistive-magnetohydrodynamic simulations of a relaxing braided magnetic field with line-tied boundary conditions, where the relaxed state is known to be non-taylor. first, our computations confirm recent analytical predictions that line helicity will be predominantly redistributed within the domain, rather than annihilated. second, we show that self-organization into a relaxed state with two discrete flux tubes may be predicted from the initial line helicity distribution. third, for this set of line-tied simulations we observe that the sub-structure within each of the final tubes is a state of uniform line helicity. this uniformization of line helicity is consistent with taylor theory applied to each tube individually. however, it is striking that the line helicity becomes significantly more uniform than the force-free parameter. | evolution of field line helicity in magnetic relaxation |
the correlation between space environment conditions and the properties of escaping ions is a central topic of mars research. although empirical correlations have been visible in the data, a physics-based interpretation, rather than statistics-based pictures, has not been established yet. as a first effort, we investigate the electric field, the direct contributor to ion acceleration, in the mars plasma environment from a hybrid plasma model (particle ions and fluid electrons). we use amitis, a hybrid model combined with an observation-based ionospheric model, to simulate the mars-solar wind interaction under nominal solar wind plasma conditions for perpendicular and parker spiral directions of the interplanetary magnetic field (imf). the simulations show following results: (1) the electric field morphology is structured by the imf direction and the different plasma domains in the solar wind-mars interaction; (2) asymmetry of the electric field between the hemispheres where the convective electric field points inward and outward, respectively, due to the mass loading and asymmetric draping of the magnetic field lines; (3) the motional electric field dominates in most regions, especially in the dayside magnetosheath; and (4) the hall term is an order of magnitude weaker and significant in the magnetotail and plasma boundaries for a perpendicular imf case. the hall term is relatively stronger for the parker spiral case. (5) the ambipolar electric field, in principle, agrees with mars atmosphere and volatile evolution measurements in the magnetosheath. | solar wind interaction with mars: electric field morphology and source terms |
orca (oscillations research with cosmics in the abyss) is the low-energy branch of km3net, the underwater cherenkov neutrino detector in the mediterranean. its primary goal is to resolve the long-standing unsolved question of the neutrino mass ordering by measuring matter oscillation effects in atmospheric neutrinos. to be deployed at the french km3net site, orca’s multi-pmt optical modules will exploit the excellent optical properties of deep seawater to reconstruct cascade and track events with a few gev of energy. this contribution reviews the methods and technology, and discusses the current expected performances. | km3net - orca: measuring the neutrino mass ordering in the mediterranean |
reno (reactor experiment for neutrino oscillation) has obtained the first measured value of effective neutrino mass difference from a spectral analysis of reactor neutrino disappearance. the measurement absolutely relies on the accurate energy calibration. several radioactive sources such as 137cs, 54mn, 68ge, 65zn, 60co, po-be, am-be, and cf-ni, are used for the energy calibration of the reno detectors. we obtained an energy conversion function from observed charges to prompt signal energy which describes a non-linear response due to the quenching effect in liquid scintillator and cherenkov radiation. we have verified the performance of the energy calibration using copious betadecay events from radioactive isotopes b12 that are produced by cosmic-muon interaction. the energy calibration was performed for the target and gamma-catcher regions separately due to their different energy responses. in this presentation we describe the methods and results of the energy calibration. | prompt energy calibration at reno |
the opera experiment reached its main goal by proving the appearance of ν_{τ} in the cngs ν_μ beam. five ν_{τ} candidates were detected with a s/b ratio of ∼ 10, allowing to reject the null hypothesis at 5.1σ . the search has been extended by loosening the selection criteria in order to improve the statistical uncertainty. one of the ν_{τ} candidates selected with the new strategy shows a double vertex topology and, after a dedicated multivariate analysis, is compatible with being a ν_{τ} interaction with charm production. based on the enlarged data sample the estimation of δ m^2_{23} in appearance mode is being performed. the search for ν_e interactions has been extended over the full data set with a more than twofold increase in statistics: data are compatible with the non-oscillation hypothesis in the three-flavour mixing model. the implications of the electron neutrino sample in the framework of the 3 + 1 sterile mode will lead to exclusion limits on sin^2 2θ_{μ e} . finally, the analysis of the annual modulation of cosmic muons is introduced. | more results from the opera experiment |
orca (oscillations research with cosmics in the abyss) is a megaton-scale cherenkov neutrino detector currently under construction by the km3net collaboration, at a depth of 2450m in the mediterranean sea. atmospheric neutrinos cross the earth along a wide range of baselines, undergoing matter effects which enhance neutrino oscillations in the few gev energy range with a dependence on the neutrino mass ordering (nmo). the orca design consists of a dense configuration of multi-pmt optical modules, exploiting the excellent optical properties of deep seawater to reconstruct both cascade events (mostly νe {ν_e} ) and track events (mostly νμ ν_μ ) down to a few gev. orca is expected to measure the nmo with a median significance greater than 3σ σ after a few years of operation. this contribution focuses on the methods and results of the sensitivity studies for the measurement of the mass ordering as well as oscillation parameters (θ23 θ_{23} , δm2 δ m^2 ). | sensitivity of orca to the neutrino mass ordering and oscillation parameters |
papers on the searches for dark matter and exotics, neutrino oscillations and detector calibration, prepared for the 35th international cosmic ray conference (icrc 2017, busan, south korea) by the antares collaboration | the antares collaboration: contributions to icrc 2017 part iii: searches for dark matter and exotics, neutrino oscillations and detector calibration |
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