text
stringlengths 89
2.49k
| category
stringclasses 19
values |
|---|---|
Verification of commercial motor performance for WEAVE at the William
Herschel Telescope: WEAVE is a 1000-fiber multi-object spectroscopic facility for the 4.2~m
William Herschel Telescope. It will feature a double-headed pick-and-place
fiber positioning robot comprising commercially available robotic axes. This
paper presents results on the performance of these axes, obtained by testing a
prototype system in the laboratory. Positioning accuracy is found to be better
than the manufacturer's published values for the tested cases, indicating that
the requirement for a maximum positioning error of 8.0~microns is achievable.
Field reconfiguration times well within the planned 60 minute observation
window are shown to be likely when individual axis movements are combined in an
efficient way. | astro-ph_IM |
Low-order wavefront control using a Zernike sensor through Lyot
coronagraphs for exoplanet imaging: Combining large segmented space telescopes, coronagraphy and wavefront
control methods is a promising solution to produce a dark hole (DH) region in
the coronagraphic image of an observed star and study planetary companions. The
thermal and mechanical evolution of such a high-contrast facility leads to
wavefront drifts that degrade the DH contrast during the observing time, thus
limiting the ability to retrieve planetary signals. Lyot-style coronagraphs are
starlight suppression systems that remove the central part of the image for an
unresolved observed star, the point spread function, with an opaque focal plane
mask (FPM). When implemented with a flat mirror containing an etched pinhole,
the mask rejects part of the starlight through the pinhole which can be used to
retrieve information about low-order aberrations. We propose an active control
scheme using a Zernike wavefront sensor (ZWFS) to analyze the light rejected by
the FPM, control low-order aberrations, and stabilize the DH contrast. The
concept formalism is first presented before characterizing the sensor behavior
in simulations and in laboratory. We then perform experimental tests to
validate a wavefront control loop using a ZWFS on the HiCAT testbed. By
controlling the first 11 Zernike modes, we show a decrease in wavefront error
standard deviation by a factor of up to 9 between open- and closed-loop
operations using the ZWFS. In the presence of wavefront perturbations, we show
the ability of this control loop to stabilize a DH contrast around 7x10^-8 with
a standard deviation of 7x10^-9. Active control with a ZWFS proves a promising
solution in Lyot coronagraphs with an FPM-filtered beam to control and
stabilize low-order wavefront aberrations and DH contrast for exoplanet imaging
with future space missions. | astro-ph_IM |
AstroDS -- A Distributed Storage for Astrophysics of Cosmic Rays.
Current Status: Currently, the processing of scientific data in astroparticle physics is
based on various distributed technologies, the most common of which are Grid
and cloud computing. The most frequently discussed approaches are focused on
large and even very large scientific experiments, such as Cherenkov Telescope
Array. We, by contrast, offer a solution designed for small to medium
experiments such as TAIGA. In such experiments, as a rule, historically
developed specific data processing methods and specialized software are used.
We have specifically designed a distributed (cloud) data storage for
astroparticle physics data collaboration in medium-sized experiments. In this
article, we discuss the current state of our work using the example of the
TAIGA and CASCADE experiments. A feature of our approach is that we provide our
users with scientific data in the form to which they are accustomed to in
everyday work on local resources. | astro-ph_IM |
How to write and develop your astronomy research paper: Writing is a vital component of a modern career in scientific research. But
how to write correctly and effectively is often not included in the training
that young astronomers receive from their supervisors and departments. We offer
a step-by-step guide to tackle this deficiency, published as a set of two
papers. In the first, we addressed how to plan and outline your paper and
decide where to publish. In the current second paper, we describe the various
sections that constitute a typical research paper in astronomy, sharing best
practice for the most efficient use of each of them. We also discuss a
selection of issues that often cause trouble to writers, from sentence to
paragraph structure, the `writing mechanics' used to develop a manuscript. Our
two-part guide is aimed primarily at master's and PhD level students who are
presented with the daunting task of writing their first scientific paper, but
more senior researchers or writing instructors may well find the ideas
presented here useful. | astro-ph_IM |
Focus diverse phase retrieval testbed development of continuous
wavefront sensing for space telescope applications: Continuous wavefront sensing on future space telescopes allows relaxation of
stability requirements while still allowing on-orbit diffraction-limited
optical performance. We consider the suitability of phase retrieval to
continuously reconstruct the phase of a wavefront from on-orbit irradiance
measurements or point spread function (PSF) images. As phase retrieval
algorithms do not require reference optics or complicated calibrations, it is a
preferable technique for space observatories, such as the Hubble Space
Telescope or the James Webb Space Telescope. To increase the robustness and
dynamic range of the phase retrieval algorithm, multiple PSF images with known
amount of defocus can be utilized. In this study, we describe a recently
constructed testbed including a 97 actuator deformable mirror, changeable
entrance pupil stops, and a light source. The aligned system wavefront error is
below ~30nm. We applied various methods to generate a known wavefront error,
such as defocus and/or other aberrations, and found the accuracy and precision
of the root mean squared error of the reconstructed wavefronts to be less than
~10nm and ~2nm, respectively. Further, we discuss the signal-to-noise ratios
required for continuous dynamic wavefront sensing. We also simulate the case of
spacecraft drifting and verify the performance of the phase retrieval algorithm
for continuous wavefront sensing in the presence of realistic disturbances. | astro-ph_IM |
An improved method for polarimetric image restoration in interferometry: Interferometric radio astronomy data require the effects of limited coverage
in the Fourier plane to be accounted for via a deconvolution process. For the
last 40 years this process, known as `cleaning', has been performed almost
exclusively on all Stokes parameters individually as if they were independent
scalar images. However, here we demonstrate for the case of the linear
polarisation $\mathcal{P}$, this approach fails to properly account for the
complex vector nature resulting in a process which is dependant on the axis
under which the deconvolution is performed. We present here an improved method,
`Generalised Complex CLEAN', which properly accounts for the complex vector
nature of polarised emission and is invariant under rotations of the
deconvolution axis. We use two Australia Telescope Compact Array datasets to
test standard and complex CLEAN versions of the H\"{o}gbom and SDI CLEAN
algorithms. We show that in general the Complex CLEAN version of each algorithm
produces more accurate clean components with fewer spurious detections and
lower computation cost due to reduced iterations than the current methods. In
particular we find that the Complex SDI CLEAN produces the best results for
diffuse polarised sources as compared with standard CLEAN algorithms and other
Complex CLEAN algorithms. Given the move to widefield, high resolution
polarimetric imaging with future telescopes such as the Square Kilometre Array,
we suggest that Generalised Complex CLEAN should be adopted as the
deconvolution method for all future polarimetric surveys and in particular that
the complex version of a SDI CLEAN should be used. | astro-ph_IM |
UVscope and its application aboard the ASTRI-Horn telescope: UVscope is an instrument, based on a multi-pixel photon detector, developed
to support experimental activities for high-energy astrophysics and cosmic ray
research. The instrument, working in single photon counting mode, is designed
to directly measure light flux in the wavelengths range 300-650~nm. The
instrument can be used in a wide field of applications where the knowledge of
the nocturnal environmental luminosity is required. Currently, one UVscope
instrument is allocated onto the external structure of the ASTRI-Horn Cherenkov
telescope devoted to the gamma-ray astronomy at very high energies. Being
co-aligned with the ASTRI-Horn camera axis, UVscope can measure the diffuse
emission of the night sky background simultaneously with the ASTRI-Horn camera,
without any interference with the main telescope data taking procedures.
UVscope is properly calibrated and it is used as an independent reference
instrument for test and diagnostic of the novel ASTRI-Horn telescope. | astro-ph_IM |
CHIME FRB: An application of FFT beamforming for a radio telescope: We have developed FFT beamforming techniques for the CHIME radio telescope,
to search for and localize the astrophysical signals from Fast Radio Bursts
(FRBs) over a large instantaneous field-of-view (FOV) while maintaining the
full angular resolution of CHIME. We implement a hybrid beamforming pipeline in
a GPU correlator, synthesizing 256 FFT-formed beams in the North-South
direction by four formed beams along East-West via exact phasing, tiling a sky
area of ~250 square degrees. A zero-padding approximation is employed to
improve chromatic beam alignment across the wide bandwidth of 400 to 800 MHz.
We up-channelize the data in order to achieve fine spectral resolution of
$\Delta\nu$=24 kHz and time cadence of 0.983 ms, desirable for detecting
transient and dispersed signals such as those from FRBs. | astro-ph_IM |
A key-formula to compute the gravitational potential of inhomogeneous
discs in cylindrical coordinates: We have established the exact expression for the gravitational potential of a
homogeneous polar cell - an elementary pattern used in hydrodynamical
simulations of gravitating discs. This formula, which is a closed-form, works
for any opening angle and radial extension of the cell. It is valid at any
point in space, i.e. in the plane of the distribution (inside and outside) as
well as off-plane, thereby generalizing the results reported by Durand (1953)
for the circular disc. The three components of the gravitational acceleration
are given. The mathematical demonstration proceeds from the "incomplete version
of Durand's formula" for the potential (based on complete elliptic integrals).
We determine first the potential due to the circular sector (i.e. a pie-slice
sheet), and then deduce that of the polar cell (from convenient radial scaling
and subtraction). As a by-product, we generate an integral theorem stating that
"the angular average of the potential of any circular sector along its tangent
circle is 2/PI times the value at the corner". A few examples are presented.
For numerical resolutions and cell shapes commonly used in disc simulations, we
quantify the importance of curvature effects by performing a direct comparison
between the potential of the polar cell and that of the Cartesian (i.e.
rectangular) cell having the same mass. Edge values are found to deviate
roughly like 2E-3 x N/256 in relative (N is the number of grid points in the
radial direction), while the agreement is typically four orders of magnitude
better for values at the cell's center. We also produce a reliable
approximation for the potential, valid in the cell's plane, inside and close to
the cell. Its remarkable accuracy, about 5E-4 x N/256 in relative, is
sufficient to estimate the cell's self-acceleration. | astro-ph_IM |
Design and Performance of the GAMMA-400 Gamma-Ray Telescope for the Dark
Matter Searches: The GAMMA-400 gamma-ray telescope is designed to measure the fluxes of gamma
rays and cosmic-ray electrons + positrons, which can be produced by
annihilation or decay of the dark matter particles, as well as to survey the
celestial sphere in order to study point and extended sources of gamma rays,
measure energy spectra of Galactic and extragalactic diffuse gamma-ray
emission, gamma-ray bursts, and gamma-ray emission from the Sun. The GAMMA-400
covers the energy range from 100 MeV to 3000 GeV. Its angular resolution is
~0.01 deg (E{\gamma} > 100 GeV), the energy resolution ~1% (E{\gamma} > 10
GeV), and the proton rejection factor ~10E6. GAMMA-400 will be installed on the
Russian space platform Navigator. The beginning of observations is planned for
2018. | astro-ph_IM |
Theory and Simulations of Refractive Substructure in Resolved
Scatter-Broadened Images: At radio wavelengths, scattering in the interstellar medium distorts the
appearance of astronomical sources. Averaged over a scattering ensemble, the
result is a blurred image of the source. However, Narayan & Goodman (1989) and
Goodman & Narayan (1989) showed that for an incomplete average, scattering
introduces refractive substructure in the image of a point source that is both
persistent and wideband. We show that this substructure is quenched but not
smoothed by an extended source. As a result, when the scatter-broadening is
comparable to or exceeds the unscattered source size, the scattering can
introduce spurious compact features into images. In addition, we derive
efficient strategies to numerically compute realistic scattered images, and we
present characteristic examples from simulations. Our results show that
refractive substructure is an important consideration for ongoing missions at
the highest angular resolutions, and we discuss specific implications for
RadioAstron and the Event Horizon Telescope. | astro-ph_IM |
SciCodes: Astronomy Research Software and Beyond: The Astrophysics Source Code Library (ASCL ascl.net), started in 1999, is a
free open registry of software used in refereed astronomy research. Over the
past few years, it has spearheaded an effort to form a consortium of scientific
software registries and repositories. In 2019 and 2020, ASCL contacted editors
and maintainers of discipline and institutional software registries and
repositories in math, biology, neuroscience, geophysics, remote sensing, and
other fields to develop a list of best practices for these research software
resources. At the completion of that project, performed as a Task Force for a
FORCE11 working group, members decided to form SciCodes as an ongoing
consortium. This presentation covered the consortium's work so far, what it is
currently working on, what it hopes to achieve for making scientific research
software more discoverable across disciplines, and how the consortium can
benefit astronomers. | astro-ph_IM |
Interstellar Now! Missions to and Sample Returns from Nearby
Interstellar Objects: The recently discovered first high velocity hyperbolic objects passing
through the Solar System, 1I/'Oumuamua and 2I/Borisov, have raised the question
about near term missions to Interstellar Objects. In situ spacecraft
exploration of these objects will allow the direct determination of both their
structure and their chemical and isotopic composition, enabling an entirely new
way of studying small bodies from outside our solar system. In this paper, we
map various Interstellar Object classes to mission types, demonstrating that
missions to a range of Interstellar Object classes are feasible, using existing
or near-term technology. We describe flyby, rendezvous and sample return
missions to interstellar objects, showing various ways to explore these bodies
characterizing their surface, dynamics, structure and composition. Interstellar
objects likely formed very far from the solar system in both time and space;
their direct exploration will constrain their formation and history, situating
them within the dynamical and chemical evolution of the Galaxy. These mission
types also provide the opportunity to explore solar system bodies and perform
measurements in the far outer solar system. | astro-ph_IM |
Impact of particles on the Planck HFI detectors: Ground-based
measurements and physical interpretation: The Planck High Frequency Instrument (HFI) surveyed the sky continuously from
August 2009 to January 2012. Its noise and sensitivity performance were
excellent, but the rate of cosmic ray impacts on the HFI detectors was
unexpectedly high. Furthermore, collisions of cosmic rays with the focal plane
produced transient signals in the data (glitches) with a wide range of
characteristics. A study of cosmic ray impacts on the HFI detector modules has
been undertaken to categorize and characterize the glitches, to correct the HFI
time-ordered data, and understand the residual effects on Planck maps and data
products. This paper presents an evaluation of the physical origins of glitches
observed by the HFI detectors. In order to better understand the glitches
observed by HFI in flight, several ground-based experiments were conducted with
flight-spare HFI bolometer modules. The experiments were conducted between 2010
and 2013 with HFI test bolometers in different configurations using varying
particles and impact energies. The bolometer modules were exposed to 23 MeV
protons from the Orsay IPN TANDEM accelerator, and to $^{241}$Am and $^{244}$Cm
$\alpha$-particle and $^{55}$Fe radioactive X-ray sources. The calibration data
from the HFI ground-based preflight tests were used to further characterize the
glitches and compare glitch rates with statistical expectations under
laboratory conditions. Test results provide strong evidence that the dominant
family of glitches observed in flight are due to cosmic ray absorption by the
silicon die substrate on which the HFI detectors reside. Glitch energy is
propagated to the thermistor by ballistic phonons, while there is also a
thermal diffusion contribution. The implications of these results for future
satellite missions, especially those in the far-infrared to sub-millimetre and
millimetre regions of the electromagnetic spectrum, are discussed. | astro-ph_IM |
Sparsity and the Bayesian Perspective: Sparsity has been recently introduced in cosmology for weak-lensing and CMB
data analysis for different applications such as denoising, component
separation or inpainting (i.e. filling the missing data or the mask). Although
it gives very nice numerical results, CMB sparse inpainting has been severely
criticized by top researchers in cosmology, based on arguments derived from a
Bayesian perspective. Trying to understand their point of view, we realize that
interpreting a regularization penalty term as a prior in a Bayesian framework
can lead to erroneous conclusions. This paper is by no means against the
Bayesian approach, which has proven to be very useful for many applications,
but warns about a Bayesian-only interpretation in data analysis, which can be
misleading in some cases. | astro-ph_IM |
Frequency chirped continuous-wave sodium laser guide stars: We numerically study a method to increase the photon return flux of
continuous-wave laser guide stars using one-dimensional atomic cooling
principles. The method relies on chirping the laser towards higher frequencies
following the change in velocity of sodium atoms due to recoil, which raises
atomic populations available for laser excitation within the Doppler
distribution. The efficiency of this effect grows with the average number of
atomic excitations between two atomic collisions in the mesosphere. We find the
parameters for maximizing the return flux and evaluate the performance of
chirping for operation at La Palma. According to our simulations, the optimal
chirp rate lies between 0.8-1.0 MHz/$\mu$s and an increase in the fluorescence
of the sodium guide star up to 60% can be achieved with current 20 W-class
guide star lasers. | astro-ph_IM |
DiskFM: A Forward Modeling Tool for Disk Analysis with Coronagraphic
Instruments: Because of bright starlight leakage in coronagraphic raw images, faint
astrophysical objects such as exoplanets can only be detected using powerful
point spread function (PSF) subtraction algorithms. However, these algorithms
have strong effects on faint objects of interest, and often prevent precise
spectroscopic analysis and scattering property measurements of circumstellar
disks. For this reason, PSF-subtraction effects is currently the main
limitations to the precise characterization of exoplanetary dust with
scattered-light imaging. Forward-modeling techniques have long been developed
for point source objects. However, forward-modeling with disks is complicated
by the fact that the disk cannot be simplified using a simple point source
convolved by the PSF as the astrophysical model; all hypothetical disk
morphologies must be explored to understand the subtle and non-linear effects
of the PSF subtraction algorithm on the shape and local geometry of these
systems. Because of their complex geometries, the forward-modeling process has
to be repeated tens or hundred of thousands of times on disks with slightly
different physical properties. All of these geometries are then compared to the
PSF-subtracted image of the data, within an MCMC or a Chi-square wrapper. In
this paper, we present here DiskFM, a new open-source algorithm included in the
PSF subtraction algorithms package pyKLIP. This code allows to produce fast
forward-modeling for a variety of observation strategies (ADI, SDI, ADI+SDI,
RDI). pyKLIP has already been used for SPHERE/IRDIS and GPI data. It is readily
available on all instruments supported by pyKLIP (SPHERE/IFS, SCExAO/CHARIS),
and can be quickly adapted for other coronagraphic instruments. | astro-ph_IM |
A Lightweight Space-based Solar Power Generation and Transmission
Satellite: We propose a novel design for a lightweight, high-performance space-based
solar power array combined with power beaming capability for operation in
geosynchronous orbit and transmission of power to Earth. We use a modular
configuration of small, repeatable unit cells, called tiles, that each
individually perform power collection, conversion, and transmission. Sunlight
is collected via lightweight parabolic concentrators and converted to DC
electric power with high efficiency III-V photovoltaics. Several CMOS
integrated circuits within each tile generates and controls the phase of
multiple independently-controlled microwave sources using the DC power. These
sources are coupled to multiple radiating antennas which act as elements of a
large phased array to beam the RF power to Earth. The power is sent to Earth at
a frequency chosen in the range of 1-10 GHz and collected with ground-based
rectennas at a local intensity no larger than ambient sunlight. We achieve
significantly reduced mass compared to previous designs by taking advantage of
solar concentration, current CMOS integrated circuit technology, and ultralight
structural elements. Of note, the resulting satellite has no movable parts once
it is fully deployed and all beam steering is done electronically. Our design
is safe, scalable, and able to be deployed and tested with progressively larger
configurations starting with a single unit cell that could fit on a cube
satellite. The design reported on here has an areal mass density of 160 g/m2
and an end-to-end efficiency of 7-14%. We believe this is a significant step
forward to the realization of space-based solar power, a concept once of
science fiction. | astro-ph_IM |
DISCO: a Spatio-Spectral Recombiner for Pupil Remapping Interferometry: Pupil-remapping is a new high-dynamic range imaging technique that has
recently demonstrated feasibility on sky. The current prototypes present
however deceiving limiting magnitude, restricting the current use to the
brightest stars in the sky. We propose to combine pupil-remapping with
spatio-spectral encoding, a technique first applied to the VEGA/CHARA
interferometer. The result is an instrument proposal, called "Dividing
Interferometer for Stars Characterizations and Observations" (DISCO). The idea
is to take profit of wavelength multiplexing when using a spectrograph in order
to pack as much as possible the available information, yet providing a
potential boost of 1.5 magnitude if used in existing prototypes. We detail in
this paper the potential of such a concept. | astro-ph_IM |
Machine learning applications in astrophysics: Photometric redshift
estimation: Machine learning has rose to become an important research tool in the past
decade, its application has been expanded to almost if not all disciplines
known to mankind. Particularly, the use of machine learning in astrophysics
research had a humble beginning in the early 1980s, it has rose and become
widely used in many sub-fields today, driven by the vast availability of free
astronomical data online. In this short review, we narrow our discussion to a
single topic in astrophysics - the estimation of photometric redshifts of
galaxies and quasars, where we discuss its background, significance, and how
machine learning has been used to improve its estimation methods in the past 20
years. We also show examples of some recent machine learning photometric
redshift work done in Malaysia, affirming that machine learning is a viable and
easy way a developing nation can contribute towards general research in
astronomy and astrophysics. | astro-ph_IM |
Systematic biases in low frequency radio interferometric data due to
calibration: the LOFAR EoR case: The redshifted 21 cm line of neutral hydrogen is a promising probe of the
Epoch of Reionization (EoR). However, its detection requires a thorough
understanding and control of the systematic errors. We study two systematic
biases observed in the LOFAR EoR residual data after calibration and
subtraction of bright discrete foreground sources. The first effect is a
suppression in the diffuse foregrounds, which could potentially mean a
suppression of the 21 cm signal. The second effect is an excess of noise beyond
the thermal noise. The excess noise shows fluctuations on small frequency
scales, and hence it can not be easily removed by foreground removal or
avoidance methods. Our analysis suggests that sidelobes of residual sources due
to the chromatic point spread function and ionospheric scintillation can not be
the dominant causes of the excess noise. Rather, both the suppression of
diffuse foregrounds and the excess noise can occur due to calibration with an
incomplete sky model containing predominantly bright discrete sources. We show
that calibrating only on bright sources can cause suppression of other signals
and introduce an excess noise in the data. The levels of the suppression and
excess noise depend on the relative flux of sources which are not included in
the model with respect to the flux of modeled sources. We discuss possible
solutions such as using only long baselines to calibrate the interferometric
gain solutions as well as simultaneous multi-frequency calibration along with
their benefits and shortcomings. | astro-ph_IM |
Performance analysis of the Least-Squares estimator in Astrometry: We characterize the performance of the widely-used least-squares estimator in
astrometry in terms of a comparison with the Cramer-Rao lower variance bound.
In this inference context the performance of the least-squares estimator does
not offer a closed-form expression, but a new result is presented (Theorem 1)
where both the bias and the mean-square-error of the least-squares estimator
are bounded and approximated analytically, in the latter case in terms of a
nominal value and an interval around it. From the predicted nominal value we
analyze how efficient is the least-squares estimator in comparison with the
minimum variance Cramer-Rao bound. Based on our results, we show that, for the
high signal-to-noise ratio regime, the performance of the least-squares
estimator is significantly poorer than the Cramer-Rao bound, and we
characterize this gap analytically. On the positive side, we show that for the
challenging low signal-to-noise regime (attributed to either a weak
astronomical signal or a noise-dominated condition) the least-squares estimator
is near optimal, as its performance asymptotically approaches the Cramer-Rao
bound. However, we also demonstrate that, in general, there is no unbiased
estimator for the astrometric position that can precisely reach the Cramer-Rao
bound. We validate our theoretical analysis through simulated digital-detector
observations under typical observing conditions. We show that the nominal value
for the mean-square-error of the least-squares estimator (obtained from our
theorem) can be used as a benchmark indicator of the expected statistical
performance of the least-squares method under a wide range of conditions. Our
results are valid for an idealized linear (one-dimensional) array detector
where intra-pixel response changes are neglected, and where flat-fielding is
achieved with very high accuracy. | astro-ph_IM |
Science with the Murchison Widefield Array: Significant new opportunities for astrophysics and cosmology have been
identified at low radio frequencies. The Murchison Widefield Array is the first
telescope in the Southern Hemisphere designed specifically to explore the
low-frequency astronomical sky between 80 and 300 MHz with arcminute angular
resolution and high survey efficiency. The telescope will enable new advances
along four key science themes, including searching for redshifted 21 cm
emission from the epoch of reionisation in the early Universe; Galactic and
extragalactic all-sky southern hemisphere surveys; time-domain astrophysics;
and solar, heliospheric, and ionospheric science and space weather. The
Murchison Widefield Array is located in Western Australia at the site of the
planned Square Kilometre Array (SKA) low-band telescope and is the only
low-frequency SKA precursor facility. In this paper, we review the performance
properties of the Murchison Widefield Array and describe its primary scientific
objectives. | astro-ph_IM |
GRID: a Student Project to Monitor the Transient Gamma-Ray Sky in the
Multi-Messenger Astronomy Era: The Gamma-Ray Integrated Detectors (GRID) is a space mission concept
dedicated to monitoring the transient gamma-ray sky in the energy range from 10
keV to 2 MeV using scintillation detectors onboard CubeSats in low Earth
orbits. The primary targets of GRID are the gamma-ray bursts (GRBs) in the
local universe. The scientific goal of GRID is, in synergy with ground-based
gravitational wave (GW) detectors such as LIGO and VIRGO, to accumulate a
sample of GRBs associated with the merger of two compact stars and study jets
and related physics of those objects. It also involves observing and studying
other gamma-ray transients such as long GRBs, soft gamma-ray repeaters,
terrestrial gamma-ray flashes, and solar flares. With multiple CubeSats in
various orbits, GRID is unaffected by the Earth occultation and serves as a
full-time and all-sky monitor. Assuming a horizon of 200 Mpc for ground-based
GW detectors, we expect to see a few associated GW-GRB events per year. With
about 10 CubeSats in operation, GRID is capable of localizing a faint GRB like
170817A with a 90% error radius of about 10 degrees, through triangulation and
flux modulation. GRID is proposed and developed by students, with considerable
contribution from undergraduate students, and will remain operated as a student
project in the future. The current GRID collaboration involves more than 20
institutes and keeps growing. On August 29th, the first GRID detector onboard a
CubeSat was launched into a Sun-synchronous orbit and is currently under test. | astro-ph_IM |
LSST is Not "Big Data": LSST promises to be the largest optical imaging survey of the sky. If we were
fortunate enough to have the equivalent of LSST today, it would represent a
"fire hose" of data that would be difficult to store, transfer, and analyze
with available compute resources.
LSST parallels the SDSS compute task which was ambitious yet tractable. By
almost any measure relative to computers that will be available (thanks to the
steady progression of Moore's Law), LSST will be a small data set. LSST will
never fill more than 22 hard drives. Individual investigators will be able to
maintain their own data copies to analyze as they choose. | astro-ph_IM |
galmask: A Python package for unsupervised galaxy masking: Galaxy morphological classification is a fundamental aspect of galaxy
formation and evolution studies. Various machine learning tools have been
developed for automated pipeline analysis of large-scale surveys, enabling a
fast search for objects of interest. However, crowded regions in the image may
pose a challenge as they can lead to bias in the learning algorithm. In this
Research Note, we present galmask, an open-source package for unsupervised
galaxy masking to isolate the central object of interest in the image. galmask
is written in Python and can be installed from PyPI via the pip command. | astro-ph_IM |
The next generation Cherenkov Telescope Array observatory: CTA: The Cherenkov Telescope Array (CTA) is a large collaborative effort aimed at
the design and operation of an observatory dedicated to the VHE gamma-ray
astrophysics in the energy range 30 GeV-100 TeV, which will improve by about
one order of magnitude the sensitivity with respect to the current major arrays
(H.E.S.S., MAGIC, and VERITAS). In order to achieve such improved performance,
for both the northern and southern CTA sites, four units of 23m diameter Large
Size Telescopes (LSTs) will be deployed close to the centre of the array with
telescopes separated by about 100m. A larger number (about 25 units) of 12m
Medium Size Telescopes (MSTs, separated by about 150m), will cover a larger
area. The southern site will also include up to 24 Schwarzschild-Couder
dual-mirror medium-size Telescopes (SCTs) with the primary mirror diameter of
9.5m. Above a few TeV, the Cherenkov light intensity is such that showers can
be detected even well outside the light pool by telescopes significantly
smaller than the MSTs. To achieve the required sensitivity at high energies, a
huge area on the ground needs to be covered by Small Size Telescopes (SSTs)
with a FOV of about 10 deg and an angular resolution of about 0.2 deg, making
the dual-mirror configuration very effective. The SST sub-array will be
composed of 50-70 telescopes with a mirror area of about 5-10 square meters and
about 300m spacing, distributed across an area of about 10 square kilometers.
We will focus on the innovative solution for the optical design of the medium
and small size telescopes based on a dual-mirror configuration. This layout
will allow us to reduce the dimension and the weight of the camera at the focal
plane of the telescope, to adopt SiPMs as light detectors thanks to the reduced
plate-scale, and to have an optimal imaging resolution on a wide FOV. | astro-ph_IM |
The CALSPEC Stars P177D and P330E: Multicolor photometric data are presented for the CALSPEC stars P177D and
P330E. Together with previously published photometry for nine other CALSPEC
standards, the photometric observations and synthetic photometry from HST/STIS
spectrophotometry agree in the B, V, R, and I bands to better than $\sim$1\%
(10 mmag). | astro-ph_IM |
Subsystem Development for the All-Sky Medium Energy Gamma-ray
Observatory (AMEGO) prototype: The gamma-ray sky from several hundred keV to $\sim$ a hundred MeV has
remained largely unexplored due to the challenging nature of detecting gamma
rays in this regime. At lower energies, Compton scattering is the dominant
interaction process whereas at higher energies pair production dominates, with
a crossover at about 10 MeV depending on the material. Thus, an instrument
designed to work in this energy range must be optimized for both Compton and
pair-production events. The All-sky Medium Energy Gamma-ray Observatory (AMEGO)
is a NASA Probe-class mission concept being submitted to the Astro2020 review.
The instrument is designed to operate from 200 keV to $>$10 GeV and is made of
four major subsystems: a plastic anti-coincidence detector for rejecting
cosmic-ray events, a silicon tracker for tracking pair-production products and
tracking and measuring the energies of Compton-scattered electrons, a CZT
calorimeter for measuring the energy and location of Compton scattered photons,
and a CsI calorimeter for measuring the energy of the pair-production products
at high energies. A prototype instrument comprising each subsystem is currently
being developed in preparation for a beam test and a balloon flight. In this
contribution we discuss the current status of the prototype subsystems. | astro-ph_IM |
Software solutions for numerical modeling of wide-field telescopes: This paper presents an integrated modeling software to analyze the PSF of
wide-field telescopes affected by misalignments. Even relatively small
misalignments in the optical system of a telescope can significantly
deteriorate the image quality by introducing large aberrations. In particular,
wide-field telescopes are critically affected by these errors, insomuch that
usually a closed-loop active optics system is adopted for a continuous
correction, rather than for sporadic alignment procedures. Typically, a
ray-tracing software such as Zemax OpticStudio is employed to accurately
analyze the system during the optical design. However, an analytical model of
the optical system is preferable when the PSF of the telescope must be
reconstructed quickly for algorithmic purposes. Here the analytical model is
derived through a hybrid approach and developed in a custom software package,
designed to be general and flexible in order to be tailored to different
optical configurations. First, leveraging on the Zemax OpticStudio API, the
ray-tracing software is integrated into a Matlab pipeline. This allows to
perform a statistical analysis by automatically simulating the system response
in a variety of misaligned working conditions. Then, the resulting dataset is
employed to populate a database of parameters describing the model. | astro-ph_IM |
Improved Acceleration of the GPU Fourier Domain Acceleration Search
Algorithm: We present an improvement of our implementation of the Correlation Technique
for the Fourier Domain Acceleration Search (FDAS) algorithm on Graphics
Processor Units (GPUs) (Dimoudi & Armour 2015; Dimoudi et al. 2017). Our new
improved convolution code which uses our custom GPU FFT code is between 2.5 and
3.9 times faster the than our cuFFT-based implementation (on an NVIDIA P100)
and allows for a wider range of filter sizes then our previous version. By
using this new version of our convolution code in FDAS we have achieved 44%
performance increase over our previous best implementation. It is also
approximately 8 times faster than the existing PRESTO GPU implementation of
FDAS (Luo 2013). This work is part of the AstroAccelerate project (Armour et
al. 2002), a many-core accelerated time-domain signal processing library for
radio astronomy. | astro-ph_IM |
Visualising three-dimensional volumetric data with an arbitrary
coordinate system: Astronomical data does not always use Cartesian coordinates. Both all-sky
observational data and simulations of rotationally symmetric systems, such as
accretion and protoplanetary discs, may use spherical polar or other coordinate
systems. Standard displays rely on Cartesian coordinates, but converting
non-Cartesian data into Cartesian format causes distortion of the data and loss
of detail. I here demonstrate a method using standard techniques from computer
graphics that avoids these problems with 3D data in arbitrary coordinate
systems. The method adds minimum computational cost to the display process and
is suitable for both realtime, interactive content and producing fixed rendered
images and videos. Proof-of-concept code is provided which works for data in
spherical polar coordinates. | astro-ph_IM |
Concept validation of a high dynamic range point-diffraction
interferometer for wavefront sensing in adaptive optics: The direct detection and imaging of exoplanets requires the use of
high-contrast adaptive optics (AO). In these systems quasi-static aberrations
need to be highly corrected and calibrated. In order to achieve this, the
pupil-modulated point-diffraction interferometer (m-PDI), was presented in an
earlier paper. This present paper focuses on m-PDI concept validation through
three experiments. First, the instrument's accuracy and dynamic range are
characterised by measuring the spatial transfer function at all spatial
frequencies and at different amplitudes. Then, using visible monochromatic
light, an adaptive optics control loop is closed on the system's systematic
bias to test for precision and completeness. In a central section of the pupil
with 72% of the total radius the residual error is 7.7nm-rms. Finally, the
control loop is run using polychromatic light with a spectral FWHM of 77nm
around the R-band. The control loop shows no drop in performance with respect
to the monochromatic case, reaching a final Strehl ratio larger than 0.7. | astro-ph_IM |
A Small Satellite Version of a Broad-band Soft X-ray Polarimeter: We describe a new implementation of a broad-band soft X-ray polarimeter,
substantially based on a previous design. This implementation, the Pioneer Soft
X-ray Polarimeter (PiSoX) is a SmallSat, designed for NASA's call for
Astrophysics Pioneers, small missions that could be CubeSats, balloon
experiments, or SmallSats. As in the REDSoX Polarimeter, the grating
arrangement is designed optimally for the purpose of polarimetry with
broad-band focussing optics by matching the dispersion of the spectrometer
channels to laterally graded multilayers (LGMLs). The system can achieve
polarization modulation factors over 90%. For PiSoX, the optics are lightweight
Si mirrors in a one-bounce parabolic configuration. High efficiency, blazed
gratings from opposite sectors are oriented to disperse to a LGML forming a
channel covering the wavelength range from 35 to 75 Angstroms (165 - 350 eV).
Upon satellite rotation, the intensities of the dispersed spectra, after
reflection and polarizing by the LGMLs, give the three Stokes parameters needed
to determine a source's linear polarization fraction and orientation. The
design can be extended to higher energies as LGMLs are developed further. We
describe examples of the potential scientific return from instruments based on
this design. | astro-ph_IM |
Variable Star Classification Using Multi-View Metric Learning: Our multi-view metric learning framework enables robust characterization of
star categories by directly learning to discriminate in a multi-faceted feature
space, thus, eliminating the need to combine feature representations prior to
fitting the machine learning model. We also demonstrate how to extend standard
multi-view learning, which employs multiple vectorized views, to the
matrix-variate case which allows very novel variable star signature
representations. The performance of our proposed methods is evaluated on the
UCR Starlight and LINEAR datasets. Both the vector and matrix-variate versions
of our multi-view learning framework perform favorably --- demonstrating the
ability to discriminate variable star categories. | astro-ph_IM |
Combined Opto-Acoustical Sensor Modules for KM3NeT: KM3NeT is a future multi-cubic-kilometre water Cherenkov neutrino telescope
currently entering a first construction phase. It will be located in the
Mediterranean Sea and comprise about 600 vertical structures called detection
units. Each of these detection units has a length of several hundred metres and
is anchored to the sea bed on one side and held taut by a buoy on the other
side. The detection units are thus subject to permanent movement due to sea
currents. Modules holding photosensors and additional equipment are equally
distributed along the detection units. The relative positions of the
photosensors has to be known with an uncertainty below $20\,$cm in order to
achieve the necessary precision for neutrino astronomy. These positions can be
determined with an acoustic positioning system: dedicated acoustic emitters
located at known positions and acoustic receivers along each detection unit.
This article describes the approach to combine an acoustic receiver with the
photosensors inside one detection module using a common power supply and data
readout. The advantage of this approach lies in a reduction of underwater
connectors and module configurations as well as in the compactification of the
detection units integrating the auxiliary devices necessary for their
successful operation. | astro-ph_IM |
Accelerating Multiframe Blind Deconvolution via Deep Learning: Ground-based solar image restoration is a computationally expensive procedure
that involves nonlinear optimization techniques. The presence of atmospheric
turbulence produces perturbations in individual images that make it necessary
to apply blind deconvolution techniques. These techniques rely on the
observation of many short exposure frames that are used to simultaneously infer
the instantaneous state of the atmosphere and the unperturbed object. We have
recently explored the use of machine learning to accelerate this process, with
promising results. We build upon this previous work to propose several
interesting improvements that lead to better models. As well, we propose a new
method to accelerate the restoration based on algorithm unrolling. In this
method, the image restoration problem is solved with a gradient descent method
that is unrolled and accelerated aided by a few small neural networks. The role
of the neural networks is to correct the estimation of the solution at each
iterative step. The model is trained to perform the optimization in a small
fixed number of steps with a curated dataset. Our findings demonstrate that
both methods significantly reduce the restoration time compared to the standard
optimization procedure. Furthermore, we showcase that these models can be
trained in an unsupervised manner using observed images from three different
instruments. Remarkably, they also exhibit robust generalization capabilities
when applied to new datasets. To foster further research and collaboration, we
openly provide the trained models, along with the corresponding training and
evaluation code, as well as the training dataset, to the scientific community. | astro-ph_IM |
AI and extreme scale computing to learn and infer the physics of higher
order gravitational wave modes of quasi-circular, spinning, non-precessing
binary black hole mergers: We use artificial intelligence (AI) to learn and infer the physics of higher
order gravitational wave modes of quasi-circular, spinning, non precessing
binary black hole mergers. We trained AI models using 14 million waveforms,
produced with the surrogate model NRHybSur3dq8, that include modes up to $\ell
\leq 4$ and $(5,5)$, except for $(4,0)$ and $(4,1)$, that describe binaries
with mass-ratios $q\leq8$, individual spins $s^z_{\{1,2\}}\in[-0.8, 0.8]$, and
inclination angle $\theta\in[0,\pi]$.Our probabilistic AI surrogates can
accurately constrain the mass-ratio, individual spins, effective spin, and
inclination angle of numerical relativity waveforms that describe such signal
manifold. We compared the predictions of our AI models with Gaussian process
regression, random forest, k-nearest neighbors, and linear regression, and with
traditional Bayesian inference methods through the PyCBC Inference toolkit,
finding that AI outperforms all these approaches in terms of accuracy, and are
between three to four orders of magnitude faster than traditional Bayesian
inference methods. Our AI surrogates were trained within 3.4 hours using
distributed training on 1,536 NVIDIA V100 GPUs in the Summit supercomputer. | astro-ph_IM |
Stellar populations in the ELT perspective: We discuss the impact that the next generation of Extremely Large Telescopes
will have on the open astrophysical problems of resolved stellar populations.
In particular, we address the interplay between multiband photometry and
spectroscopy. | astro-ph_IM |
Detectability of Galactic Faraday Rotation in Multi-wavelength CMB
Observations: A Cross-Correlation Analysis of CMB and Radio Maps: We introduce a new cross-correlation method to detect and verify the
astrophysical origin of Faraday Rotation (FR) in multiwavelength surveys. FR is
well studied in radio astronomy from radio point sources but the $\lambda^{2}$
suppression of FR makes detecting and accounting for this effect difficult at
millimeter and sub-millimeter wavelengths. Therefore statistical methods are
used to attempt to detect FR in the cosmic microwave background (CMB). Most
estimators of the FR power spectrum rely on single frequency data. In contrast,
we investigate the correlation of polarized CMB maps with FR measure maps from
radio point sources. We show a factor of $\sim30$ increase in sensitivity over
single frequency estimators and predict detections exceeding $10\sigma$
significance for a CMB-S4 like experiment. Improvements in observations of FR
from current and future radio polarization surveys will greatly increase the
usefulness of this method. | astro-ph_IM |
Using weighting algorithms to refine source direction determinations in
all-sky gravitational wave burst searches with two-detector networks: I explore the possibility of resurrecting an old, non-Bayesian computational
approach for inferring the source direction of a gravitational wave from the
output of a two-detector network. The method gives the beam pattern response
functions and time delay, and performs well even in the presence of noise and
unexpected signal forms. I further suggest an improvement to this method in the
form of a weighting algorithm that usefully improves its accuracy beyond what
can be achieved with simple best-fit methods, validating the new procedure with
several small-scale simulations. The approach is identified as complimentary to
-- rather than in competition with -- the now-standard Bayesian approach
typically used by the LIGO network in parameter determination. Finally, I
briefly discuss the possible applications of this method in the world of
three-or-more detector networks and some directions for future work. | astro-ph_IM |
Correcting for the ionosphere in the uv-plane: In radio astronomy, the correlator measures intensity in visibility space. In
addition, the EoR power spectrum measured by an experiment such as the MWA is
constructed in visibility space. Thus, correcting for the ionosphere in the
uv-plane instead of real space could potentially save computation. In this
paper, we study this technique. The mathematical formula for obtaining the
unperturbed data from the ionospherically reflected data is non-local in the
uv-plane. Moreover, an analytic solution for the unperturbed intensity may only
be obtained for a limited number of expansions of the ionospheric
perturbations. We numerically study one of these expansions (with perturbations
as sinusoidal modes). Obtaining an analytic solution for this expansion
required a Taylor expansion, and we investigate the optimal order of this
expansion. We also propose a number of potential computation saving techniques,
and evaluate their pros and cons. | astro-ph_IM |
Sparse aperture masking at the VLT I. Faint companion detection limits
for the two debris disk stars HD 92945 and HD 141569: Observational data on companion statistics around young stellar systems is
needed to flesh out the formation pathways for extrasolar planets and brown
dwarfs. Aperture masking is a new technique that is able to address an
important part of this discovery space. We observed the two debris disk systems
HD 92945 and HD 141569 with sparse aperture masking (SAM), a new mode offered
on the NaCo instrument at the VLT. A search for faint companions was performed
using a detection strategy based on the analysis of closure phases recovered
from interferograms recorded on the Conica camera. Our results demonstrate that
SAM is a very competitive mode in the field of companion detection. We obtained
5 sigma high-contrast detection limits at lambda/D of 2.5x10^{-3} (\Delta L' =
6.5) for HD 92945 and 4.6x10^{-3} (\Delta L' = 5.8) for HD 141569. According to
brown dwarf evolutionary models, our data impose an upper mass boundary for any
companion for the two stars to, respectively, 18 and 22 Jupiter masses at
minimum separations of 1.5 and 7 AU. The detection limits is mostly independent
of angular separation, until reaching the diffraction limit of the telescope.
We have placed upper limits on the existence of companions to our target
systems that fall close to the planetary mass regime. This demonstrates the
potential for SAM mode to contribute to studies of faint companions. We
furthermore show that the final dynamic range obtained is directly proportional
to the error on the closure phase measurement. At the present performance
levels of 0.28 degree closure phase error, SAM is among the most competitive
techniques for recovering companions at scales of one to several times the
diffraction limit of the telescope. Further improvements to the detection
threshold can be expected with more accurate phase calibration. | astro-ph_IM |
Millimeter/submillimeter VLBI with a Next Generation Large Radio
Telescope in the Atacama Desert: The proposed next generation Event Horizon Telescope (ngEHT) concept
envisions the imaging of various astronomical sources on scales of
microarcseconds in unprecedented detail with at least two orders of magnitude
improvement in the image dynamic ranges by extending the Event Horizon
Telescope (EHT). A key technical component of ngEHT is the utilization of large
aperture telescopes to anchor the entire array, allowing the connection of less
sensitive stations through highly sensitive fringe detections to form a dense
network across the planet. Here, we introduce two projects for planned next
generation large radio telescopes in the 2030s on the Chajnantor Plateau in the
Atacama desert in northern Chile, the Large Submillimeter Telescope (LST) and
the Atacama Large Aperture Submillimeter Telescope (AtLAST). Both are designed
to have a 50-meter diameter and operate at the planned ngEHT frequency bands of
86, 230 and 345\,GHz. A large aperture of 50\,m that is co-located with two
existing EHT stations, the Atacama Large Millimeter/Submillimeter Array (ALMA)
and the Atacama Pathfinder Experiment (APEX) Telescope in the excellent
observing site of the Chajnantor Plateau, will offer excellent capabilities for
highly sensitive, multi-frequency, and time-agile millimeter very long baseline
interferometry (VLBI) observations with accurate data calibration relevant to
key science cases of ngEHT. In addition to ngEHT, its unique location in Chile
will substantially improve angular resolutions of the planned Next Generation
Very Large Array in North America or any future global millimeter VLBI arrays
if combined. LST and AtLAST will be a key element enabling transformative
science cases with next-generation millimeter/submillimeter VLBI arrays. | astro-ph_IM |
The KAGRA underground environment and lessons for the Einstein Telescope: The KAGRA gravitational-wave detector in Japan is the only operating detector
hosted in an underground infrastructure. Underground sites promise a greatly
reduced contribution of the environment to detector noise thereby opening the
possibility to extend the observation band to frequencies well below 10 Hz. For
this reason, the proposed next-generation infrastructure Einstein Telescope in
Europe would be realized underground aiming for an observation band that
extends from 3 Hz to several kHz. However, it is known that ambient noise in
the low-frequency band 10 Hz - 20 Hz at current surface sites of the Virgo and
LIGO detectors is predominantly produced by the detector infrastructure. It is
of utmost importance to avoid spoiling the quality of an underground site with
noisy infrastructure, at least at frequencies where this noise can turn into a
detector-sensitivity limitation. In this paper, we characterize the KAGRA
underground site to determine the impact of its infrastructure on environmental
fields. We find that while excess seismic noise is observed, its contribution
in the important band below 20 Hz is minor preserving the full potential of
this site to realize a low-frequency gravitational-wave detector. Moreover, we
estimate the Newtonian-noise spectra of surface and underground seismic waves
and of the acoustic field inside the caverns. We find that these will likely
remain a minor contribution to KAGRA's instrument noise in the foreseeable
future. | astro-ph_IM |
Real-time Data Ingestion at the Keck Observatory Archive (KOA): Since February of this year, KOA began to prepare, transfer, and ingest data
as they were acquired in near-real time; in most cases data are available to
observers through KOA within one minute of acquisition. Real-time ingestion
will be complete for all active instruments by the end of Summer 2022. The
observatory is supporting the development of modern Python data reduction
pipelines, which when delivered, will automatically create science-ready data
sets at the end of each night for ingestion into the archive. This presentation
will describe the infrastructure developed to support real-time data ingestion,
itself part of a larger initiative at the Observatory to modernize end-to-end
operations.
During telescope operations, the software at WMKO is executed automatically
when a newly acquired file is recognized through monitoring a keyword-based
observatory control system; this system is used at Keck to execute virtually
all observatory functions. The monitor uses callbacks built into the control
system to begin data preparation of files for transmission to the archive on an
individual basis: scheduling scripts or file system related triggers are
unnecessary. An HTTP-based system called from the Flask micro-framework enables
file transfers between WMKO and NExScI and triggers data ingestion at NExScI.
The ingestion system at NEXScI is a compact (4 KLOC), highly fault-tolerant,
Python-based system. It uses a shared file system to transfer data from WMKO to
NExScI. The ingestion code is instrument agnostic, with instrument parameters
read from configuration files. It replaces an unwieldy (50 KLOC) C-based system
that had been in use since 2004. | astro-ph_IM |
Inferring the properties of a population of compact binaries in presence
of selection effects: Shortly after a new class of objects is discovered, the attention shifts from
the properties of the individual sources to the question of their origin: do
all sources come from the same underlying population, or several populations
are required? What are the properties of these populations? As the detection of
gravitational waves is becoming routine and the size of the event catalog
increases, finer and finer details of the astrophysical distribution of compact
binaries are now within our grasp. This Chapter presents a pedagogical
introduction to the main statistical tool required for these analyses:
hierarchical Bayesian inference in the presence of selection effects. All key
equations are obtained from first principles, followed by two examples of
increasing complexity. Although many remarks made in this Chapter refer to
gravitational-wave astronomy, the write-up is generic enough to be useful to
researchers and graduate students from other fields. | astro-ph_IM |
Design of the KOSMOS oil-coupled spectrograph camera lenses: We present the design details of oil-coupled lens groups used in the KOSMOS
spectrograph camera. The oil-coupled groups use silicone rubber O-rings in a
unique way to accurately center lens elements with high radial and axial
stiffness while also allowing easy assembly. The O-rings robustly seal the oil
within the lens gaps to prevent oil migration. The design of an expansion
diaphragm to compensate for differential expansion due to temperature changes
is described. The issues of lens assembly, lens gap shimming, oil filling and
draining, bubble mitigation, material compatibility, mechanical inspection, and
optical testing are discussed. | astro-ph_IM |
Introducing Astronomy into Mozambican Society: Mozambique has been proposed as a host for one of the future Square Kilometre
Array stations in Southern Africa. However, Mozambique does not possess a
university astronomy department and only recently has there been interest in
developing one. South Africa has been funding students at the MSc and PhD
level, as well as researchers. Additionally, Mozambicans with Physics degrees
have been funded at the MSc level. With the advent of the International Year of
Astronomy, there has been a very strong drive, from these students, to
establish a successful astronomy department in Mozambique. The launch of the
commemorations during the 2008 World Space Week was very successful and
Mozambique is to be used to motivate similar African countries who lack funds
but are still trying to take part in the International Year of Astronomy. There
hare been limited resources and funding, however there is a strong will to
carry this momentum into 2009 and, with this, influence the Government to
introduce Astronomy into its national curriculum and at University level.
Mozambique's motto for the International Year of Astronomy is "Descobre o teu
Universo". | astro-ph_IM |
Electrode level Monte Carlo model of radiation damage effects on
astronomical CCDs: Current optical space telescopes rely upon silicon Charge Coupled Devices
(CCDs) to detect and image the incoming photons. The performance of a CCD
detector depends on its ability to transfer electrons through the silicon
efficiently, so that the signal from every pixel may be read out through a
single amplifier. This process of electron transfer is highly susceptible to
the effects of solar proton damage (or non-ionizing radiation damage). This is
because charged particles passing through the CCD displace silicon atoms,
introducing energy levels into the semi-conductor bandgap which act as
localized electron traps. The reduction in Charge Transfer Efficiency (CTE)
leads to signal loss and image smearing. The European Space Agency's
astrometric Gaia mission will make extensive use of CCDs to create the most
complete and accurate stereoscopic map to date of the Milky Way. In the context
of the Gaia mission CTE is referred to with the complementary quantity Charge
Transfer Inefficiency (CTI = 1-CTE). CTI is an extremely important issue that
threatens Gaia's performances. We present here a detailed Monte Carlo model
which has been developed to simulate the operation of a damaged CCD at the
pixel electrode level. This model implements a new approach to both the charge
density distribution within a pixel and the charge capture and release
probabilities, which allows the reproduction of CTI effects on a variety of
measurements for a large signal level range in particular for signals of the
order of a few electrons. A running version of the model as well as a brief
documentation and a few examples are readily available at
http://www.strw.leidenuniv.nl/~prodhomme/cemga.php as part of the CEMGA java
package (CTI Effects Models for Gaia). | astro-ph_IM |
Giant Radio Array for Neutrino Detection (GRAND): GRAND is a newly proposed series of radio arrays with a combined area of
200,000 square km, to be deployed in mountainous areas. Its primary goal is to
measure cosmic ultra-high-energy tau-neutrinos (E>1 EeV), through the
interaction of these neutrinos in rock and the decay of the tau-lepton in the
atmosphere. This decay creates an air shower, whose properties can be inferred
from the radio signal it creates. The huge area of GRAND makes it the most
sensitive instrument proposed to date, ensured to measure neutrinos in all
reasonable models of cosmic ray production and propagation. At the same time,
GRAND will be a very versatile observatory with enormous exposure to
ultra-high-energy cosmic rays and photons. This talk covers the scientific
motivation, as well as the staged approach required in the R\&D stages to get
to a final design that will make the construction, deployment and operation of
this vast detector affordable. | astro-ph_IM |
New SST Optical Sensor of Pampilhosa da Serra: studies on image
processing algorithms and multi-filter characterization of Space Debris: As part of the Portuguese Space Surveillance and Tracking (SST) System, two
new Wide Field of View (2.3deg x 2.3deg) small aperture (30cm) telescopes will
be deployed in 2021, at the Pampilhosa da Serra Space Observatory (PASO),
located in the center of the continental Portuguese territory, in the heart of
a certified Dark Sky area. These optical systems will provide added value
capabilities to the Portuguese SST network, complementing the optical
telescopes currently in commissioning in Madeira and Azores. These telescopes
are optimized for GEO and MEO survey operations and besides the required SST
operational capability, they will also provide an important development
component to the Portuguese SST network. The telescopes will be equipped with
filter wheels, being able to perform observations in several optical bands
including white light, BVRI bands and narrow band filters such as H(alpha) and
O[III] to study potential different objects' albedos. This configuration
enables us to conduct a study on space debris classification$/$characterization
using combinations of different colors aiming the production of improved color
index schemes to be incorporated in the automatic pipelines for classification
of space debris. This optical sensor will also be used to conduct studies on
image processing algorithms, including source extraction and classification
solutions through the application of machine learning techniques. Since SST
dedicated telescopes produce a large quantity of data per observation night,
fast, efficient and automatic image processing techniques are mandatory. A
platform like this one, dedicated to the development of Space Surveillance
studies, will add a critical capability to keep the Portuguese SST network
updated, and as a consequence it may provide useful developments to the
European SST network as well. | astro-ph_IM |
The Solar Probe ANalyzers -- Electrons on Parker Solar Probe: Electrostatic analyzers of different designs have been used since the
earliest days of the space age, beginning with the very earliest solar wind
measurements made by Mariner 2 en route to Venus in 1962. The Parker Solar
Probe (PSP) mission, NASA's first dedicated mission to study the innermost
reaches of the heliosphere, makes its thermal plasma measurements using a suite
of instruments called the Solar Wind Electrons, Alphas, and Protons (SWEAP)
investigation. SWEAP's electron Parker Solar Probe Analyzer (SPAN-E)
instruments are a pair of top-hat electrostatic analyzers on PSP that are
capable of measuring the electron distribution function in the solar wind from
2 eV to 30 keV. For the first time, in-situ measurements of thermal electrons
provided by SPAN-E will help reveal the heating and acceleration mechanisms
driving the evolution of the solar wind at the points of acceleration and
heating, closer than ever before to the Sun. This paper details the design of
the SPAN-E sensors and their operation, data formats, and measurement caveats
from Parker Solar Probe's first two close encounters with the Sun. | astro-ph_IM |
Optical Design and Characterization of 40-GHz Detector and Module for
the BICEP Array: Families of cosmic inflation models predict a primordial gravitational-wave
background that imprints B-mode polarization pattern in the Cosmic Microwave
Background (CMB). High sensitivity instruments with wide frequency coverage and
well-controlled systematic errors are needed to constrain the faint B-mode
amplitude. We have developed antenna-coupled Transition Edge Sensor (TES)
arrays for high-sensitivity polarized CMB observations over a wide range of
millimeter-wave bands. BICEP Array, the latest phase of the BICEP/Keck
experiment series, is a multi-receiver experiment designed to search for
inflationary B-mode polarization to a precision $\sigma$(r) between 0.002 and
0.004 after 3 full years of observations, depending on foreground complexity
and the degree of lensing removal. We describe the electromagnetic design and
measured performance of BICEP Array low-frequency 40-GHz detector, their
packaging in focal plane modules, and optical characterization including
efficiency and beam matching between polarization pairs. We summarize the
design and simulated optical performance, including an approach to improve the
optical efficiency due to mismatch losses. We report the measured beam maps for
a new broad-band corrugation design to minimize beam differential ellipticity
between polarization pairs caused by interactions with the module housing
frame, which helps minimize polarized beam mismatch that converts CMB
temperature to polarization ($T \rightarrow P$) anisotropy in CMB maps. | astro-ph_IM |
Physically constrained causal noise models for high-contrast imaging of
exoplanets: The detection of exoplanets in high-contrast imaging (HCI) data hinges on
post-processing methods to remove spurious light from the host star. So far,
existing methods for this task hardly utilize any of the available domain
knowledge about the problem explicitly. We propose a new approach to HCI
post-processing based on a modified half-sibling regression scheme, and show
how we use this framework to combine machine learning with existing scientific
domain knowledge. On three real data sets, we demonstrate that the resulting
system performs clearly better (both visually and in terms of the SNR) than one
of the currently leading algorithms. If further studies can confirm these
results, our method could have the potential to allow significant discoveries
of exoplanets both in new and archival data. | astro-ph_IM |
Automated Real-Time Classification and Decision Making in Massive Data
Streams from Synoptic Sky Surveys: The nature of scientific and technological data collection is evolving
rapidly: data volumes and rates grow exponentially, with increasing complexity
and information content, and there has been a transition from static data sets
to data streams that must be analyzed in real time. Interesting or anomalous
phenomena must be quickly characterized and followed up with additional
measurements via optimal deployment of limited assets. Modern astronomy
presents a variety of such phenomena in the form of transient events in digital
synoptic sky surveys, including cosmic explosions (supernovae, gamma ray
bursts), relativistic phenomena (black hole formation, jets), potentially
hazardous asteroids, etc. We have been developing a set of machine learning
tools to detect, classify and plan a response to transient events for astronomy
applications, using the Catalina Real-time Transient Survey (CRTS) as a
scientific and methodological testbed. The ability to respond rapidly to the
potentially most interesting events is a key bottleneck that limits the
scientific returns from the current and anticipated synoptic sky surveys.
Similar challenge arise in other contexts, from environmental monitoring using
sensor networks to autonomous spacecraft systems. Given the exponential growth
of data rates, and the time-critical response, we need a fully automated and
robust approach. We describe the results obtained to date, and the possible
future developments. | astro-ph_IM |
Long term measurements from the Mátra Gravitational and Geophysical
Laboratory: Summary of the long term data taking, related to one of the proposed next
generation ground-based gravitational detector's location is presented here.
Results of seismic and infrasound noise, electromagnetic attenuation and cosmic
muon radiation measurements are reported in the underground Matra Gravitational
and Geophysical Laboratory near Gy\"ongy\"osoroszi, Hungary. The collected
seismic data of more than two years is evaluated from the point of view of the
Einstein Telescope, a proposed third generation underground gravitational wave
observatory. Applying our results for the site selection will significantly
improve the signal to nose ratio of the multi-messenger astrophysics era,
especially at the low frequency regime. | astro-ph_IM |
Imfit: A Fast, Flexible New Program for Astronomical Image Fitting: I describe a new, open-source astronomical image-fitting program called
Imfit, specialized for galaxies but potentially useful for other sources, which
is fast, flexible, and highly extensible. A key characteristic of the program
is an object-oriented design which allows new types of image components (2D
surface-brightness functions) to be easily written and added to the program.
Image functions provided with Imfit include the usual suspects for galaxy
decompositions (Sersic, exponential, Gaussian), along with Core-Sersic and
broken-exponential profiles, elliptical rings, and three components which
perform line-of-sight integration through 3D luminosity-density models of disks
and rings seen at arbitrary inclinations.
Available minimization algorithms include Levenberg-Marquardt, Nelder-Mead
simplex, and Differential Evolution, allowing trade-offs between speed and
decreased sensitivity to local minima in the fit landscape. Minimization can be
done using the standard chi^2 statistic (using either data or model values to
estimate per-pixel Gaussian errors, or else user-supplied error images) or
Poisson-based maximum-likelihood statistics; the latter approach is
particularly appropriate for cases of Poisson data in the low-count regime. I
show that fitting low-S/N galaxy images using chi^2 minimization and
individual-pixel Gaussian uncertainties can lead to significant biases in
fitted parameter values, which are avoided if a Poisson-based statistic is
used; this is true even when Gaussian read noise is present. | astro-ph_IM |
Experimental study on Modified Linear Quadratic Gaussian Control for
Adaptive Optics: To achieve high resolution imaging the standard control algorithm used for
classical adaptive optics (AO) is the simple but efficient
proportional-integral (PI) controller. The goal is to minimize the root mean
square (RMS) error of the residual wave front. However, with the PI controller
one does not reach this minimum. A possibility to achieve is to use Linear
Quadratic Gaussian Control (LQG). In practice, however this control algorithm
still encounters one unexpected problem, leading to the divergence of control
in AO. In this paper we propose a Modified LQG (MLQG) to solve this issue. The
controller is analyzed explicitly. Test in the lab shows strong stability and
high precision compared to the classical control. | astro-ph_IM |
Simulations of astrometric planet detection in Alpha Centauri by
intensity interferometry: Recent dynamical studies indicate that the possibility of an Earth-like
planet around $\alpha\;$Cen A or B should be taken seriously. Such a planet, if
it exists, would perturb the orbital astrometry by $<10 \ {\mu}\rm as$, which
is $10^{-6}$ of the separation between the two stars. We assess the feasibility
of detecting such perturbations using ground-based intensity interferometry. We
simulate a dedicated setup consisting of four 40-cm telescopes equipped with
photon counters and correlators with time resolution $0.1\,\rm ns$, and a sort
of matched filter implemented through an aperture mask. The astrometric error
from one night of observing $\alpha\;$Cen AB is $\approx0.5\,\rm mas$. The
error decreases if longer observing times and multiple spectral channels are
used, as $(\hbox{channels}\times\hbox{nights})^{-1/2}$. | astro-ph_IM |
The Habitable Zone Planet Finder: A Proposed High Resolution NIR
Spectrograph for the Hobby Eberly Telescope to Discover Low Mass Exoplanets
around M Dwarfs: The Habitable Zone Planet Finder (HZPF) is a proposed instrument for the 10m
class Hobby Eberly telescope that will be capable of discovering low mass
planets around M dwarfs. HZPF will be fiber-fed, provide a spectral resolution
R~ 50,000 and cover the wavelength range 0.9-1.65{\mu}m, the Y, J and H NIR
bands where most of the flux is emitted by mid-late type M stars, and where
most of the radial velocity information is concentrated. Enclosed in a chilled
vacuum vessel with active temperature control, fiber scrambling and mechanical
agitation, HZPF is designed to achieve a radial velocity precision < 3m/s, with
a desire to obtain <1m/s for the brightest targets. This instrument will enable
a study of the properties of low mass planets around M dwarfs; discover planets
in the habitable zones around these stars, as well serve as an essential radial
velocity confirmation tool for astrometric and transit detections around late M
dwarfs. Radial velocity observation in the near-infrared (NIR) will also enable
a search for close in planets around young active stars, complementing the
search space enabled by upcoming high-contrast imaging instruments like GPI,
SPHERE and PALM3K. Tests with a prototype Pathfinder instrument have already
demonstrated the ability to recover radial velocities at 7-10 m/s precision
from integrated sunlight and ~15-20 m/s precision on stellar observations at
the HET. These tests have also demonstrated the ability to work in the NIR Y
and J bands with an un-cooled instrument. We will also discuss lessons learned
about calibration and performance from our tests and how they impact the
overall design of the HZPF. | astro-ph_IM |
High contrast imaging at the photon noise limit with self-calibrating
WFS/C systems: High contrast imaging (HCI) systems rely on active wavefront control (WFC) to
deliver deep raw contrast in the focal plane, and on calibration techniques to
further enhance contrast by identifying planet light within the residual
speckle halo. Both functions can be combined in an HCI system and we discuss a
path toward designing HCI systems capable of calibrating residual starlight at
the fundamental contrast limit imposed by photon noise. We highlight the value
of deploying multiple high-efficiency wavefront sensors (WFSs) covering a wide
spectral range and spanning multiple optical locations. We show how their
combined information can be leveraged to simultaneously improve WFS sensitivity
and residual starlight calibration, ideally making it impossible for an image
plane speckle to hide from WFS telemetry. We demonstrate residual starlight
calibration in the laboratory and on-sky, using both a coronagraphic setup, and
a nulling spectro-interferometer. In both case, we show that bright starlight
can calibrate residual starlight. | astro-ph_IM |
Interferometric Beam Combination with a Triangular Tricoupler Photonic
Chip: Beam combiners are important components of an optical/infrared astrophysical
interferometer, with many variants as to how to optimally combine two or more
beams of light to fringe-track and obtain the complex fringe visibility. One
such method is the use of an integrated optics chip that can instantaneously
provide the measurement of the visibility without temporal or spatial
modulation of the optical path. Current asymmetric planar designs are complex,
resulting in a throughput penalty, and so here we present developments into a
three dimensional triangular tricoupler that can provide the required
interferometric information with a simple design and only three outputs. Such a
beam combiner is planned to be integrated into the upcoming $\textit{Pyxis}$
interferometer, where it can serve as a high-throughput beam combiner with a
low size footprint. Results into the characterisation of such a coupler are
presented, highlighting a throughput of 85$\pm$7% and a flux splitting ratio
between 33:33:33 and 52:31:17 over a 20% bandpass. We also show the response of
the chip to changes in optical path, obtaining an instantaneous complex
visibility and group delay estimate at each input delay. | astro-ph_IM |
Archival Legacy Investigations of Circumstellar Environments (ALICE):
Statistical assessment of point source detections: The ALICE program, for Archival Legacy Investigation of Circumstellar
Environment, is currently conducting a virtual survey of about 400 stars, by
re-analyzing the HST-NICMOS coronagraphic archive with advanced post-processing
techniques. We present here the strategy that we adopted to identify detections
and potential candidates for follow-up observations, and we give a preliminary
overview of our detections. We present a statistical analysis conducted to
evaluate the confidence level on these detection and the completeness of our
candidate search. | astro-ph_IM |
A Condition Monitoring Concept Studied at the MST Prototype for the
Cherenkov Telescope Array: The Cherenkov Telescope Array (CTA) is a future ground-based gamma-ray
observatory that will provide unprecedented sensitivity and angular resolution
for the detection of gamma rays with energies above a few tens of GeV. In
comparison to existing instruments (like H.E.S.S., MAGIC, and VERITAS) the
sensitivity will be improved by installing two extended arrays of telescopes in
the northern and southern hemisphere, respectively. A large number of planned
telescopes (>100 in total) motivates the application of predictive maintenance
techniques to the individual telescopes. A constant and automatic condition
monitoring of the mechanical telescope structure and of the drive system
(motors, gears) is considered for this purpose. The condition monitoring system
aims at detecting degradations well before critical errors occur; it should
help to ensure long-term operation and to reduce the maintenance efforts of the
observatory. We present approaches for the condition monitoring of the
structure and the drive system of Medium-Sized Telescopes (MSTs), respectively.
The overall concept has been developed and tested at the MST prototype for CTA
in Berlin. The sensors used, the joint data acquisition system, possible
analysis methods (like Operational Modal Analysis, OMA, and Experimental Modal
Analysis, EMA) and first performance results are discussed. | astro-ph_IM |
KLLR: A scale-dependent, multivariate model class for regression
analysis: The underlying physics of astronomical systems governs the relation between
their measurable properties. Consequently, quantifying the statistical
relationships between system-level observable properties of a population offers
insights into the astrophysical drivers of that class of systems. While purely
linear models capture behavior over a limited range of system scale, the fact
that astrophysics is ultimately scale-dependent implies the need for a more
flexible approach to describing population statistics over a wide dynamic
range. For such applications, we introduce and implement a class of
Kernel-Localized Linear Regression (KLLR) models. KLLR is a natural extension
to the commonly-used linear models that allows the parameters of the linear
model -- normalization, slope, and covariance matrix -- to be scale-dependent.
KLLR performs inference in two steps: (1) it estimates the mean relation
between a set of independent variables and a dependent variable and; (2) it
estimates the conditional covariance of the dependent variables given a set of
independent variables. We demonstrate the model's performance in a simulated
setting and showcase an application of the proposed model in analyzing the
baryonic content of dark matter halos. As a part of this work, we publicly
release a Python implementation of the KLLR method. | astro-ph_IM |
The Venus ground-based image Active Archive: a database of amateur
observations of Venus in ultraviolet and infrared light: The Venus ground-based image Active Archive is an online database designed to
collect ground-based images of Venus in such a way that they are optimally
useful for science. The Archive was built to support ESA's Venus Amateur
Observing Project, which utilises the capabilities of advanced amateur
astronomers to collect filtered images of Venus in ultraviolet, visible and
near-infrared light. These images complement the observations of the Venus
Express spacecraft, which cannot continuously monitor the northern hemisphere
of the planet due to its elliptical orbit with apocentre above the south pole.
We present the first set of observations available in the Archive and assess
the usability of the dataset for scientific purposes. | astro-ph_IM |
Optimal Dithering Configuration Mitigating
Rayleigh-Backscattering-Induced Distortion in Radioastronomic Optical Fiber
Systems: In the context of Radioastronomic applications where the Analog
Radio-over-Fiber technology is used for the antenna downlink, detrimental
nonlinearity effects arise because of the interference between the forward
signal generated by the laser and the Rayleigh backscattered one which is
re-forwarded by the laser itself toward the photodetector.
The adoption of the so called dithering technique, which involves the direct
modulation of the laser with a sinusoidal tone and takes advantage of the laser
chirping phenomenon, has been proved to reduce such Rayleigh Back Scattering -
induced nonlinearities. The frequency and the amplitude of the dithering tone
should both be as low as possible, in order to avoid undesired collateral
effects on the received spectrum as well as keep at low levels the global
energy consumption.
Through a comprehensive analysis of dithered Radio over Fiber systems, it is
demonstrated that a progressive reduction of the dithering tone frequency
affects in a peculiar fashion both the chirping characteristics of the field
emitted by the laser and the spectrum pattern of the received signal at the
fiber end.
Accounting for the concurrent effects caused by such phenomena, optimal
operating conditions are identified for the implementation of the dithering
tone technique in radioastronomic systems. | astro-ph_IM |
H2 distribution during 2-phase Molecular Cloud Formation: We performed high-resolution, 3D MHD simulations and we compared to
observations of translucent molecular clouds. We show that the observed
populations of rotational levels of H2 can arise as a consequence of the
multi-phase structure of the ISM. | astro-ph_IM |
Characterization and Physical Explanation of Energetic Particles on
Planck HFI Instrument: The Planck High Frequency Instrument (HFI) has been surveying the sky
continuously from the second Lagrangian point (L2) between August 2009 and
January 2012. It operates with 52 high impedance bolometers cooled at 100mK in
a range of frequency between 100 GHz and 1THz with unprecedented sensivity, but
strong coupling with cosmic radiation. At L2, the particle flux is about 5
$cm^{-2} s^{-1}$ and is dominated by protons incident on the spacecraft.
Protons with an energy above 40MeV can penetrate the focal plane unit box
causing two different effects: glitches in the raw data from direct interaction
of cosmic rays with detectors (producing a data loss of about 15% at the end of
the mission) and thermal drifts in the bolometer plate at 100mK adding
non-gaussian noise at frequencies below 0.1Hz. The HFI consortium has made
strong efforts in order to correct for this effect on the time ordered data and
final Planck maps. This work intends to give a view of the physical explanation
of the glitches observed in the HFI instrument in-flight. To reach this goal,
we performed several ground-based experiments using protons and $\alpha$
particles to test the impact of particles on the HFI spare bolometers with a
better control of the environmental conditions with respect to the in-flight
data. We have shown that the dominant part of glitches observed in the data
comes from the impact of cosmic rays in the silicon die frame supporting the
micro-machinced bolometric detectors propagating energy mainly by ballistic
phonons and by thermal diffusion. The implications of these results for future
satellite missions will be discussed. | astro-ph_IM |
Stellar distances from spectroscopic observations: a new technique: A Bayesian approach to the determination of stellar distances from
photometric and spectroscopic data is presented and tested both on pseudodata,
designed to mimic data for stars observed by the RAVE survey, and on the real
stars from the Geneva-Copenhagen survey. It is argued that this method is
optimal in the sense that it brings to bear all available information and that
its results are limited only by observational errors and the underlying physics
of stars. The method simultaneously returns the metallicities, ages and masses
of programme stars. Remarkably, the uncertainty in the output metallicity is
typically 44 per cent smaller than the uncertainty in the input metallicity. | astro-ph_IM |
4MOST - 4-metre Multi-Object Spectroscopic Telescope: The 4MOST consortium is currently halfway through a Conceptual Design study
for ESO with the aim to develop a wide-field (>3 square degree, goal >5 square
degree), high-multiplex (>1500 fibres, goal 3000 fibres) spectroscopic survey
facility for an ESO 4m-class telescope (VISTA). 4MOST will run permanently on
the telescope to perform a 5 year public survey yielding more than 20 million
spectra at resolution R~5000 ({\lambda}=390-1000 nm) and more than 2 million
spectra at R~20,000 (395-456.5 nm & 587-673 nm). The 4MOST design is especially
intended to complement three key all-sky, space-based observatories of prime
European interest: Gaia, eROSITA and Euclid. Initial design and performance
estimates for the wide-field corrector concepts are presented. We consider two
fibre positioner concepts, a well-known Phi-Theta system and a new R-Theta
concept with a large patrol area. The spectrographs are fixed configuration
two-arm spectrographs, with dedicated spectrographs for the high- and
low-resolution. A full facility simulator is being developed to guide trade-off
decisions regarding the optimal field-of-view, number of fibres needed, and the
relative fraction of high-to-low resolution fibres. Mock catalogues with
template spectra from seven Design Reference Surveys are simulated to verify
the science requirements of 4MOST. The 4MOST consortium aims to deliver the
full 4MOST facility by the end of 2018 and start delivering high-level data
products for both consortium and ESO community targets a year later with yearly
increments. | astro-ph_IM |
Creating A Galactic Plane Atlas With Amazon Web Services: This paper describes by example how astronomers can use cloud-computing
resources offered by Amazon Web Services (AWS) to create new datasets at scale.
We have created from existing surveys an atlas of the Galactic Plane at 16
wavelengths from 1 {\mu}m to 24 {\mu}m with pixels co-registered at spatial
sampling of 1 arcsec. We explain how open source tools support management and
operation of a virtual cluster on AWS platforms to process data at scale, and
describe the technical issues that users will need to consider, such as
optimization of resources, resource costs, and management of virtual machine
instances. | astro-ph_IM |
Driving unmodelled gravitational-wave transient searches using
astrophysical information: Transient gravitational-wave searches can be divided into two main families
of approaches: modelled and unmodelled searches, based on matched filtering
techniques and time-frequency excess power identification respectively. The
former, mostly applied in the context of compact binary searches, relies on the
precise knowledge of the expected gravitational-wave phase evolution. This
information is not always available at the required accuracy for all plausible
astrophysical scenarios, e.g., in presence of orbital precession, or
eccentricity. The other search approach imposes little priors on the targetted
signal. We propose an intermediate route based on a modification of unmodelled
search methods in which time-frequency pattern matching is constrained by
astrophysical waveform models (but not requiring accurate prediction for the
waveform phase evolution). The set of astrophysically motivated patterns is
conveniently encapsulated in a graph, that encodes the time-frequency pixels
and their co-occurrence. This allows the use of efficient graph-based
optimization techniques to perform the pattern search in the data. We show in
the example of black-hole binary searches that such an approach leads to an
averaged increase in the distance reach (+7-8\%) for this specific source over
standard unmodelled searches. | astro-ph_IM |
Speckle Suppression Through Dual Imaging Polarimetry, and a Ground-Based
Image of the HR 4796A Circumstellar Disk: We demonstrate the versatility of a dual imaging polarimeter working in
tandem with a Lyot coronagraph and Adaptive Optics to suppress the highly
static speckle noise pattern--the greatest hindrance to ground-based direct
imaging of planets and disks around nearby stars. Using a double difference
technique with the polarimetric data, we quantify the level of speckle
suppression, and hence improved sensitivity, by placing an ensemble of
artificial faint companions into real data, with given total brightness and
polarization. For highly polarized sources within 0.5 arcsec, we show that we
achieve 3 to 4 magnitudes greater sensitivity through polarimetric speckle
suppression than simply using a coronagraph coupled to a high-order Adaptive
Optics system. Using such a polarimeter with a classical Lyot coronagraph at
the 3.63m AEOS telescope, we have obtained a 6.5 sigma detection in the H-band
of the 76 AU diameter circumstellar debris disk around the star HR 4796A. Our
data represent the first definitive, ground-based, near-IR polarimetric image
of the HR 4796A debris disk and clearly show the two outer ansae of the disk,
evident in Hubble Space Telescope NICMOS/STIS imaging. We derive a lower limit
to the fractional linear polarization of 29% caused by dust grains in the disk.
In addition, we fit simple morphological models of optically thin disks to our
data allowing us to constrain the dust disk scale height to 2.5{+5.0}_{-1.3} AU
and scattering asymmetry parameter (g=0.20^{+.07}_{-.10}). These values are
consistent with several lines of evidence suggesting that the HR 4796A disk is
dominated by a micron-sized dust population, and are indeed typical of disks in
transition between those surrounding the Herbig Ae stars to those associated
with Vega-like stars. | astro-ph_IM |
Wide-band Profile Domain Pulsar Timing Analysis: We extend profile domain pulsar timing to incorporate wide-band effects such
as frequency-dependent profile evolution and broadband shape variation in the
pulse profile. We also incorporate models for temporal variations in both pulse
width and in the separation in phase of the main pulse and interpulse. We
perform the analysis with both nested sampling and Hamiltonian Monte Carlo
methods. In the latter case we introduce a new parameterisation of the
posterior that is extremely efficient in the low signal-to-noise regime and can
be readily applied to a wide range of scientific problems. We apply this
methodology to a series of simulations, and to between seven and nine yr of
observations for PSRs J1713$+$0747, J1744$-$1134, and J1909$-$3744 with
frequency coverage that spans 700-3600MHz. We use a smooth model for profile
evolution across the full frequency range, and compare smooth and piecewise
models for the temporal variations in DM. We find the profile domain framework
consistently results in improved timing precision compared to the standard
analysis paradigm by as much as 40% for timing parameters. Incorporating
smoothness in the DM variations into the model further improves timing
precision by as much as 30%. For PSR J1713+0747 we also detect pulse shape
variation uncorrelated between epochs, which we attribute to variation
intrinsic to the pulsar at a level consistent with previously published
analyses. Not accounting for this shape variation biases the measured arrival
times at the level of $\sim$30ns, the same order of magnitude as the expected
shift due to gravitational-waves in the pulsar timing band. | astro-ph_IM |
Microarcsecond VLBI pulsar astrometry with PSR$π$ II. parallax
distances for 57 pulsars: We present the results of PSR$\pi$, a large astrometric project targeting
radio pulsars using the Very Long Baseline Array (VLBA). From our astrometric
database of 60 pulsars, we have obtained parallax-based distance measurements
for all but 3, with a parallax precision of typically 40 $\mu$as and
approaching 10 $\mu$as in the best cases. Our full sample doubles the number of
radio pulsars with a reliable ($\gtrsim$5$\sigma$) model-independent distance
constraint. Importantly, many of the newly measured pulsars are well outside
the solar neighbourhood, and so PSR$\pi$ brings a near-tenfold increase in the
number of pulsars with a reliable model-independent distance at $d>2$ kpc.
Using our sample along with previously published results, we show that even the
most recent models of the Galactic electron density distribution model contain
significant shortcomings, particularly at high Galactic latitudes. When
comparing our results to pulsar timing, two of the four millisecond pulsars in
our sample exhibit significant discrepancies in the estimates of proper motion
obtained by at least one pulsar timing array. With additional VLBI observations
to improve the absolute positional accuracy of our reference sources and an
expansion of the number of millisecond pulsars, we will be able to extend the
comparison of proper motion discrepancies to a larger sample of pulsar
reference positions, which will provide a much more sensitive test of the
applicability of the solar system ephemerides used for pulsar timing. Finally,
we use our large sample to estimate the typical accuracy attainable for
differential astrometry with the VLBA when observing pulsars, showing that for
sufficiently bright targets observed 8 times over 18 months, a parallax
uncertainty of 4 $\mu$as per arcminute of separation between the pulsar and
calibrator can be expected. | astro-ph_IM |
The Design and Performance of IceCube DeepCore: The IceCube neutrino observatory in operation at the South Pole, Antarctica,
comprises three distinct components: a large buried array for ultrahigh energy
neutrino detection, a surface air shower array, and a new buried component
called DeepCore. DeepCore was designed to lower the IceCube neutrino energy
threshold by over an order of magnitude, to energies as low as about 10 GeV.
DeepCore is situated primarily 2100 m below the surface of the icecap at the
South Pole, at the bottom center of the existing IceCube array, and began
taking physics data in May 2010. Its location takes advantage of the
exceptionally clear ice at those depths and allows it to use the surrounding
IceCube detector as a highly efficient active veto against the principal
background of downward-going muons produced in cosmic-ray air showers. DeepCore
has a module density roughly five times higher than that of the standard
IceCube array, and uses photomultiplier tubes with a new photocathode featuring
a quantum efficiency about 35% higher than standard IceCube PMTs. Taken
together, these features of DeepCore will increase IceCube's sensitivity to
neutrinos from WIMP dark matter annihilations, atmospheric neutrino
oscillations, galactic supernova neutrinos, and point sources of neutrinos in
the northern and southern skies. In this paper we describe the design and
initial performance of DeepCore. | astro-ph_IM |
Radio Astronomy Data Transfer and eVLBI using KAREN: Kiwi Advanced Research and Education Network (KAREN) has been used to
transfer large volumes of radio astronomical data between the Radio
Astronomical Observatory at Warkworth, New Zealand and various international
organizations involved in joint projects and VLBI observations. Here we report
on the current status of connectivity and on the results of testing different
data transfer protocols. We investigate new UDP protocols such as 'tsunami' and
UDT and demonstrate that the UDT protocol is more efficient than 'tsunami' and
'ftp'. We also report on the tests on direct data streaming from the radio
telescope receiving system to the correlation centre without intermediate
buffering or recording (real-time eVLBI). | astro-ph_IM |
CUTE solutions for two-point correlation functions from large
cosmological datasets: In the advent of new large galaxy surveys, which will produce enormous
datasets with hundreds of millions of objects, new computational techniques are
necessary in order to extract from them any two-point statistic, the
computational time of which grows with the square of the number of objects to
be correlated. Fortunately technology now provides multiple means to massively
parallelize this problem. Here we present a free-source code specifically
designed for this kind of calculations. Two implementations are provided: one
for execution on shared-memory machines using OpenMP and one that runs on
graphical processing units (GPUs) using CUDA. The code is available at
http://members.ift.uam-csic.es/dmonge/CUTE.html. | astro-ph_IM |
Using the Astrophysics Source Code Library: Find, cite, download, parse,
study, and submit: The Astrophysics Source Code Library (ASCL) contains 3000 metadata records
about astrophysics research software and serves primarily as a registry of
software, though it also can and does accept code deposit. Though the ASCL was
started in 1999, many astronomers, especially those new to the field, are not
very familiar with it. This hands-on virtual tutorial was geared to new users
of the resource to teach them how to use the ASCL, with a focus on finding
software and information about software not only in this resource, but also by
using Google and NASA's Astrophysics Data System (ADS). With computational
methods so important to research, finding these methods is useful for examining
(for transparency) and possibly reusing the software (for reproducibility or to
enable new research). Metadata about software is useful for, for example,
knowing how to cite software when it is used for research and studying trends
in the computational landscape. Though the tutorial was primarily aimed at new
users, advanced users were also likely to learn something new. | astro-ph_IM |
MAGIC-II Camera Slow Control Software: The Imaging Atmospheric Cherenkov Telescope MAGIC I has recently been
extended to a stereoscopic system by adding a second 17 m telescope, MAGIC-II.
One of the major improvements of the second telescope is an improved camera.
The Camera Control Program is embedded in the telescope control software as an
independent subsystem.
The Camera Control Program is an effective software to monitor and control
the camera values and their settings and is written in the visual programming
language LabVIEW. The two main parts, the Central Variables File, which stores
all information of the pixel and other camera parameters, and the Comm Control
Routine, which controls changes in possible settings, provide a reliable
operation. A safety routine protects the camera from misuse by accidental
commands, from bad weather conditions and from hardware errors by automatic
reactions. | astro-ph_IM |
On the use of asymmetric PSF on NIR images of crowded stellar fields: We present data collected using the camera PISCES coupled with the Firt Light
Adaptive Optics (FLAO) mounted at the Large Binocular Telescope (LBT). The
images were collected using two natural guide stars with an apparent magnitude
of R<13 mag. During these observations the seeing was on average ~0.9". The AO
performed very well: the images display a mean FWHM of 0.05 arcsec and of 0.06
arcsec in the J- and in the Ks-band, respectively. The Strehl ratio on the
quoted images reaches 13-30% (J) and 50-65% (Ks), in the off and in the central
pointings respectively. On the basis of this sample we have reached a J-band
limiting magnitude of ~22.5 mag and the deepest Ks-band limiting magnitude ever
obtained in a crowded stellar field: Ks~23 mag.
J-band images display a complex change in the shape of the PSF when moving at
larger radial distances from the natural guide star. In particular, the stellar
images become more elongated in approaching the corners of the J-band images
whereas the Ks-band images are more uniform. We discuss in detail the strategy
used to perform accurate and deep photometry in these very challenging images.
In particular we will focus our attention on the use of an updated version of
ROMAFOT based on asymmetric and analytical Point Spread Functions.
The quality of the photometry allowed us to properly identify a feature that
clearly shows up in NIR bands: the main sequence knee (MSK). The MSK is
independent of the evolutionary age, therefore the difference in magnitude with
the canonical clock to constrain the cluster age, the main sequence turn off
(MSTO), provides an estimate of the absolute age of the cluster. The key
advantage of this new approach is that the error decreases by a factor of two
when compared with the classical one. Combining ground-based Ks with space
F606W photometry, we estimate the absolute age of M15 to be 13.70+-0.80 Gyr. | astro-ph_IM |
The ATA Digital Processing Requirements are Driven by RFI Concerns: As a new generation radio telescope, the Allen Telescope Array (ATA) is a
prototype for the square kilometer array (SKA). Here we describe recently
developed design constraints for the ATA digital signal processing chain as a
case study for SKA processing. As radio frequency interference (RFI) becomes
increasingly problematical for radio astronomy, radio telescopes must support a
wide range of RFI mitigation strategies including online adaptive RFI nulling.
We observe that the requirements for digital accuracy and control speed are not
driven by astronomical imaging but by RFI. This can be understood from the fact
that high dynamic range and digital precision is necessary to remove strong RFI
signals from the weak astronomical background, and because RFI signals may
change rapidly compared with celestial sources. We review and critique lines of
reasoning that lead us to some of the design specifications for ATA digital
processing, including these: beamformer coefficients must be specified with at
least 1{\deg} precision and at least once per millisecond to enable flexible
RFI excision. | astro-ph_IM |
Measurement of South Pole ice transparency with the IceCube LED
calibration system: The IceCube Neutrino Observatory, approximately 1 km^3 in size, is now
complete with 86 strings deployed in the Antarctic ice. IceCube detects the
Cherenkov radiation emitted by charged particles passing through or created in
the ice. To realize the full potential of the detector, the properties of light
propagation in the ice in and around the detector must be well understood. This
report presents a new method of fitting the model of light propagation in the
ice to a data set of in-situ light source events collected with IceCube. The
resulting set of derived parameters, namely the measured values of scattering
and absorption coefficients vs. depth, is presented and a comparison of IceCube
data with simulations based on the new model is shown. | astro-ph_IM |
Multiplexing lobster-eye optics: a concept for wide-field X-ray
monitoring: We propose a concept of multiplexing lobster-eye (MuLE) optics to achieve
significant reductions in the number of focal plane imagers in lobster-eye (LE)
wide-field X-ray monitors. In the MuLE configuration, an LE mirror is divided
into several segments and the X-rays reflected on each of these segments are
focused on a single image sensor in a multiplexed configuration. If each LE
segment assumes a different rotation angle, the azimuthal rotation angle of a
cross-like image reconstructed from a point source by the LE optics identifies
the specific segment that focuses the X-rays on the imager. With a focal length
of 30 cm and LE segments with areas of 10 x 10 cm^2, ~1 sr of the sky can be
covered with 36 LE segments and only four imagers (with total areas of 10 x 10
cm^2). A ray tracing simulation was performed to evaluate the nine-segment MuLE
configuration. The simulation showed that the flux (0.5 to 2 keV) associated
with the 5-sigma detection limit was ~2 x 10^-10 erg cm^-2 s^-1 (10 mCrab) for
a transient with a duration of 100 s. The simulation also showed that the
direction of the transient for flux in the range of 14 to 17 mCrab at 0.6 keV
was determined correctly with 99.7% confidence limit. We conclude that the MuLE
configuration can become an effective on-board device for small satellites for
future X-ray wide-field transient monitoring. | astro-ph_IM |
Multi-messenger Astronomy: a Bayesian approach: After the discovery of the gravitational waves and the observation of
neutrinos of cosmic origin, we have entered a new and exciting era where cosmic
rays, neutrinos, photons and gravitational waves will be used simultaneously to
study the highest energy phenomena in the Universe. Here we present a fully
Bayesian approach to the challenge of combining and comparing the wealth of
measurements from existing and upcoming experimental facilities. We discuss the
procedure from a theoretical point of view and using simulations, we also
demonstrate the feasibility of the method by incorporating the use of
information provided by different theoretical models and different experimental
measurements. | astro-ph_IM |
Proximity Operators for Phase Retrieval: We present a new formulation of a family of proximity operators that
generalize the projector step for phase retrieval. These proximity operators
for noisy intensity measurements can replace the classical "noise free"
projection in any projection-based algorithm. They are derived from a maximum
likelihood formulation and admit closed form solutions for both the Gaussian
and the Poisson cases. In addition, we extend these proximity operators to
undersampled intensity measurements. To assess their performance, these
operators are exploited in a classical Gerchberg Saxton algorithm. We present
numerical experiments showing that the reconstructed complex amplitudes with
these proximity operators perform always better than using the classical
intensity projector while their computational overhead is moderate. | astro-ph_IM |
Progress with the LOFAR Imaging Pipeline: One of the science drivers of the new Low Frequency Array (LOFAR) is
large-area surveys of the low-frequency radio sky. Realizing this goal requires
automated processing of the interferometric data, such that fully calibrated
images are produced by the system during survey operations. The LOFAR Imaging
Pipeline is the tool intended for this purpose, and is now undergoing
significant commissioning work. The pipeline is now functional as an automated
processing chain. Here we present several recent LOFAR images that have been
produced during the still ongoing commissioning period. These early LOFAR
images are representative of some of the science goals of the commissioning
team members. | astro-ph_IM |
Diffractive Microlensing: A New Probe of the Local Universe: Diffraction is important when nearby substellar objects gravitationally lens
distant stars. If the wavelength of the observation is comparable to the
Schwarzschild radius of lensing object, diffraction leaves an observable
imprint on the lensing signature. The SKA may have sufficient sensitivity to
detect the typical sources, giant stars in the bulge. The diffractive
signatures in a lensing event break the degeneracies between the mass of the
lens, its distance and proper motion. | astro-ph_IM |
Compensation of tropospheric and ionospheric effects in gravitational
sessions of the spacecraft RadioAstron: The possibility of compensating atmospheric influence in an experiment on
precision measurement of gravitational redshift using the "RadioAstron"
spacecraft (SC) is discussed. When a signal propagates from a ground-based
tracking station to a spacecraft and back, interaction with the ionosphere and
troposphere makes considerable contribution to the frequency shift. A brief
overview of the physical effects determining this contribution is given, and
the principles of calculation and compensation of the corresponding frequency
distortions of radio signals are described. Then these approaches are used to
reduce the atmospheric frequency shift of the "RadioAstron" spacecraft signal.
The spacecraft hardware allows working in two communication modes: "one-way"
and "two-way", in addition, two communication channels at different frequencies
work simultaneously. "One-way" (SC - ground-based tracking station)
communication mode, a signal is synchronized by the on board hydrogen frequency
standard. The "two-way" (SC - ground-based tracking station - SC ) mode is
synchronized by the ground hydrogen standard. The calculations performed allow
us to compare the quality of compensation of atmospheric fluctuations performed
by various methods and choose the optimal one. | astro-ph_IM |
RISTRETTO: coronagraph and AO designs enabling High Dispersion
Coronagraphy at 2 lambda/D: RISTRETTO is the evolution of the original idea of coupling the VLT
instruments SPHERE and ESPRESSO, aiming at High Dispersion Coronagraphy.
RISTRETTO is a visitor instrument that should enable the characterization of
the atmospheres of nearby exoplanets in reflected light, by using the technique
of high-contrast, high-resolution spectroscopy. Its goal is to observe Prox Cen
b and other planets placed at about 35mas from their star, i.e. 2lambda/D at
lambda=750nm. The instrument is composed of an extreme adaptive optics, a
coronagraphic Integral Field Unit, and a diffraction-limited spectrograph
(R=140.000, lambda=620-840 nm).
We present the status of our studies regarding the coronagraphic IFU and the
XAO system. The first in particular is based on a modified version of the PIAA
apodizer, allowing nulling on the first diffraction ring. Our proposed design
has the potential to reach > 50% coupling and <1E-4 contrast at 2lambda/D in
median seeing conditions. | astro-ph_IM |
The angular resolution of GRAPES-3 EAS array after correction for the
shower front curvature: The angular resolution of an extensive air shower (EAS) array plays a
critical role in determining its sensitivity for the detection of point
$\gamma$-ray sources in the multi-TeV energy range. The GRAPES-3 an EAS array
located at Ooty in India (11.4$^{\circ}$N, 76.7$^{\circ}$E, 2200 m altitude) is
designed to study $\gamma$-rays in the TeV-PeV energy range. It comprises of a
dense array of 400 plastic scintillators deployed over an area of 25000 m$^2$
and a large area (560 m$^2$) muon telescope. A new statistical method allowed
real time determination of the propagation delay of each detector in the
GRAPES-3 array. The shape of shower front is known to be curved and here the
details of a new method developed for accurate measurement of the shower front
curvature is presented. These two developments have led to a sizable
improvement in the angular resolution of GRAPES-3 array. It is shown that the
curvature depends on the size and age of an EAS. By employing two different
techniques, namely, the odd-even and the left-right methods, independent
estimates of the angular resolution are obtained. The odd-even method estimates
the best achievable resolution of the array. For obtaining the angular
resolution, the left-right method is used after implementing the size and age
dependent curvature corrections. A comparison of the angular resolution as a
function of EAS energy by these two methods shows them be virtually
indistinguishable. The angular resolution of GRAPES-3 array is 47$^{\prime}$
for energies E$>$5 TeV and improves to 17$^{\prime}$ at E$>$100 TeV and finally
approaching 10$^{\prime}$ at E$>$500 TeV. | astro-ph_IM |
The Graphical User Interface of the Operator of the Cherenkov Telescope
Array: The Cherenkov Telescope Array (CTA) is the next generation gamma-ray
observatory. CTA will incorporate about 100 imaging atmospheric Cherenkov
telescopes (IACTs) at a southern site, and about 20 in the north. Previous IACT
experiments have used up to five telescopes. Subsequently, the design of a
graphical user interface (GUI) for the operator of CTA poses an interesting
challenge. In order to create an effective interface, the CTA team is
collaborating with experts from the field of Human-Computer Interaction. We
present here our GUI prototype. The back-end of the prototype is a Python Web
server. It is integrated with the observation execution system of CTA, which is
based on the Alma Common Software (ACS). The back-end incorporates a redis
database, which facilitates synchronization of GUI panels. redis is also used
to buffer information collected from various software components and databases.
The front-end of the prototype is based on Web technology. Communication
between Web server and clients is performed using Web Sockets, where graphics
are generated with the d3.js Javascript library. | astro-ph_IM |
Gemini Planet Imager Observational Calibrations II: Detector Performance
and Calibration: The Gemini Planet Imager is a newly commissioned facility instrument designed
to measure the near-infrared spectra of young extrasolar planets in the solar
neighborhood and obtain imaging polarimetry of circumstellar disks. GPI's
science instrument is an integral field spectrograph that utilizes a HAWAII-2RG
detector with a SIDECAR ASIC readout system. This paper describes the detector
characterization and calibrations performed by the GPI Data Reduction Pipeline
to compensate for effects including bad/hot/cold pixels, persistence,
non-linearity, vibration induced microphonics and correlated read noise. | astro-ph_IM |
First tests of a 1 megapixel near-infrared avalanche photodiode array
for ultra-low background space astronomy: Spectroscopy of Earth-like exoplanets and ultra-faint galaxies are priority
science cases for the coming decades. Here, broadband source flux rates are
measured in photons per square meter per hour, imposing extreme demands on
detector performance, including dark currents lower than 1 e-/pixel/kilosecond,
read noise less than 1 e-/pixel/frame, and large formats. There are currently
no infrared detectors that meet these requirements. The University of Hawaii
and industrial partners are developing one promising technology, linear mode
avalanche photodiodes (LmAPDs), using fine control over the HgCdTe bandgap
structure to enable noise-free charge amplification and minimal glow.
Here we report first results of a prototype megapixel format LmAPD operated
in our cryogenic testbed. At 50 Kelvin, we measure a dark current of about 3
e-/pixel/kilosecond, which is due to an intrinsic dark current consistent with
zero (best estimate of 0.1 e-/pixel/kilosecond) and a ROIC glow of 0.08
e-/pixel/frame. The read noise of these devices is about 10 e-/pixel/frame at 3
volts, and decreases by 30% with each additional volt of bias, reaching 2 e- at
8 volts. Upcoming science-grade devices are expected to substantially improve
upon these figures, and address other issues uncovered during testing. | astro-ph_IM |
Temporal spectrum of multi-conjugate adaptive optics residuals and
impact of tip-tilt anisoplanatism on astrometric observations: Multi-conjugate adaptive optics (MCAO) will assist a new era of ground-based
astronomical observations with the extremely large telescopes and the Very
Large Telescope. High precision relative astrometry is among the main science
drivers of these systems and challenging requirements have been set for the
astrometric measurements. A clear understanding of the astrometric error budget
is needed and the impact of the MCAO correction has to be taken into account.
In this context, we propose an analytical formulation to estimate the residual
phase produced by an MCAO correction in any direction of the scientific field
of view. The residual phase, computed in the temporal frequency domain, allows
to consider the temporal filtering of the turbulent phase from the MCAO loop
and to extract the temporal spectrum of the residuals, as well as to include
other temporal effects such as the scientific integration time. The formulation
is kept general and allows to consider specific frameworks by setting the
telescope diameter, the turbulence profile, the guide stars constellation, the
deformable mirrors configuration, the modes sensed and corrected and the
tomographic reconstruction algorithm. The formalism is presented for both a
closed loop and a pseudo-open loop control. We use our results to investigate
the effect of tip-tilt residuals on MCAO-assisted astrometric observations. We
derive an expression for the differential tilt jitter power spectrum that also
includes the dependence on the scientific exposure time. Finally, we
investigate the contribution of the differential tilt jitter error on the
future astrometric observations with MAVIS and MAORY. | astro-ph_IM |
Physical properties of the interstellar medium using high-resolution
Chandra spectra: O K-edge absorption: Chandra high-resolution spectra toward eight low-mass Galactic binaries have
been analyzed with a photoionization model that is capable of determining the
physical state of the interstellar medium. Particular attention is given to the
accuracy of the atomic data. Hydrogen column densities are derived with a
broadband fit that takes into account pileup effects, and in general are in
good agreement with previous results. The dominant features in the oxygen-edge
region are O I and O II K$\alpha$ absorption lines whose simultaneous fits lead
to average values of the ionization parameter of $\log\xi=-2.90$ and oxygen
abundance of $A_{\rm O}=0.70$. The latter is relative to the standard by
Grevesse & Sauval (1998), but a rescaling with the revision by Asplund et al.
(2009) would lead to an average abundance value fairly close to solar. The low
average oxygen column density ($N_{\rm O}=9.2 \times 10^{17}$ cm$^{-2}$)
suggests a correlation with the low ionization parameters, the latter also
being in evidence in the column density ratios OII/OI and OIII/OI that are
estimated to be less than 0.1. We do not find conclusive evidence for
absorption by any other compound but atomic oxygen. | astro-ph_IM |
Upgrading electron temperature and electron density diagnostic diagrams
of forbidden line emission: Diagnostic diagrams of forbidden lines have been a useful tool for observers
in astrophysics for many decades now. They are used to obtain information on
the basic physical properties of thin gaseous nebulae. Some diagnostic diagrams
are in wavelength domains which were difficult to take either due to missing
wavelength coverage or low resolution of older spectrographs. Furthermore, most
of the diagrams were calculated using just the species involved as a single
atom gas, although several are affected by well-known fluorescence mechanisms
as well. Additionally the atomic data have improved up to the present time. Aim
of this work was a recalculation of well-known, but also of sparsely used,
unnoted diagnostics diagrams. The new diagrams provide observers with modern,
easy-to-use recipes to determine electron temperature and densities. The new
diagnostic diagrams are calculated using large grids of parameter space in the
photoionization code CLOUDY. For a given basic parameter (e.g. electron density
or temperature) the solutions with cooling-heating-equilibrium are chosen to
derive the diagnostic diagrams. Empirical numerical functions are fitted to
provide formulas usable in e.g. data reduction pipelines. The resulting
diagrams differ significantly from those used up to now and will improve the
thermodynamic calculations. To our knowledge, for the first time detailed
directly applicable fit formulas are given, leading to electron temperature or
density from the line ratios. | astro-ph_IM |
Time-Dependent Behavior of Linear Polarization in Unresolved
Photospheres, With Applications for The Hanle Effect: Aims: This paper extends previous studies in modeling time varying linear
polarization due to axisymmetric magnetic fields in rotating stars. We use the
Hanle effect to predict variations in net line polarization, and use geometric
arguments to generalize these results to linear polarization due to other
mechanisms. Methods: Building on the work of Lopez Ariste et al., we use simple
analytic models of rotating stars that are symmetric except for an axisymmetric
magnetic field to predict the polarization lightcurve due to the Hanle effect.
We highlight the effects for the variable line polarization as a function of
viewing inclination and field axis obliquity. Finally, we use geometric
arguments to generalize our results to linear polarization from the weak
transverse Zeeman effect. Results: We derive analytic expressions to
demonstrate that the variable polarization lightcurve for an oblique magnetic
rotator is symmetric. This holds for any axisymmetric field distribution and
arbitrary viewing inclination to the rotation axis. Conclusions: For the
situation under consideration, the amplitude of the polarization variation is
set by the Hanle effect, but the shape of the variation in polarization with
phase depends largely on geometrical projection effects. Our work generalizes
the applicability of results described in Lopez Ariste et al., inasmuch as the
assumptions of a spherical star and an axisymmetric field are true, and
provides a strategy for separating the effects of perspective from the Hanle
effect itself for interpreting polarimetric lightcurves. | astro-ph_IM |
Subsets and Splits
No saved queries yet
Save your SQL queries to embed, download, and access them later. Queries will appear here once saved.