arXiv subjects

Jennifer L. West

Publications and source records attributed to Jennifer L. West.

17 recordsLinked to original sources

arXiv2026

Advancing Astrophysics with the SKA II

Advancing Astrophysics with the SKA II (AASKAII), written by our science community, outlines the transformative scientific advances that will be enabled by the SKA telescopes. In the decade since the publication of the previous edition, telescope designs have matured, construction has commenced, and the SKA Organisation has evolved into the SKA Observatory (SKAO). At the same time, observations from SKA precursor and pathfinder telescopes have provided new insights into longstanding scientific challenges while revealing entirely new phenomena. Published in advance of the first science verification campaign for the SKA Observatory, this volume looks ahead to the coming decades of discovery and innovation in radio astronomy. AASKAII spans the broad range of scientific research enabled by the SKA telescopes, SKA-Mid and SKA-Low. The contributions are organised into six thematic categories according to their scientific focus. The opening section presents overview chapters from the SKA Science Working Groups, around which our community is organised. Each overview provides the broader context that connects the contributions in this volume to the key scientific questions being pursued by their respective communities.

astro-ph.IM↗

arXiv2026

Cosmic Magnetism Science with the SKA

Magnetic fields are a fundamental component of astrophysical systems, yet many key questions about their origin, amplification, and role in structure formation and evolution remain unresolved. The SKA will mark a transformational step forward in addressing these questions, enabling studies of cosmic magnetism across a large range of spatial scales and environments. This overview summarizes the main science cases in the Cosmic Magnetism Science Working Group, which cover a huge breadth of scales from the smallest scales governing planet and star formation, all the way up to the large-scale structure of the Universe. The chapter summarizes the main observational techniques for studying magnetic fields, including direct polarization imaging, Faraday rotation, rotation-measure grids, and Zeeman splitting. We also address fundamental considerations of these studies including SKA-Low vs SKA-Mid and wide-area vs deep observing strategies.

astro-ph.GA↗

arXiv2026

Faraday Tomography with the SKA: A New Era of Cosmic Magnetism Studies

The Square Kilometre Array (SKA) represents a significant advancement in radio astronomy, enabling detailed study of cosmic magnetism through Faraday Rotation and Faraday Measurement Synthesis. This chapter provides a comprehensive review of these techniques, tracing their development and illustrating their crucial role in investigating magnetic fields across different cosmic environments. We focus on Array Assembly 4 (AA4), the final deployment stage, featuring 197 dishes (133 SKA, 64 MeerKAT) with 40 km maximum baselines and frequency coverage from 350 MHz to 15.4 GHz (goal: 24 GHz). These capabilities enable high-resolution Faraday tomography of magnetic structures in galaxies, clusters, and the cosmic web. We also highlight the earlier AA* stage with 144 dishes (80 SKA, 64 MeerKAT). Band 1 (350--1050 MHz) offers the finest Faraday depth resolution among the dish bands (full resolution $\sim$2.5 rad m$^{-2}$) with a maximum observable depth around 174,708 rad m$^{-2}$. Other bands (Band 2, Band 5a, Band 5b) enable multi-scale studies, probing larger structures with lower resolution but higher maximum depths. We discuss synergies between Faraday Measurement Synthesis and Image Synthesis techniques, showing how they complement each other in reconstructing magnetic field structures. Simulation results demonstrate the SKA's potential for high-resolution observations. We explore possible enhancements beyond the baseline configuration and synergies with other observatories, emphasizing multi-wavelength astronomy. This chapter aims to provide a thorough account of how the SKA's phased deployment from AA* to AA4 will transform our understanding of cosmic magnetic fields and drive new discoveries in astrophysics.

astro-ph.IM↗

arXiv2026

Radio detection of supernova remnant G310.7-5.4 with $γ$-ray counterpart: Abeona SNR

G310.7-5.4 is a supernova remnant (SNR) candidate identified as a faint shell in the second epoch Molonglo Galactic Plane Survey (MGPS-2), but this has not been followed up with multi-wavelength observations until now. It is an example of an SNR at high Galactic latitude showing spatially coinciding $γ$-ray emission. Here, we make the first detailed investigation of the radio emission from the G310.7-5.4 region, aiming to characterise the radio structure, polarisation measurements and the coinciding GeV emission. We used recent radio continuum observations at 943.5 MHz from the EMU and the POSSUM surveys with ASKAP, as well as 16.5 years of Fermi-LAT observations. We furthermore considered the multiwavelength context of the object by investigating observations previously conducted with other instruments, such as infrared and X-ray surveys. We confirm the SNR candidate as a new supernova remnant, dubbed Abeona. We detect the presence of a faint, extended, bilateral radio shell of the size of around 30' diameter and ASKAP radio flux density of $1.5^{+1.5}_{-0.1}$ Jy with no obvious infrared counterparts. With a radio surface brightness of about $2.4^{+2.4}_{-0.1}\times10^{-22}$ W m$^{-2}$ Hz$^{-1}$ sr$^{-1}$, this SNR is one of the faintest radio SNRs known. The northern part of the shell shows linearly polarised radio emission, characteristic of synchrotron emission in SNRs. The physical size of the SNR is estimated to be around $42^{+42}_{-21}$ pc, which would give a distance of around $4.9^{+4.9}_{-2.5}$ kpc. Furthermore, the spatially coincident $γ$-ray source 4FGL J1413.9-6705 shows an energy flux of $1.26\pm0.35\times 10^{-6}$ MeV cm$^{-2}$ s$^{-1}$ with a significance of 5.7 $σ$ between 100 MeV and 100 GeV. The SNR is also put in context with known high-latitude SNRs with $γ$-ray counterparts and compared with their observational properties.

astro-ph.HE↗

arXiv2026

The Salamander: A case study of the magnetic field and peculiar morphology of G309.8-2.6 through radio polarimetry

Studying the interaction between core-collapse supernova remnants (SNRs) and their surrounding environments is essential to understanding the mechanism for energy transfer to the interstellar medium (ISM) and the intrinsic physical properties of these remnants. In this paper, we focus on G309.8-2.6. Our new observations reveal that this object includes an SNR shell with a relic pulsar wind nebula (PWN) that extends well beyond the emission that has been previously observed in X-rays. We present new radio continuum and polarization images of G309.8-2.6 from the Evolutionary Map of the Universe (EMU) and Polarization Sky Survey of the Universe's Magnetism (POSSUM) surveys with the Australian Square Kilometre Array Pathfinder (ASKAP). The images reveal the complex and peculiar morphology of G309.8-2.6. The linear polarization displays an atypical S-shaped morphology and a highly ordered magnetic field. The rotation measure (RM) map shows a large-scale gradient or possible sign reversal, depending on the foreground RM. We reprocessed archival X-ray observations from Chandra and eROSITA, and retrieved archival H$α$ and infrared observations. We performed a joint analysis of the multiwavelength data and proposed scenarios to explain the unusual shape. Our results place new constraints on the magnetic field of G309.8-2.6, including its environment, and demonstrate the power of polarization observations in probing the properties of SNRs.

astro-ph.GA↗

arXiv2026

GMIMS-DRAGONS: A Faraday Depth Survey of the Northern Sky Covering 350-1030 MHz

Polarized synchrotron emission at meter to centimeter wavelengths provides an effective tracer of the Galactic magnetic field. Calculating Faraday depth, the most useful parameter for mapping the line-of-sight magnetic field, requires observations covering wide frequency bands with many channels. As part of the Global Magneto-Ionic Medium Survey (GMIMS), we have observed polarized emission spanning 350-1030 MHz over the northern sky, in the declination range ${-20^{\circ}}\leqδ\leq{90^{\circ}}$. We used the 15 m telescope at the Dominion Radio Astrophysical Observatory (DRAO), equipped to receive orthogonal circular polarizations, with the Onsala Space Observatory band 1 feed developed for the SKA Project. Angular resolution varies across the band from $1.3^{\circ}$ to $3.6^{\circ}$. A digital spectrometer provided 42 kHz frequency resolution. Data were taken with the telescope moving rapidly in azimuth and are absolutely calibrated in intensity. Approximately 25% of the data were lost due to radio-frequency interference. The resolution in Faraday depth is $\sim6$ rad m$^{-2}$, and features as wide as $\sim38$ rad m$^{-2}$ are represented. The median sensitivity of the Faraday depth cube is 11 mK. Approximately 55% of sight-lines in this survey show Faraday complexity. This dataset, called ``DRAO GMIMS of the Northern Sky'' (DRAGONS), is the first to probe Faraday depth of the northern sky in its frequency range and will support many scientific investigations. The data will be used to calibrate surveys with higher angular resolution, particularly Galactic foreground maps from the Canadian Hydrogen Intensity Mapping Experiment, and to provide information on large structures for aperture-synthesis telescopes, particularly the DRAO Synthesis Telescope. The data are available through the Canadian Astronomy Data Centre.

astro-ph.IM↗

arXiv2026

RM-Tools: Software for Analyzing Polarized Radio Spectra

Polarization observations using modern radio telescopes cover large numbers of frequency channels over broad bandwidths, and require advanced techniques to extract reliable scientific results. We present RM-Tools, analysis software for deriving polarization properties, such as Faraday rotation measures, from spectropolarimetric observations of linearly polarized radio sources. The software makes use of techniques such as rotation measure synthesis and QU-model fitting, along with many features to simplify and enhance the analysis of radio polarization data. RM-Tools is currently the main software that large-area polarization sky surveys such as POSSUM and VLASS deploy for science-ready data processing. The software code is freely available online and can be used with data from a wide range of telescopes.

astro-ph.IM↗

arXiv2025

A three-dimensional model for the reversal in the local large-scale interstellar magnetic field

We probe the three-dimensional geometry of the large-scale Galactic magnetic field within 1 kpc of the Sun using the Dominion Radio Astrophysical Observatory (DRAO) Global Magneto-Ionic Medium Survey (GMIMS) of the Northern Sky (DRAGONS). DRAGONS is a new full polarization survey of the Northern sky from 350 to 1030 MHz covering declinations -20° < $δ$ < 90° and a component of GMIMS. The first moment of the Faraday depth spectra produced from DRAGONS above 500 MHz reveals large-angular-scale Faraday depth structures with signs that alternate only once in the Southern Galactic hemisphere and twice in the Northern hemisphere, patterns shared by other Faraday rotation datasets. DRAGONS is the first survey to achieve high Faraday depth resolution while maintaining sensitivity to broad Faraday depth structures, enabling the first use of Galactic longitude-Faraday depth plots. These plots reveal Faraday-complex structures across the sky, indicating a slab-like scenario in which emission and Faraday rotation are mixed. This complexity is overlaid on the same large-scale Faraday depth patterns that appear in the first moment map. We model these patterns as a magnetic reversal slicing through the disk on a diagonal and passing above the Sun in Galactic coordinates. We describe this reversal as a plane with a normal vector parallel to the line directed along ($\ell$, b) = (168.5°, -60°) and estimate its distance to be between 0.25 and 0.55 kpc. Our results show that much of the observed Faraday sky may be dominated by the local magnetic field configuration.

astro-ph.GA↗

arXiv2025

A pioneering experiment combining single-antenna and aperture-synthesis data to measure Faraday rotation with GMIMS and the CGPS

Structures in the magnetoionic medium exist across a wide range of angular sizes owing to large-scale magnetic fields coherent over the Galactic spiral arms combined with small-scale fluctuations in the magnetic field and electron density resulting from energy injection processes such as supernovae. For the first time, we produce diffuse Galactic synchrotron emission Faraday rotation maps covering all spatial scales down to $3'$ resolution for magnetic field studies. These maps complement total and polarized intensity maps combining single-antenna and interferometric data that have been produced, such as the Canadian Galactic Plane Survey (CGPS). Combined maps have sensitivity to large scales from the single-antenna component and angular resolution from the interferometric component. We combine Global Magneto-Ionic Medium Survey High-Band North single-antenna and CGPS aperture-synthesis polarization data after spatial filtering, producing Stokes $Q$ and $U$ maps for the four CGPS frequency channels. We calculate rotation measures (RMs) for all pixels using a linear fit to polarization angle versus wavelength squared. Smooth polarized emission regions require the large-scale sensitivity of the single-antenna to illuminate the Faraday rotation, while aperture synthesis reveals small-scale RM variability. While these maps show magnetic field structures on the full range of spatial scales they probe, the RM values should be interpreted with caution, as the narrow $λ^2$ coverage limits sensitivity to Faraday complexity. Despite this limitation of the CGPS 35 MHz bandwidth, we demonstrate that useful Faraday rotation information can be obtained from the combined dataset, highlighting the important synergy between future broadband interferometric and single-antenna polarization surveys.

astro-ph.GA↗

arXiv2025

Teleios (G305.4-2.2) -- the mystery of a perfectly shaped new Galactic supernova remnant

We present the serendipitous radio-continuum discovery of a likely Galactic supernova remnant (SNR) G305.4-2.2. This object displays a remarkable circular symmetry in shape, making it one of the most circular Galactic SNRs known. Nicknamed Teleios due to its symmetry, it was detected in the new Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) radio-continuum images with an angular size of 1320"x1260" and PA = 0 deg. While there is a hint of possible H$α$ and gamma-ray emission, Teleios is exclusively seen at radio-continuum frequencies. Interestingly, Teleios is not only almost perfectly symmetric, but it also has one of the lowest surface brightnesses discovered among Galactic SNRs and a steep spectral index of $α=-0.6\pm 0.3$. Our estimates from HI studies and the Sigma-D relation place Teleios as a type Ia SNR at a distance of either ~2.2 kpc of ~7.7 kpc. This indicates two possible scenarios, either a young (under 1000 yr) or an older SNR (over 10000 yr). With a corresponding diameter of 14/48 pc, our evolutionary studies place Teleios at the either early or late Sedov phase, depending on the distance estimate. However, our modelling also predicts X-ray emission, which we do not see in the present generation of eROSITA images. We also explored a type Iax explosion scenario that points to a much closer distance of <1 kpc and Teleios size of only ~3.3 pc, which would be similar to the only known type Iax remnant SN1181. Unfortunately, all examined scenarios have their challenges, and no definitive supernova (SN) origin type can be established at this stage. Teleios's symmetrical shape suggests expansion into a rarefied and isotropic ambient medium. The low radio surface brightness and the lack of pronounced polarisation can be explained by a high level of ambient rotation measure (RM), with the largest RM being observed at centre.

astro-ph.HE↗

arXiv2025

ASKAP observations of the radio shell in the composite supernova remnant G310.6-1.6

We report the observations of the radio shell of the supernova remnant (SNR) G310.6-1.6 at 943 MHz from the Evolutionary Map of the Universe (EMU) and the Polarization Sky Survey of the Universe's Magnetism (POSSUM) surveys by using the Australian Square Kilometre Array Pathfinder (ASKAP). We detect polarized emission from the central pulsar wind nebula (PWN) with rotation measures varying from -696 rad m$^{-2}$ to -601 rad m$^{-2}$. We measure the integrated flux density of the shell to be 36.4 +/- 2.2 mJy at 943 MHz and derive a spectral index of -0.4 +/- 0.1 for the PWN and -0.7 +/- 0.3 for the SNR shell. From the combined radio and X-ray observations, the object can be identified as a supernova explosion of about 2500 yr ago with energy of about 1.3 x 10$^{50}$ erg, suggesting an ejected mass of about 10 M_sun. The circular radio shell outside the circular hard X-ray shell is unique among Galactic SNRs. We discuss several possible scenarios, including blast wave, reverse shock, and pulsar-fed emission, but find that none of them can fully explain the observed characteristics of the shell. This poses a challenge for understanding the evolution of SNRs. The results of this paper demonstrate the potential of the ASKAP EMU and POSSUM surveys in discovering more objects of small angular size and low surface brightness.

astro-ph.GA↗

arXiv2024

Faraday tomography with CHIME: the `tadpole' feature G137+7

A direct consequence of Faraday rotation is that the polarized radio sky does not resemble the total intensity sky at long wavelengths. We analyze G137+7, which is undetectable in total intensity but appears as a depolarization feature. We use the first polarization maps from the Canadian Hydrogen Intensity Mapping Experiment. Our $400-729$ MHz bandwidth and angular resolution, $17'$ to $30'$, allow us to use Faraday synthesis to analyze the polarization structure. In polarized intensity and polarization angle maps, we find a "tail" extending $10^\circ$ from the "head" and designate the combined object the "tadpole". Similar polarization angles, distinct from the background, indicate that the head and tail are physically associated. The head appears as a depolarized ring in single channels, but wideband observations show that it is a Faraday rotation feature. Our investigations of H I and H$α$ find no connections to the tadpole. The tail suggests motion of either the gas or an ionizing star through the ISM; the B2(e) star HD 20336 is a candidate. While the head features a coherent, $\sim -8$ rad m$^2$ Faraday depth, Faraday synthesis also identifies multiple components in both the head and tail. We verify the locations of the components in the spectra using QU fitting. Our results show that $\sim$octave-bandwidth Faraday rotation observations at $\sim 600$ MHz are sensitive to low-density ionized or partially-ionized gas which is undetectable in other tracers.

astro-ph.GA↗

arXiv2023

The Rapid ASKAP Continuum Survey III: Spectra and Polarisation In Cutouts of Extragalactic Sources (SPICE-RACS) First Data Release

The Australian SKA Pathfinder (ASKAP) radio telescope has carried out a survey of the entire Southern Sky at 887.5MHz. The wide area, high angular resolution, and broad bandwidth provided by the low-band Rapid ASKAP Continuum Survey (RACS-low) allow the production of a next-generation rotation measure (RM) grid across the entire Southern Sky. Here we introduce this project as Spectral and Polarisation in Cutouts of Extragalactic sources from RACS (SPICE-RACS). In our first data release, we image 30 RACS-low fields in Stokes $I$, $Q$, $U$ at 25'' angular resolution, across 744 to 1032MHz with 1MHz spectral resolution. Using a bespoke, highly parallelised, software pipeline we are able to rapidly process wide-area spectro-polarimetric ASKAP observations. Notably, we use 'postage stamp' cutouts to assess the polarisation properties of \ncomponents\ radio components detected in total intensity. We find that our Stokes $Q$ and $U$ images have an rms noise of ~80$μ$Jy/PSF, and our correction for instrumental polarisation leakage allows us to characterise components with >1% polarisation fraction over most of the field of view. We produce a broadband polarised radio component catalogue that contains \nrms\ RM measurements over an area of ~1300deg^2 with an average error in RM of 1.6+1.1-1.0rad/m^2, and an average linear polarisation fraction 3.4+3.0-1.6%. We determine this subset of components using the conditions that the polarised signal-to-noise ratio is $>8$, the polarisation fraction is above our estimated polarised leakage, and the Stokes $I$ spectrum has a reliable model. Our catalogue provides an areal density of $4\pm2$ RMs/deg^2; an increase of $\sim4$ times over the previous state-of-the-art (Taylor et al. 2009). Meaning that, having used just 3% of the RACS-low sky area, we have produced the 3rd largest RM catalogue to date. This catalogue has broad applications for studying...

astro-ph.GA↗

arXiv2021

Multiwavelength view of SPT-CL J2106-5844. The radio galaxies and the thermal and relativistic plasmas in a massive galaxy cluster merger at z~1.13

SPT-CL J2106-5844 is among the most massive galaxy clusters at z>1 yet discovered. While initially used in cosmological tests to assess the compatibility with $Λ$CDM cosmology of such a massive virialized object at this redshift, more recent studies indicate SPT-CL J2106-5844 is undergoing a major merger, and is not an isolated system with a singular, well-defined halo. We use sensitive, high spatial resolution measurements from ALMA and ACA of the thermal SZ effect to reconstruct the pressure distribution of the intracluster medium in this system. These measurements are coupled with radio observations from the EMU pilot survey, using ASKAP and the ATCA to search for diffuse nonthermal emission. Further, to better constrain the thermodynamic structure of the cluster, we complement our analysis with reprocessed archival $Chandra$ observations. We fit the ALMA+ACA SZ data in $uv$-space using a Bayesian forward modelling technique. The ASKAP and ATCA data are processed and imaged to specifically highlight any potential diffuse radio emission. In the ALMA+ACA SZ data, we reliably identify at high significance two main gas components associated with the mass clumps inferred from weak lensing. Our statistical test excludes at the ~9.9$σ$ level the possibility of describing the system with a single SZ component. While the components had been more difficult to identify in the X-ray data alone, we find that the bimodal gas distribution is supported by the X-ray hardness distribution. The EMU radio observations reveal a diffuse radio structure ~400 kpc in projected extent along the northwest-southeast direction, indicative of strong activity from the active galactic nucleus within the brightest cluster galaxy. Interestingly, a putative optical star-forming filamentary structure detected in the HST image is in an excellent alignment with the radio structure, albeit on a smaller scale.

astro-ph.CO↗

arXiv2020

Magnetism Science with the Square Kilometre Array

The Square Kilometre Array (SKA) will answer fundamental questions about the origin, evolution, properties, and influence of magnetic fields throughout the Universe. Magnetic fields can illuminate and influence phenomena as diverse as star formation, galactic dynamics, fast radio bursts, active galactic nuclei, large-scale structure, and Dark Matter annihilation. Preparations for the SKA are swiftly continuing worldwide, and the community is making tremendous observational progress in the field of cosmic magnetism using data from a powerful international suite of SKA pathfinder and precursor telescopes. In this contribution, we revisit community plans for magnetism research using the SKA, in the light of these recent rapid developments. We focus in particular on the impact that new radio telescope instrumentation is generating, thus advancing our understanding of key SKA magnetism science areas, as well as the new techniques that are required for processing and interpreting the data. We discuss these recent developments in the context of the ultimate scientific goals for the SKA era.

astro-ph.GA↗

arXiv2006

Using the medical image processing package, ImageJ, for astronomy

At the most fundamental level, all digital images are just large arrays of numbers that can easily be manipulated by computer software. Specialized digital imaging software packages often use routines common to many different applications and fields of study. The freely available, platform independent, image-processing package ImageJ has many such functions. We highlight ImageJ's capabilities by presenting methods of processing sequences of images to produce a star trail image and a single high quality planetary image.

astro-ph↗

arXiv2006

The Fragmenting Superbubble Associated with the HII Region W4

New observations at high latitudes above the HII region W4 show that the structure formerly identified as a chimney candidate, an opening to the Galactic halo, is instead a superbubble in the process of fragmenting and possibly evolving into a chimney. Data at high Galactic latitudes (b > 5 degrees) above the W3/W4 star forming region at 1420 and 408 MHz Stokes I (total power) and 1420 MHz Stokes Q and U (linear polarization) reveal an egg-shaped structure with morphological correlations between our data and the H-alpha data of Dennison, Topasna, & Simonetti. Polarized intensity images show depolarization extending from W4 up the walls of the superbubble, providing strong evidence that the radio continuum is generated by thermal emission coincident with the H-alpha emission regions. We conclude that the parts of the HII region hitherto known as W4 and the newly revealed thermal emission are all ionized by the open cluster OCl 352. Assuming a distance of 2.35 kpc, the ovoid structure is 164 pc wide and extends 246 pc above the mid-plane of the Galaxy. The shell's emission decreases in total-intensity and polarized intensity in various locations, appearing to have a break at its top and another on one side. Using a geometric analysis of the depolarization in the shell's walls, we estimate that a magnetic field line-of-sight component of 3 to 5 uG exists in the shell. We explore the connection between W4 and the Galactic halo, considering whether sufficient radiation can escape from the fragmenting superbubble to ionize the kpc-scale H-alpha loop discovered by Reynolds, Sterling & Haffner.

astro-ph↗