Stokes QU-fitting is a powerful tool for interpreting spectropolarimetric radio continuum observations by linking them to physical models, enabling estimates of the magnetic fields in, for example, the Milky Way, galaxy clusters, and radio jets. We present a comprehensive investigation into the effectiveness and limitations of QU-fitting within the ASKAP Polarization Sky Survey of the Universe's Magnetism (POSSUM) survey frequency ranges (800-1088 MHz and 1296-1440 MHz) with projections to other spectropolarimetric radio observations. We simulate different physical polarization sources: Faraday simple, Burn slab, internal turbulence, external turbulence, and two-component models in the POSSUM frequencies, and assess their observational degeneracies and fit accuracies. Our results highlight the model-dependent nature of reliable fitting and identify specific regions of parameter space where model selection, and therefore characterization of the physical medium, becomes ambiguous. For QU-fitting we find the Bayes factor, computed using the marginal likelihood, outperforms more traditionally used goodness-of-fit metrics such as Bayesian Information Criterion, Akaike Information Criterion, and X-2 for model selection. We provide empirical relationships to delineate the boundaries where model distinguishability is impossible. Finally, we evaluate how accurately QU-fitting recovers model parameters and their associated uncertainties, thereby assessing its ability to correctly characterize the Faraday-rotating medium in both point and extended sources in Faraday depth space.

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 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 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 and Polarization Sky Survey of the Universe's Magnetism surveys with the Australian Square Kilometre Array Pathfinder. 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 alpha 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.

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 and can be used with data from a wide range of telescopes.

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.

The Polarisation Sky Survey of the Universe's Magnetism (POSSUM) will conduct a sensitive $\sim$1 GHz radio polarization survey covering 20 000 square degrees of the Southern sky with the Australian Square Kilometre Array Pathfinder (ASKAP). In anticipation of the full survey, we analyze pilot observations of low-band (800-1087 MHz), mid-band (1316-1439 MHz), and combined-band observations for an extragalactic field and a Galactic-plane field (low-band only). Using the POSSUM processing pipeline, we produce prototype RM catalogs that are filtered to construct prototype RM grids. We assess typical RM grid densities and RM uncertainties and their dependence on frequency, bandwidth, and Galactic latitude. We present a median filter method for separating foreground diffuse emission from background components, and find that after application of the filter, 99.5% of measured RMs of simulated sources are within 3$\sigma$ of their true RM, with a typical loss of polarized intensity of 5% $\pm$ 5%. We find RM grid densities of 35.1, 30.6, 37.2, and 13.5 RMs per square degree and median uncertainties on RM measurements of 1.55, 12.82, 1.06, and 1.89 rad m$^{-2}$ for the median-filtered low-band, mid-band, combined-band, and Galactic observations, respectively. We estimate that the full POSSUM survey will produce an RM catalog of $\sim$775 000 RMs with median-filtered low-band observations and $\sim$877 000 RMs with median-filtered combined-band observations. We construct a structure function from the Galactic RM catalog, which shows a break at $0.7^{\circ}$, corresponding to a physical scale of 12-24 pc for the nearest spiral arm.

AbstractThe Australian SKA Pathfinder (ASKAP) radio telescope has carried out a survey of the entire Southern Sky at 887.5 MHz. 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 StokesI,Q,Uat 25$^{\prime\prime}$angular resolution, across 744–1032 MHz with 1 MHz 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 105912 radio components detected in total intensity. We find that our StokesQandUimages have anrmsnoise of$\sim$80$\unicode{x03BC}$Jy PSF$^{-1}$, and our correction for instrumental polarisation leakage allows us to characterise components with$\gtrsim$1% polarisation fraction over most of the field of view. We produce a broadband polarised radio component catalogue that contains 5818 RM measurements over an area of$\sim$1300 deg$^{2}$with an average error in RM of$1.6^{+1.1}_{-1.0}$rad 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 StokesIspectrum has a reliable model. Our catalogue provides an areal density of$4\pm2$RMs deg$^{-2}$; an increase of$\sim$4 times over the previous state-of-the-art (Taylor, Stil, Sunstrum 2009, ApJ, 702, 1230). 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 astrophysical magnetic fields; notably revealing remarkable structure in the Galactic RM sky. We will explore this Galactic structure in a follow-up paper. We will also apply the techniques described here to produce an all-Southern-sky RM catalogue from RACS observations. Finally, we make our catalogue, spectra, images, and processing pipeline publicly available.

Neutral hydrogen (H i) bubbles and shells are common in the interstellar medium (ISM). Studying their properties provides insight into the characteristics of the local ISM as well as the galaxy in which the bubbles reside. We report the detection of magnetic fields associated with superbubbles in the nearby irregular galaxy, the Small Magellanic Cloud (SMC). Using the Polarisation Sky Survey of the Universe's Magnetism (POSSUM) pilot survey, we obtain a high-density grid (approximate to 25 sources, deg(-2)) of Faraday rotation measure (RM) from polarized sources behind the SMC. This provides a sufficiently large number of RM measurements to study the magnetic properties of three of the largest H i shells previously identified in the SMC. The RM profiles as a function of distance from the shell centre show characteristic patterns at angular scales comparable to the shell size. We demonstrate that this can be explained by magnetohydrodynamic simulation models of bubbles expanding in magnetized environments. From the observations, we estimate the line-of-sight magnetic field strength at the edges of the shells is enhanced by similar to 1 mu G with respect to their centres. This is an order of magnitude larger than the field strength in the ambient medium (similar to 1 mu G) estimated based on the expansion velocity of the shells. This paper highlights the power of densely mapped RM grids in studying the magnetic properties of galactic substructures beyond the Milky Way.

Abstract We identified discrepancies between the rotation measure (RM) catalogs by Clegg et al. and Minter & Spangler and their corresponding rows in the Van Eck et al. consolidated catalog. The discrepancies, in the case of Minter & Spangler, are caused by the improper coordinate conversions between J2000 and B1950 Equatorial coordinate systems used between the catalogs. The discrepancies associated with Clegg et al. are due to differences in the number of significant figures reported between the original and consolidated catalogs, which result in overlapping points in the consolidated catalog. These errors affect studies that rely on the accuracy of the coordinate data. After proper unit conversions, we found that coordinate values differed by up to 1° between the Minter & Spangler and consolidated catalogs. Employing a combination of manual data extraction and automated coordinate conversion tools, we corrected and updated those RM coordinate values to align with the standardized format used in the Van Eck et al. consolidated catalog.

ABSTRACT Measurements of the polarization of radio emission are subject to a number of depolarization effects such as bandwidth depolarization, which is caused by the averaging effect of a finite channel bandwidth combined with the frequency-dependent polarization caused by Faraday rotation. There have been very few mathematical treatments of bandwidth depolarization, especially in the context of the rotation measure (RM) synthesis method for analysing radio polarization data. We have found a simple equation for predicting if bandwidth depolarization is significant for a given observational configuration. We have derived and tested three methods of modifying RM synthesis to correct for bandwidth depolarization. From these tests we have developed a new algorithm that can detect bandwidth-depolarized signals with higher signal-to-noise ratio than conventional RM synthesis and recover the correct source polarization properties (RM and polarized intensity). We have verified that this algorithm works as expected with real data from the LOw-Frequency ARray (LOFAR) Two-metre Sky Survey. To make this algorithm available to the community, we have added it as a new tool in the RM-Tools polarization analysis package.

A Faraday rotation measure (RM) catalogue, or RM Grid, is a valuable resource for the study of cosmic magnetism. Using the second data release (DR2) from the LOFAR Two-metre Sky Survey (LoTSS), we have produced a catalogue of 2461 extragalactic high-precision RM values across 5720 deg(2) of sky (corresponding to a polarized source areal number density of similar to 0.43 deg(-2)). The linear polarization and RM properties were derived using RM synthesis from the Stokes Q and U channel images at an angular resolution of 20 arcsec across a frequency range of 120 to 168 MHz with a channel bandwidth of 97.6 kHz. The fraction of total intensity sources (>1 mJy beam(-1)) found to be polarized was similar to 0.2 per cent. The median detection threshold was 0.6 mJy beam(-1) (8 sigma(QU)), with a median RM uncertainty of 0.06 rad m(-2) (although a systematic uncertainty of up to 0.3 rad m(-2) is possible, after the ionosphere RM correction). The median degree of polarization of the detected sources is 1.8 per cent, with a range of 0.05 per cent to 31 per cent. Comparisons with cm-wavelength RMs indicate minimal amounts of Faraday complexity in the LoTSS detections, making them ideal sources for RM Grid studies. Host galaxy identifications were obtained for 88 per cent of the sources, along with redshifts for 79 per cent (both photometric and spectroscopic), with the median redshift being 0.6. The focus of the current catalogue was on reliability rather than completeness, and we expect future versions of the LoTSS RM Grid to have a higher areal number density. In addition, 25 pulsars were identified, mainly through their high degrees of linear polarization.

ABSTRACT We use data from the pilot observations of the EMU/POSSUM surveys to study the ‘missing supernova remnant (SNR) problem’, the discrepancy between the number of Galactic SNRs that have been observed, and the number that are estimated to exist. The Evolutionary Map of the Universe (EMU) and the Polarization Sky Survey of the Universe’s Magnetism (POSSUM) are radio sky surveys that are conducted using the Australian Square Kilometre Array Pathfinder (ASKAP). We report on the properties of seven known SNRs in the joint Galactic pilot field, with an approximate longitude and latitude of 323° ≤ l ≤ 330° and −4° ≤ b ≤ 2°, respectively, and identify 21 SNR candidates. Of these, four have been previously identified as SNR candidates, three were previously listed as a single SNR, 13 have not been previously studied, and one has been studied in the infrared. These are the first discoveries of Galactic SNR candidates with EMU/POSSUM and, if confirmed, they will increase the SNR density in this field by a factor of 4. By comparing our SNR candidates to the known Galactic SNR population, we demonstrate that many of these sources were likely missed in previous surveys due to their small angular size and/or low surface brightness. We suspect that there are SNRs in this field that remain undetected due to limitations set by the local background and confusion with other radio sources. The results of this paper demonstrate the potential of the full EMU/POSSUM surveys to uncover more of the missing Galactic SNR population.

The study of fast radio bursts (FRBs) is of great importance, and is a topic that has been extensively researched, particularly in recent years. While the extreme nature of FRBs can serve as a tool for researchers to probe the intergalactic medium and study exotic aspects of the Universe, keeping track of FRB properties is challenged by the frequent detection of new bursts. We introduce the FRBSTATS platform, which provides an up-to-date and user-friendly web interface to an open-access catalogue of published FRBs, along with a statistical overview of the observed events. The platform supports the retrieval of fundamental FRB data either directly through the FRBSTATS API, or in the form of a CSV/JSON-parsed database, while enabling the plotting of parameters and their distributions, for a variety of visualizations. These features allow researchers to conduct population studies and comparisons with astrophysical models, describing the origin and emission mechanism behind these sources. So far, the inferred redshift estimates of 813 bursts have been computed, providing the first public database that includes redshift estimates inferred from dispersion measure entries for nearly all observed FRBs, as well as host redshifts (where available). Lastly, the platform provides a visualization tool that illustrates associations between primary bursts and repeaters, complementing basic repeater information provided by the Transient Name Server. In this work, we present the structure of the platform, the established version control system, as well as the strategy for keeping such an open database up to date. Additionally, we introduce a novel, computationally-efficient, clustering-based approach that enables unsupervised classification of hundreds of bursts into repeaters and non-repeaters, resulting in the discovery of one new FRB repeater.

Faraday rotation measure (RM) is arguably the most practical observational tracer of magnetic fields in the diffuse circumgalactic medium (CGM). We sample synthetic Faraday rotation skies of Milky Way-like galaxies in TNG50 of the IllustrisTNG project by placing an observer inside the galaxies at a solar circle-like position. Our synthetic RM grids emulate specifications of current and upcoming surveys; the NRAO VLA Sky Survey (NVSS), the Polarisation Sky Survey of the Universe's Magnetism (POSSUM), and a future Square Kilometre Array (SKA1-mid) polarisation survey. It has been suggested that magnetic fields regulate the survival of high-velocity clouds. However, there is only a small number of observational detections of magnetised clouds thus far. In the first part of the paper, we test conditions for the detection of magnetised circumgalactic clouds. Based on the synthetic RM samplings of clouds in the simulations, we predict upcoming polarimetric surveys will open opportunities for the detection of even low-mass and distant clouds. In the second part of the paper, we investigate the imprint of the CGM in the all-sky RM distribution. We test whether the RM variation produced by the CGM is correlated with global galaxy properties, such as distance to a satellite, specific star formation rate, neutral hydrogen covering fraction, and accretion rate to the supermassive black hole. We argue that the observed fluctuation in the RM measurements on scales less than 1 degree, which has been considered an indication of intergalactic magnetic fields, might in fact incorporate a significant contribution of the Milky Way CGM.

Faraday rotation measures (RMs) have been used for many studies of cosmic magnetism, and in most cases having more RMs is beneficial for those studies. This has lead to development of RM surveys that have produced large catalogs, as well as meta-catalogs collecting RMs from many different publications. However, it has been difficult to take full advantage of all these RMs as the individual catalogs have been published in many different places, and in many different formats. In addition, the polarization spectra used to determine these RMs are rarely published, limiting the ability to re-analyze data as new methods or additional observations become available. We propose a standard convention for RM catalogs, RMTable2023, and a standard for source-integrated polarized spectra of radio sources, PolSpectra2023. These standards are intended to maximize the value and utility of these data for researchers and to make them easier to access. To demonstrate the use of the RMTable2023 standard, we have produced a consolidated catalog of 55 819 RMs collected from 42 published catalogs.

This is a catalog of Faraday rotation measures (and other related properties) of astronomical radio sources, consolidated from many published catalogs in the astronomical literature from 1980 to the present day. These catalogs have been converted to the RMTable standard and stored in 3 formats: FITS binary table, tab-seperated-value ASCII, and VOTable XML. These catalog files can be read by any suitable reader, but we have created a Python module, RMTable (https://github.com/CIRADA-Tools/RMTable), which streamlines the process of interacting with and creating new RMTables.

This work gives an update to existing reconstructions of the Galactic Faraday rotation sky by processing almost all Faraday rotation data sets available at the end of the year 2020. Observations of extra-Galactic sources in recent years have, among other regions, further illuminated the previously under-constrained southern celestial sky, as well as parts of the inner disc of the Milky Way. This has culminated in an all-sky data set of 55,190 data points, which is a significant expansion on the 41,330 used in previous works, hence making an updated separation of the Galactic component a promising venture. The increased source density allows us to present our results in a resolution of about $1.3\cdot 10^{-2}\, \mathrm{deg}^2$ ($46.8\,\mathrm{arcmin}^2$), which is a twofold increase compared to previous works. As for previous Faraday rotation sky reconstructions, this work is based on information field theory, a Bayesian inference scheme for field-like quantities which handles noisy and incomplete data. In contrast to previous reconstructions, we find a significantly thinner and pronounced Galactic disc with small-scale structures exceeding values of several thousand $\mathrm{rad}\,\mathrm{m}^{-2}$. The improvements can mainly be attributed to the new catalog of Faraday data, but are also supported by advances in correlation structure modeling within numerical information field theory. We furthermore give a detailed discussion on statistical properties of the Faraday rotation sky and investigate correlations to other data sets.

Understanding how the Galactic magnetic field threads the multiphase interstellar medium (ISM) remains a considerable challenge, as different magnetic field tracers probe dissimilar phases and field components. We search for evidence of a common magnetic field shared between the ionized and neutral ISM by comparing 1.4 GHz radio continuum polarization and H i line emission from the Galactic Arecibo L -Band Feed Array Continuum Transit Survey (GALFACTS) and Galactic Arecibo L -Band Feed Array H i (GALFA-H i ) survey, respectively. We compute the polarization gradient of the continuum emission and search for associations with diffuse/translucent H i structures. The polarization gradient is sensitive to changes in the integrated product of the thermal electron density and line-of-sight field strength ( B ∥ ) in warm ionized gas, while narrow H i structures highlight the plane-of-sky field orientation in cold neutral gas. We identified one region in the high Galactic latitude Arecibo sky, G216+26 centered on ( ℓ , b ) ∼ (216°, +26°), containing filaments in the polarization gradient that are aligned with narrow H i structures roughly parallel to the Galactic plane. We present a comparison of multiphase observations and magnetic field tracers of this region, demonstrating that the warm ionized and cold neutral media are connected likely via a common magnetic field. We quantify the physical properties of a polarization gradient filament associated with H α emission, measuring a line-of-sight field strength B ∥ = 6 ± 4 μ G and a plasma beta β = 2.1 − 2.1 + 3.1 . We discuss the lack of widespread multiphase magnetic field alignments and consider whether this region is associated with a short-timescale or physically rare phenomenon. This work highlights the utility of multitracer analyses for understanding the magnetized ISM.

Context. Faraday tomography of a field centred on the extragalactic point source 3C 196 with the LOw Frequency ARray (LOFAR) revealed an intertwined structure of diffuse polarised emission with straight depolarisation canals and tracers of the magnetised and multi-phase interstellar medium (ISM), such as dust and line emission from atomic hydrogen (HI). Aims. This study aims at extending the multi-tracer analysis of LOFAR data to three additional fields in the surroundings of the 3C 196 field. For the first time, we study the three-dimensional structure of the LOFAR emission by determining the distance to the depolarisation canals. Methods. We used the rolling Hough transform to compare the orientation of the depolarisation canals with that of the filamentary structure seen in HI, and based on starlight and dust polarisation data, with that of the plane-of-the-sky magnetic field. Stellar parallaxes from Gaia complemented the starlight polarisation with the corresponding distances. Results. Faraday tomography of the three fields shows a rich network of diffuse polarised emission at Faraday depths between − 10 and + 15 rad m−2. A complex system of straight depolarisation canals resembles that of the 3C 196 field. The depolarisation canals align both with the HI filaments and with the magnetic field probed by dust. The observed alignment suggests that an ordered magnetic field organises the multiphase ISM over a large area (~20°). In one field, two groups of stars at distances below and above 200 pc, respectively, show distinct magnetic field orientations. These are both comparable with the orientations of the depolarisation canals in the same field. We conclude that the depolarisation canals likely trace the same change in the magnetic field as probed by the stars, which corresponds to the edge of the Local Bubble.