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1. Early Science from POSSUM: Shocks, turbulence, and a massive new reservoir of ionised gas in the Fornax cluster

   https://doi.org/10.1017/pasa.2021.4  

   Anderson, C. S.; Heald, G. H.; Eilek, J. A.; Lenc, E.; Gaensler, B. M.; Rudnick, Lawrence; Van Eck, C. L.; O’Sullivan, S. P.; Stil, J. M.; Chippendale, A.; et al (January 2021, Publications of the Astronomical Society of Australia)

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   Abstract We present the first Faraday rotation measure (RM) grid study of an individual low-mass cluster—the Fornax cluster—which is presently undergoing a series of mergers. Exploiting commissioning data for the POlarisation Sky Survey of the Universe’s Magnetism (POSSUM) covering a $${\sim}34$$ square degree sky area using the Australian Square Kilometre Array Pathfinder (ASKAP), we achieve an RM grid density of $${\sim}25$$ RMs per square degree from a 280-MHz band centred at 887 MHz, which is similar to expectations for forthcoming GHz-frequency $${\sim}3\pi$$ -steradian sky surveys. These data allow us to probe the extended magnetoionic structure of the cluster and its surroundings in unprecedented detail. We find that the scatter in the Faraday RM of confirmed background sources is increased by $$16.8\pm2.4$$ rad m −2 within 1 $$^\circ$$ (360 kpc) projected distance to the cluster centre, which is 2–4 times larger than the spatial extent of the presently detectable X-ray-emitting intracluster medium (ICM). The mass of the Faraday-active plasma is larger than that of the X-ray-emitting ICM and exists in a density regime that broadly matches expectations for moderately dense components of the Warm-Hot Intergalactic Medium. We argue that forthcoming RM grids from both targeted and survey observations may be a singular probe of cosmic plasma in this regime. The morphology of the global Faraday depth enhancement is not uniform and isotropic but rather exhibits the classic morphology of an astrophysical bow shock on the southwest side of the main Fornax cluster, and an extended, swept-back wake on the northeastern side. Our favoured explanation for these phenomena is an ongoing merger between the main cluster and a subcluster to the southwest. The shock’s Mach angle and stand-off distance lead to a self-consistent transonic merger speed with Mach 1.06. The region hosting the Faraday depth enhancement also appears to show a decrement in both total and polarised radio emission compared to the broader field. We evaluate cosmic variance and free-free absorption by a pervasive cold dense gas surrounding NGC 1399 as possible causes but find both explanations unsatisfactory, warranting further observations. Generally, our study illustrates the scientific returns that can be expected from all-sky grids of discrete sources generated by forthcoming all-sky radio surveys. 

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2. A circular polarization survey for radio stars with the Australian SKA Pathfinder

   https://doi.org/10.1093/mnras/stab299  

   Pritchard, Joshua; Murphy, Tara; Zic, Andrew; Lynch, Christene; Heald, George; Kaplan, David L; Anderson, Craig; Banfield, Julie; Hale, Catherine; Hotan, Aidan; et al (March 2021, Monthly Notices of the Royal Astronomical Society)

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   ABSTRACT We present results from a circular polarization survey for radio stars in the Rapid ASKAP Continuum Survey (RACS). RACS is a survey of the entire sky south of δ = +41○ being conducted with the Australian Square Kilometre Array Pathfinder telescope (ASKAP) over a 288 MHz wide band centred on 887.5 MHz. The data we analyse include Stokes I and V polarization products to an RMS sensitivity of 250 μJy PSF−1. We searched RACS for sources with fractional circular polarization above 6 per cent, and after excluding imaging artefacts, polarization leakage, and known pulsars we identified radio emission coincident with 33 known stars. These range from M-dwarfs through to magnetic, chemically peculiar A- and B-type stars. Some of these are well-known radio stars such as YZ CMi and CU Vir, but 23 have no previous radio detections. We report the flux density and derived brightness temperature of these detections and discuss the nature of the radio emission. We also discuss the implications of our results for the population statistics of radio stars in the context of future ASKAP and Square Kilometre Array surveys. 

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3. The ASKAP EMU Early Science Project: radio continuum survey of the Small Magellanic Cloud

   https://doi.org/10.1093/mnras/stz2650  

   Joseph, T D; Filipović, M D; Crawford, E J; Bojičić, I; Alexander, E L; Wong, G F; Andernach, H; Leverenz, H; Norris, R P; Alsaberi, R Z; et al (November 2019, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present two new radio continuum images from the Australian Square Kilometre Array Pathfinder (ASKAP) survey in the direction of the Small Magellanic Cloud (SMC). These images are part of the Evolutionary Map of the Universe (EMU) Early Science Project (ESP) survey of the Small and Large Magellanic Clouds. The two new source lists produced from these images contain radio continuum sources observed at 960 MHz (4489 sources) and 1320 MHz (5954 sources) with a bandwidth of 192 MHz and beam sizes of 30.0 × 30.0 arcsec2 and 16.3 × 15.1 arcsec2, respectively. The median root mean square (RMS) noise values are 186 $$\mu$$Jy beam−1 (960 MHz) and 165 $$\mu$$Jy beam−1 (1320 MHz). To create point source catalogues, we use these two source lists, together with the previously published Molonglo Observatory Synthesis Telescope (MOST) and the Australia Telescope Compact Array (ATCA) point source catalogues to estimate spectral indices for the whole population of radio point sources found in the survey region. Combining our ASKAP catalogues with these radio continuum surveys, we found 7736 point-like sources in common over an area of 30 deg2. In addition, we report the detection of two new, low surface brightness supernova remnant candidates in the SMC. The high sensitivity of the new ASKAP ESP survey also enabled us to detect the bright end of the SMC planetary nebula sample, with 22 out of 102 optically known planetary nebulae showing point-like radio continuum emission. Lastly, we present several morphologically interesting background radio galaxies. 

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4. Classical novae in the ASKAP pilot surveys

   https://doi.org/10.1017/pasa.2023.21  

   Gulati, Ashna; Murphy, Tara; Kaplan, David L.; Soria, Roberto; Leung, James K.; Wang, Yuanming; Pritchard, Joshua; Lenc, Emil; Duchesne, Stefan W.; O’Brien, Andrew (January 2023, Publications of the Astronomical Society of Australia)

   Abstract We present a systematic search for radio counterparts of novae using the Australian Square Kilometer Array Pathfinder (ASKAP). Our search used the Rapid ASKAP Continuum Survey, which covered the entire sky south of declination $$+41^{\circ}$$ ( $$\sim$$ $34000$ square degrees) at a central frequency of 887.5 MHz, the Variables and Slow Transients Pilot Survey, which covered $$\sim$$ $5000$ square degrees per epoch (887.5 MHz), and other ASKAP pilot surveys, which covered $$\sim$$ 200–2000 square degrees with 2–12 h integration times. We crossmatched radio sources found in these surveys over a two–year period, from 2019 April to 2021 August, with 440 previously identified optical novae, and found radio counterparts for four novae: V5668 Sgr, V1369 Cen, YZ Ret, and RR Tel. Follow-up observations with the Australian Telescope Compact Array confirm the ejecta thinning across all observed bands with spectral analysis indicative of synchrotron emission in V1369 Cen and YZ Ret. Our light-curve fit with the Hubble Flow model yields a value of $$1.65\pm 0.17 \times 10^{-4} \rm \:M_\odot$$ for the mass ejected in V1369 Cen. We also derive a peak surface brightness temperature of $$250\pm80$$ K for YZ Ret. Using Hubble Flow model simulated radio lightcurves for novae, we demonstrate that with a 5 $$\sigma$$ sensitivity limit of 1.5 mJy in 15-min survey observations, we can detect radio emission up to a distance of 4 kpc if ejecta mass is in the range $$10^{-3}\rm \:M_\odot$$ , and upto 1 kpc if ejecta mass is in the range $$10^{-5}$$ – $$10^{-3}\rm \:M_\odot$$ . Our study highlights ASKAP’s ability to contribute to future radio observations for novae within a distance of 1 kpc hosted on white dwarfs with masses $0.4$ – $$1.25\:\rm M_\odot$$ , and within a distance of 4 kpc hosted on white dwarfs with masses $0.4$ – $$1.0\:\rm M_\odot$$ . 

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5. A comprehensive search for the radio counterpart of GW190814 with the Australian Square Kilometre Array Pathfinder

   https://doi.org/10.1093/mnras/stab3628  

   Dobie, D.; Stewart, A.; Hotokezaka, K.; Murphy, Tara; Kaplan, D. L.; Buckley, D. A. H.; Cooke, J.; Ho, A. Y. Q.; Lenc, E.; Leung, J. K.; et al (December 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present results from a search for the radio counterpart to the possible neutron star–black hole merger GW190814 with the Australian Square Kilometre Array Pathfinder. We have carried out 10 epochs of observation spanning 2–655 d post-merger at a frequency of 944 MHz. Each observation covered 30 deg2, corresponding to 87 per cent of the posterior distribution of the merger’s sky location. We conducted an untargeted search for radio transients in the field, as well as a targeted search for transients associated with known galaxies. We find one radio transient, ASKAP J005022.3−230349, but conclude that it is unlikely to be associated with the merger. We use our observations to place constraints on the inclination angle of the merger and the density of the surrounding environment by comparing our non-detection to model predictions for radio emission from compact binary coalescences. This survey is also the most comprehensive widefield search (in terms of sensitivity and both areal and temporal coverage) for radio transients to-date and we calculate the radio transient surface density at 944 MHz. 

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