Search

Neutral hydrogen (HI) 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 ($\approx 25 \,\rm sources\,deg^{-2}$) 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 HI 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 magneto-hydrodynamic simulation models of bubbles expanding in magnetised environments. From the observations, we estimate the line-of-sight magnetic field strength at the edges of the shells is enhanced by $\sim1\,\rm \mu G$ with respect to their centres. This is an order of magnitude larger than the field strength in the ambient medium ($\sim 0.1\,\rm \mu G$) 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., Comment: 15 pages, 7 figures, Accepted for publication in MNRAS

We present the first results from a new backend on the Australian Square Kilometre Array Pathfinder, the Commensal Realtime ASKAP Fast Transient COherent (CRACO) upgrade. CRACO records millisecond time resolution visibility data, and searches for dispersed fast transient signals including fast radio bursts (FRB), pulsars, and ultra-long period objects (ULPO). With the visibility data, CRACO can localise the transient events to arcsecond-level precision after the detection. Here, we describe the CRACO system and report the result from a sky survey carried out by CRACO at 110ms resolution during its commissioning phase. During the survey, CRACO detected two FRBs (including one discovered solely with CRACO, FRB 20231027A), reported more precise localisations for four pulsars, discovered two new RRATs, and detected one known ULPO, GPM J1839-10, through its sub-pulse structure. We present a sensitivity calibration of CRACO, finding that it achieves the expected sensitivity of 11.6 Jy ms to bursts of 110 ms duration or less. CRACO is currently running at a 13.8 ms time resolution and aims at a 1.7 ms time resolution before the end of 2024. The planned CRACO has an expected sensitivity of 1.5 Jy ms to bursts of 1.7 ms duration or less, and can detect 10x more FRBs than the current CRAFT incoherent sum system (i.e., 0.5-2 localised FRBs per day), enabling us to better constrain he models for FRBs and use them as cosmological probes., Comment: 26 pages, 19 figures, 9 tables, Accepted for publication in PASA

We study the radio continuum emission of four galaxies experiencing ram-pressure stripping in four clusters of the Shapley supercluster at redshift z~0.05. Multi-band (235-1367 MHz) radio data, complemented by integral-field spectroscopy, allow us to detect and analyse in detail the non-thermal component both in the galaxy discs and the radio continuum tails. Three galaxies present radio continuum tails which are tens of kiloparsecs long. By deriving the radio spectral index in the inner and outer tails and comparing our findings with the distribution of the extraplanar ionised gas and the results of N-body/hydrodynamical simulations, we demonstrate that these tails are caused by the ram pressure which, together with the ionised gas, sweeps the magnetic field from the galaxy discs. We suggest that the radio continuum emission in these tails can be differently powered by (i) in situ star formation; (ii) relativistic electrons stripped from the disc; (iii) shock excitation or a combination of them. All the ram-pressure stripped galaxies are found in environments where cluster-cluster interactions occurred and/or are ongoing thus strongly supporting the thesis that cluster and group collisions and mergers may locally increase the ram pressure and trigger hydrodynamical interactions between the intracluster medium and the interstellar medium of galaxies., Comment: 18 pages, 14 figures, accepted for publication in MNRAS

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., Comment: 51 pages, 19 figures, 5 tables

We present the detailed analysis of the thermal, diffuse emission of the proto-intracluster medium (ICM) detected in the halo of the Spiderweb Galaxy at z=2.16, within a radius of $\sim$ 150 kpc. We combined deep X-ray data from Chandra and millimeter observations of the Sunyaev-Zeldovich (SZ) effect obtained by ALMA. Thanks to independent measurements of the pressure profile from ALMA SZ observation and the electron density profile from the available X-ray data, we derived, for the first time, the temperature profile in the ICM of a z>2 protocluster. It reveals the presence of a strong cool core (comparable to the local ones) that may host a significant mass deposition flow, consistent with measured local star formation values. We also find mild evidence of an asymmetry in the X-ray surface brightness distribution, which may be tentatively associated with a cavity carved into the proto-ICM by the radio jets or, alternatively, may be due to the young dynamical status of the halo. The cooling time of baryons in the core of the Spiderweb Protocluster is estimated to be $\sim$ 0.1 Gyr, implying that the baryon cycle in the first stages of the protocluster formation is characterised by a high-duty cycle and a very active environment. In the case of the Spiderweb protocluster, we are witnessing the presence of a strongly peaked core that is possibily hosting a cooling flow with a mass deposition rate up to 250-1000 $M_{\odot}$/yr, responsible for feeding both the central supermassive black hole and the high star formation rate observed in the Spiderweb Galaxy. This phase is expected to be rapidly followed by active galactic nucleus feedback events, whose onset may have already left an imprint in the radio and X-ray appearance of the Spiderweb protocluster, eventually driving the ICM into a self-regulated, long-term evolution in less than one Gyr.

We present WALLABY pilot data release 1, the first public release of HI pilot survey data from the Wide-field ASKAP L-band Legacy All-sky Blind Survey (WALLABY) on the Australian Square Kilometre Array Pathfinder. Phase 1 of the WALLABY pilot survey targeted three $60~{\rm deg}^2$ regions on the sky in the direction of the Hydra and Norma galaxy clusters and the NGC 4636 galaxy group, covering the redshift range of z < 0.08. The source catalogue, images and spectra of nearly 600 extragalactic HI detections and kinematic models for 109 spatially resolved galaxies are available. As the pilot survey targeted regions containing nearby group and cluster environments, the median redshift of the sample of z ~ 0.014 is relatively low compared to the full WALLABY survey. The median galaxy HI mass is $2.3 \times 10^{9}~M_{\odot}$. The target noise level of 1.6 mJy per $30''$ beam and 18.5 kHz channel translates into a $5\sigma$ HI mass sensitivity for point sources of about $5.2 \times 10^{8} \, (D_{\rm L} / \mathrm{100~Mpc})^{2} \, M_{\odot}$ across 50 spectral channels (~200 km/s) and a $5\sigma$ HI column density sensitivity of about $8.6 \times 10^{19} \, (1 + z)^{4}~\mathrm{cm}^{-2}$ across 5 channels (~20 km/s) for emission filling the $30''$ beam. As expected for a pilot survey, several technical issues and artefacts are still affecting the data quality. Most notably, there are systematic flux errors of up to several 10% caused by uncertainties about the exact size and shape of each of the primary beams as well as the presence of sidelobes due to the finite deconvolution threshold. In addition, artefacts such as residual continuum emission and bandpass ripples have affected some of the data. The pilot survey has been highly successful in uncovering such technical problems, most of which are expected to be addressed and rectified before the start of the full WALLABY survey., Comment: 24 pages, 13 figures, 5 tables, accepted for publication in PASA

We present the X-ray imaging and spectral analysis of the diffuse emission around the Spiderweb galaxy at z=2.16 and of its nuclear emission, based on a deep (700 ks) Chandra observation. We characterize the nuclear emission and computed the contamination in the surrounding regions due to the wings of the instrument PSF. Then, we quantified the extended emission within 12". We find that the Spiderweb galaxy hosts a mildly absorbed quasar, with modest yet significant variability on a timescale of ~1 year. We find that the emission in the jet regions is well described by a power law with Gamma~2-2.5, and it is consistent with IC upscattering of the CMB photons by the relativistic electrons. We also find a roughly symmetric, diffuse emission within a radius of ~100 kpc. This emission is consistent with thermal bremsstrahlung from a hot ICM with a temperature of kT=2.0_{-0.4}^{+0.7} keV, and a metallicity of Z<1.6Z_sun. The average electron density within 100 kpc is n_e=(1.51+-0.24+-0.14)E-2 cm^{-3}, corresponding to an upper limit for the total ICM mass of <=(1.76+-0.30+-0.17)E+12 M_sun (where error bars are 1 sigma statistical and systematic, respectively). If we apply hydrostatic equilibrium to the ICM, we measure a total gravitational mass M(<100 kpc)=(1.5^{+0.5}_{-0.3})E+13 M_sun and, extrapolating at larger radii, we estimate a total mass M_{500}=(3.2^{+1.1}_{-0.6})E+13 M_sun within a radius of r_{500}=(220+-30) kpc. We conclude that the Spiderweb protocluster shows significant diffuse emission within a radius of 12 arcsec, whose major contribution is provided by IC scattering associated with the radio jets. Outside the jet regions, we also identified thermal emission within a radius of ~100 kpc, revealing the presence of hot, diffuse baryons that may represent the embryonic virialized halo of the forming cluster., Comment: 19 pages, 16 figures, accepted in Astronomy & Astrophysics

(Abridged) We present an analysis of the 700 ks Chandra ACIS-S observation of the field around the Spiderweb Galaxy at z=2.156, focusing on the nuclear activity in the associated large-scale environment. We identify unresolved X-ray sources down to flux limits of 1.3X10^{-16} and 3.9X10^{-16} erg/s/cm^2 in the soft and hard band, respectively. We search for counterparts in the optical, NIR and submm bands to identify X-ray sources belonging to the protocluster. We detect 107 X-ray unresolved sources within 5 arcmin (corresponding to 2.5 Mpc) of J1140-2629, among which 13 have optical counterparts with spectroscopic redshift 2.11~1.84+-0.04. The best-fit intrinsic absorption for 5 protocluster X-ray members is N_H>10^{23} cm^{-2}, while other 6 have upper limits of the order of fewX10^{22} cm^{-2}. Two sources can only be fitted with very flat \Gamma<=1, and are therefore considered Compton-thick candidates. Their 0.5-10 keV rest frame luminosities are larger than 2X10^{43} erg/s, significantly greater than X-ray luminosities expected from star formation activity. The X-ray luminosity function of AGN in the volume associated to the Spiderweb protocluster in the range 10^{43}10.5, corresponding to an enhancement of 6.0^{+9.0}_{-3.0} with respect to the COSMOS field at comparable redshifts and stellar mass range. We conclude that the galaxy population in the Spiderweb Protocluster is characterized by enhanced X-ray nuclear activity triggered by environmental effects on Mpc scales., Comment: Astronomy & Astrophysics. Minor changes in this version. Version accepted in A&A

We present the most sensitive and detailed view of the neutral hydrogen (HI) emission associated with the Small Magellanic Cloud (SMC), through the combination of data from the Australian Square Kilometre Array Pathfinder (ASKAP) and Parkes (Murriyang), as part of the Galactic Australian Square Kilometre Array Pathfinder (GASKAP) pilot survey. These GASKAP-HI pilot observations, for the first time, reveal HI in the SMC on similar physical scales as other important tracers of the interstellar medium, such as molecular gas and dust. The resultant image cube possesses an rms noise level of 1.1 K (1.6 mJy/beam) per 0.98 km s$^{-1}$ spectral channel with an angular resolution of 30$''$ ($\sim$10 pc). We discuss the calibration scheme and the custom imaging pipeline that utilizes a joint deconvolution approach, efficiently distributed across a computing cluster, to accurately recover the emission extending across the entire $\sim$25 deg$^2$ field-of-view. We provide an overview of the data products and characterize several aspects including the noise properties as a function of angular resolution and the represented spatial scales by deriving the global transfer function over the full spectral range. A preliminary spatial power spectrum analysis on individual spectral channels reveals that the power-law nature of the density distribution extends down to scales of 10 pc. We highlight the scientific potential of these data by comparing the properties of an outflowing high velocity cloud with previous ASKAP+Parkes HI test observations., Comment: Accepted for publication in PASA, 34 pages, 18 figures, 5 tables

The Variables and Slow Transients Survey (VAST) on the Australian Square Kilometre Array Pathfinder (ASKAP) is designed to detect highly variable and transient radio sources on timescales from 5 seconds to $\sim 5$ years. In this paper, we present the survey description, observation strategy and initial results from the VAST Phase I Pilot Survey. This pilot survey consists of $\sim 162$ hours of observations conducted at a central frequency of 888~MHz between 2019 August and 2020 August, with a typical rms sensitivity of 0.24~mJy~beam$^{-1}$ and angular resolution of $12-20$ arcseconds. There are 113 fields, \red{each of which was observed for 12 minutes integration time}, with between 5 and 13 repeats, with cadences between 1 day and 8 months. The total area of the pilot survey footprint is 5\,131 square degrees, covering six distinct regions of the sky. An initial search of two of these regions, totalling 1\,646 square degrees, revealed 28 highly variable and/or transient sources. Seven of these are known pulsars, including the millisecond pulsar J2039--5617. Another seven are stars, four of which have no previously reported radio detection (SCR~J0533--4257, LEHPM~2-783, UCAC3~89--412162 and 2MASS J22414436--6119311). Of the remaining 14 sources, two are active galactic nuclei, six are associated with galaxies and the other six have no multiwavelength counterparts and are yet to be identified.

We present the data and initial results from the first Pilot Survey of the Evolutionary Map of the Universe (EMU), observed at 944 MHz with the Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The survey covers 270 \sqdeg of an area covered by the Dark Energy Survey, reaching a depth of 25--30 \ujybm\ rms at a spatial resolution of $\sim$ 11--18 arcsec, resulting in a catalogue of $\sim$ 220,000 sources, of which $\sim$ 180,000 are single-component sources. Here we present the catalogue of single-component sources, together with (where available) optical and infrared cross-identifications, classifications, and redshifts. This survey explores a new region of parameter space compared to previous surveys. Specifically, the EMU Pilot Survey has a high density of sources, and also a high sensitivity to low surface-brightness emission. These properties result in the detection of types of sources that were rarely seen in or absent from previous surveys. We present some of these new results here., Comment: Accepted by PASA

We present a comprehensive multi-frequency catalogue of radio sources behind the Large Magellanic Cloud between 0.2 and 20 GHz, gathered from a combination of new and legacy radio continuum surveys. This catalogue covers an area of $\sim$144~deg$^2$ at angular resolutions from 45 arcsec to $\sim$3 arcmin. We find 6434 discrete radio sources in total, of which 3789 are detected at two or more radio frequencies. We estimate the median spectral index ($\alpha$; where $S_{v}\sim\nu^\alpha$) of $\alpha = -0.89 $ and mean of $-0.88 \pm 0.48$ for 3636 sources detected exclusively at two frequencies (0.843 and 1.384 GHz) with similar resolution (FWHM $\sim$40-45 arcsec). The large frequency range of the surveys makes it an effective tool to investigate Gigahertz Peak Spectrum (GPS), Compact Steep Spectrum (CSS) and Infrared Faint Radio sources populations within our sample. We find 10 GPS candidates with peak frequencies near 5 GHz, from which we estimate their linear size. 1866 sources from our catalogue are (CSS) candidates with $\alpha <-0.8$. We found six candidates for High Frequency Peaker (HFP) sources, whose radio fluxes peak above 5 GHz and no sources with unconstrained peaks and $\alpha~>0.5$. We found optical counterparts for 343 of the radio continuum sources, of which 128have a redshift measurement. Finally, we investigate the population of 123 Infrared Faint Radio Sources (IFRSs) found in this study., Comment: Accepted for publication in MNRAS

In this paper we describe the system design and capabilities of the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope at the conclusion of its construction project and commencement of science operations. ASKAP is one of the first radio telescopes to deploy phased array feed (PAF) technology on a large scale, giving it an instantaneous field of view that covers 31 square degrees at 800 MHz. As a two-dimensional array of 36x12m antennas, with baselines ranging from 22m to 6km, ASKAP also has excellent snapshot imaging capability and 10 arcsecond resolution. This, combined with 288 MHz of instantaneous bandwidth and a unique third axis of rotation on each antenna, gives ASKAP the capability to create high dynamic range images of large sky areas very quickly. It is an excellent telescope for surveys between 700 MHz and 1800 MHz and is expected to facilitate great advances in our understanding of galaxy formation, cosmology and radio transients while opening new parameter space for discovery of the unknown., Comment: 38 pages, 24 figures, accepted for publication in PASA

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 $\sim34$ square degree sky area using the Australian Square Kilometre Array Pathfinder (ASKAP), we achieve an RM grid density of $\sim25$ RMs per square degree from a 280 MHz band centred at 887 MHz, which is similar to expectations for forthcoming GHz-frequency all-sky surveys. We thereby probe the extended magnetoionic structure of the cluster 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 degree (360 kpc) projected distance to the cluster centre, which is 2--4 times more extended than the presently-detectable X-ray-emitting intracluster medium (ICM). The Faraday-active plasma is more massive than the X-ray-emitting ICM, with an average density that broadly matches expectations for the Warm-Hot Intergalactic Medium. The morphology of the Faraday depth enhancement 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 is an ongoing merger between the main cluster and a sub-cluster 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 shows a decrement in both total and polarised intensity. We fail to identify a satisfactory explanation for this; further observations are warranted. 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., Comment: Accepted for publication in PASA. 27 pages, 14 figures, 1 table

The low-frequency linearly-polarised radio source population is largely unexplored. However, a renaissance in low-frequency polarimetry has been enabled by pathfinder and precursor instruments for the Square Kilometre Array. In this second paper from the POlarised GaLactic and Extragalactic All-Sky Murchison Widefield Array (MWA) Survey -- the POlarised GLEAM Survey, or POGS -- we present the results from our all-sky MWA Phase I Faraday Rotation Measure survey. Our survey covers nearly the entire Southern sky in the Declination range $-82^{\circ}$ to $+30^{\circ}$ at a resolution between around three and seven arcminutes (depending on Declination) using data in the frequency range 169$-$231 MHz. We have performed two targeted searches: the first covering 25,489 square degrees of sky, searching for extragalactic polarised sources; the second covering the entire sky South of Declination $+30^{\circ}$, searching for known pulsars. We detect a total of 517 sources with 200 MHz linearly-polarised flux densities between 9.9 mJy and 1.7 Jy, of which 33 are known radio pulsars. All sources in our catalogues have Faraday rotation measures in the range $-328.07$ rad m$^{-2}$ to $+279.62$ rad m$^{-2}$. The Faraday rotation measures are broadly consistent with results from higher-frequency surveys, but with typically more than an order of magnitude improvement in the precision, highlighting the power of low-frequency polarisation surveys to accurately study Galactic and extragalactic magnetic fields. We discuss the properties of our extragalactic and known-pulsar source population, how the sky distribution relates to Galactic features, and identify a handful of new pulsar candidates among our nominally extragalactic source population., Comment: Replacement of previous version. Only change is minor updates to catalogues (see ancillary files) which now contain bib code of finalised manuscript (published in PASA). Manuscript has 31 pages, 10 figures, 5 tables. Four Appendices are included in the ancillary material, showing further Figures, continuum spectra for a handful of selected sources, and RM spectra for all 517 sources

We present a Wide-field ASKAP L-Band Legacy All-sky Blind surveY (WALLABY) study of the nearby ($v_{\rm sys}$ = 915 km s$^{-1}$) spiral galaxy IC 5201 using the Australian Square Kilometre Array Pathfinder (ASKAP). IC 5201 is a blue, barred spiral galaxy that follows the known scaling relations between stellar mass, SFR, HI mass and diameter. We create a four-beam mosaicked HI image cube, from 175 hours of observations made with a 12-antenna sub-array. The RMS noise level of the cube is 1.7 mJy beam$^{-1}$ per channel, equivalent to a column density of $N_{\rm HI}$ = 1.4 $\times$ 10$^{20}$ cm$^{-2}$ over 25 km s$^{-1}$. We report 9 extragalactic HI detections $-$ 5 new HI detections including the first velocity measurements for 2 galaxies. These sources are IC 5201, 3 dwarf satellite galaxies, 2 galaxies and a tidal feature belonging to the NGC 7232/3 triplet and 2 potential infalling galaxies to the triplet. There is evidence of a previous tidal interaction between IC 5201 and the irregular satellite AM 2220$-$460. A close fly-by is likely responsible for the asymmetric optical morphology of IC 5201 and warping its disc, resulting in the irregular morphology of AM 2220$-$460. We quantify the HI kinematics of IC 5201, presenting its rotation curve as well as showing that the warp starts at 14 kpc along the major axis, increasing as a function of radius with a maximum difference in position angle of 20$^\circ$. There is no evidence of stripped HI, triggered or quenched star formation in the system as measured using DECam optical and $GALEX$ UV photometry., Comment: 19 pages, 11 figures, accepted in MNRAS

This paper reports on the atomic hydrogen gas (HI) observations of the spiral galaxy NGC 1566 using the newly commissioned Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope. We measure an integrated HI flux density of $180.2$ Jy km s$^{-1}$ emanating from this galaxy, which translates to an HI mass of $1.94\times10^{10}$M$_\circ$ at an assumed distance of $21.3$ Mpc. Our observations show that NGC 1566 has an asymmetric and mildly warped HI disc. The HI-to-stellar mass fraction of NGC 1566 is $0.29$, which is high in comparison with galaxies that have the same stellar mass ($10^{10.8}$M$_\circ$). We also derive the rotation curve of this galaxy to a radius of $50$ kpc and fit different mass models to it. The NFW, Burkert and pseudo-isothermal dark matter halo profiles fit the observed rotation curve reasonably well and recover dark matter fractions of $0.62$, $0.58$ and $0.66$, respectively. Down to the column density sensitivity of our observations ($N_{HI} = 3.7\times10^{19}$ cm$^{-2}$), we detect no HI clouds connected to, or in the nearby vicinity of, the HI disc of NGC 1566 nor nearby interacting systems. We conclude that, based on a simple analytic model, ram pressure interactions with the IGM can affect the HI disc of NGC 1566 and is possibly the reason for the asymmetries seen in the HI morphology of NGC 1566., Comment: Accepted for publication in MNRAS

We investigate the previously proposed possibility that multi-epoch broadband polarimetry could act as a complement or limited proxy for VLBI observations of blazars, in that the number of polarised emission components in the jet, and some of their properties and those of the foreground environment, might be inferred from the object's time-varying 1D Faraday depth spectrum (FDS) alone. We report on a pilot-scale experiment designed to establish the basic plausibility and utility of this idea. We analyse temporal changes in the complex polarisation spectra of nine spatially unresolved (at arcsecond scales) blazars in two epochs separated by $\sim$5 years, using data taken with the Australia Telescope Compact Array. The data allow for precise modelling, and we demonstrate that all objects in our sample show changes in their polarisation spectrum that cannot be accounted for by uncertainties in calibration or observational effects. By associating polarised emission components across epochs, we infer changes in their number, intrinsic fractional polarisation, intrinsic polarisation angle, rotation measure, and depolarisation characteristics. We attribute these changes to evolution in the structure of the blazar jets, most likely located at distances of up to tens of parsecs from the central active galactic nuclei. Our results suggest that continued work in this area is warranted; in particular, it will be important to determine the frequency ranges and temporal cadence most useful for scientifically exploiting the effects., Comment: 12 pages of text, 13 figures, 4 tables (29 pages total). Accepted for publication in MNRAS (Jan 2019)

Cold neutral gas is a key ingredient for growing the stellar and central black hole mass in galaxies throughout cosmic history. We have used the Atacama Large Millimetre Array (ALMA) to detect a rare example of redshifted $^{12}$CO(2-1) absorption in PKS B1740-517, a young ($t \sim 1.6 \times 10^{3}$ yr) and luminous ($L_{\rm 5 GHz} \sim 6.6 \times 10^{43}$ erg s$^{-1}$ ) radio galaxy at $z = 0.44$ that is undergoing a tidal interaction with at least one lower-mass companion. The coincident HI 21-cm and molecular absorption have very similar line profiles and reveal a reservoir of cold gas ($M_{\rm gas} \sim 10^{7} - 10^{8}$ M$_{\odot}$), likely distributed in a disc or ring within a few kiloparsecs of the nucleus. A separate HI component is kinematically distinct and has a very narrow line width ($\Delta{v}_{\rm FWHM} \lesssim 5$ km s$^{-1}$), consistent with a single diffuse cloud of cold ($T_{\rm k} \sim 100$ K) atomic gas. The $^{12}$CO(2-1) absorption is not associated with this component, which suggests that the cloud is either much smaller than 100 pc along our sight-line and/or located in low-metallicity gas that was possibly tidally stripped from the companion. We argue that the gas reservoir in PKS B1740-517 may have accreted onto the host galaxy $\sim$50 Myr before the young radio AGN was triggered, but has only recently reached the nucleus. This is consistent with the paradigm that powerful luminous radio galaxies are triggered by minor mergers and interactions with low-mass satellites and represent a brief, possibly recurrent, active phase in the life cycle of massive early type galaxies., Comment: 15 pages, 7 figures, accepted for publication in MNRAS

The low-frequency polarisation properties of radio sources are poorly studied, particularly in statistical samples. However, the new generation of low-frequency telescopes, such as the Murchison Widefield Array (MWA; the precursor for the low-frequency component of the Square Kilometre Array) offers an opportunity to probe the physics of radio sources at very low radio frequencies. In this paper, we present a catalogue of linearly-polarised sources detected at 216 MHz, using data from the Galactic and Extragalactic All-sky MWA (GLEAM) survey. Our catalogue covers the Declination range $-17^{\circ}$ to $-37^{\circ}$ and 24 hours in Right Ascension, at a resolution of around 3 arcminutes. We detect 81 sources (including both a known pulsar and new pulsar candidate) with linearly-polarised flux densities in excess of 18 mJy across a survey area of approximately 6400 square degrees, corresponding to a surface density of 1 source per 79 square degrees. The level of Faraday rotation measured for our sources is broadly consistent with those recovered at higher frequencies, with typically more than an order of magnitude improvement in the uncertainty compared to higher-frequency measurements. However, our catalogue is likely incomplete at low Faraday rotation measures, due to our practice of excluding sources in the region where instrumental leakage appears. The majority of sources exhibit significant depolarisation compared to higher frequencies; however, a small sub-sample repolarise at 216 MHz. We also discuss the polarisation properties of four nearby, large-angular-scale radio galaxies, with a particular focus on the giant radio galaxy ESO 422$-$G028, in order to explain the striking differences in polarised morphology between 216 MHz and 1.4 GHz., Comment: 32 pages, 13 figures, 3 tables. Accepted for publication in PASA

We present observations and analysis of the polarized radio emission from the nearby radio galaxy Fornax A over 1.28--3.1 GHz, using data from the Australia Telescope Compact Array (ATCA). In this, the first of two associated papers, we use modern broadband polarimetric techniques to examine the nature and origin of conspicuous low-polarization (low-$p$) patches in the lobes. We resolve the low-$p$ patches, and find that their low fractional polarization is associated with complicated frequency-dependent interference in the polarized signal generated by Faraday effects along the line of sight. The low-$p$ patches are spatially correlated with interfaces in the magnetic structure of the lobe, across which the line-of-sight-projected magnetic field changes direction. Spatial correlations with the sky-projected magnetic field orientation and structure in total intensity are also identified and discussed. We argue that the low-$p$ patches, along with associated reversals in the line-of-sight magnetic field and other related phenomena, are best explained by the presence of $\mathcal{O}(10^9)$ $M_\odot$ of magnetized thermal plasma in the lobes, structured in shells or filaments, and likely advected from the ISM of NCG 1316 or its surrounding ICM. Our study underscores the power and utility of spatially-resolved, broadband, full-polarization radio observations to reveal new facets of flow behaviors and magneto-ionic structure in radio lobes and their interplay with the surrounding environment., Comment: 32 pages, 21 figures. Accepted for publication in The Astrophysical Journal on 10 Feb. 2018

We present techniques developed to calibrate and correct Murchison Widefield Array (MWA) low frequency (72-300 MHz) radio observations for polarimetry. The extremely wide field-of-view, excellent instantaneous (u, v)-coverage and sensitivity to degree-scale structure that the MWA provides enable instrumental calibration, removal of instrumental artefacts, and correction for ionospheric Faraday rotation through imaging techniques. With the demonstrated polarimetric capabilities of the MWA, we discuss future directions for polarimetric science at low frequencies to answer outstanding questions relating to polarised source counts, source depolarisation, pulsar science, low-mass stars, exoplanets, the nature of the interstellar and intergalactic media, and the solar environment., Comment: 18 pages, 11 figures, accepted for publication in PASA

We present the results from a broadband (1 to 3 GHz), spectro-polarimetry study of the integrated emission from 100 extragalactic radio sources with the ATCA, selected to be highly linearly polarized at 1.4 GHz. We use a general purpose, polarization model-fitting procedure that describes the Faraday rotation measure (RM) and intrinsic polarization structure of up to three distinct polarized emission regions or 'RM components' of a source. Overall, 37%/52%/11% of sources are best fit by one/two/three RM components. However, these fractions are dependent on the signal-to-noise ratio (S/N) in polarization (more RM components more likely at higher S/N). In general, our analysis shows that sources with high integrated degrees of polarization at 1.4 GHz have low Faraday depolarization, are typically dominated by a single RM component, have a steep spectral index, and a high intrinsic degree of polarization. After classifying our sample into radiative-mode and jet-mode AGN, we find no significant difference between the Faraday rotation or Faraday depolarization properties of jet-mode and radiative-mode AGN. However, there is a statistically significant difference in the intrinsic degree of polarization between the two types, with the jet-mode sources having more intrinsically ordered magnetic field structures than the radiative-mode sources. We also find a preferred perpendicular orientation of the intrinsic magnetic field structure of jet-mode AGN with respect to the jet direction, while no clear preference is found for the radiative-mode sources., Comment: 29 pages (including Appendix), 28 figures, 7 tables. Accepted for publication in MNRAS

We present a broadband polarization analysis of 36 discrete polarized radio sources over a very broad, densely-sampled frequency band. Our sample was selected on the basis of polarization behaviour apparent in narrowband archival data at 1.4 GHz: half the sample show complicated frequency-dependent polarization behaviour (i.e. Faraday complexity) at these frequencies, while half show comparatively simple behaviour (i.e. they appear Faraday simple). We re-observed the sample using the Australia Telescope Compact Array (ATCA) in full polarization, with 6 GHz of densely sampled frequency coverage spanning 1.3 to 10 GHz. We have devised a general polarization modelling technique that allows us to identify multiple polarized emission components in a source, and to characterize their properties. We detect Faraday complex behaviour in almost every source in our sample. Several sources exhibit particularly remarkable polarization behaviour. By comparing our new and archival data, we have identified temporal variability in the broadband integrated polarization spectra of some sources. In a number of cases, the characteristics of the polarized emission components, including the range of Faraday depths over which they emit, their temporal variability, spectral index, and the linear extent of the source, allow us to argue that the spectropolarimetric data encodes information about the magnetoionic environment of active galactic nuclei themselves. Furthermore, the data place direct constraints on the geometry and magnetoionic structure of this material. We discuss the consequences of restricted frequency bands on the detection and interpretation of polarization structures, and implications for upcoming spectropolarimetric surveys., Comment: Accepted for publication in ApJ on 31 March 2016. 48 pages total (14 pages text), 63 figures, 6 tables

We present a broadband spectropolarimetric survey of 563 discrete, mostly unresolved radio sources between 1.3 \& 2.0 GHz using data taken with the Australia Telescope Compact Array (ATCA). We have used rotation measure synthesis to identify Faraday complex polarized sources --- i.e. objects whose frequency-dependent polarization behaviour indicates the presence of material possessing complicated magnetoionic structure along the line of sight (LOS). For sources classified as Faraday complex, we have analyzed a number of their radio and multiwavelength properties to determine whether they differ from Faraday simple polarized sources (i.e. sources for which LOS magnetoionic structures are comparatively simple) in these properties. We use this information to constrain the physical nature of the magnetoionic structures responsible for generating the observed complexity. We detect Faraday complexity in 12\% of polarized sources at $\sim1'$ resolution, but demonstrate that underlying signal-to-noise limitations mean the true percentage is likely to be significantly higher in the polarized radio source population. We find that the properties of Faraday complex objects are diverse, but that complexity is most often associated with depolarization of extended radio sources possessing a relatively steep total intensity spectrum. We find an association between Faraday complexity and LOS structure in the Galactic interstellar medium (ISM), and claim that a significant proportion of the Faraday complexity we observe may be generated at interfaces of the ISM associated with ionization fronts near neutral hydrogen structures. Galaxy clusters environments and internally generated Faraday complexity provide possible alternative explanations in some cases., Comment: 16 pages, 44 figures, 8 tables. Accepted for publication by The Astrophysical Journal on October 29, 2015

(abridged) We run a Faraday structure determination data challenge to benchmark the currently available algorithms including Faraday synthesis (previously called RM synthesis in the literature), wavelet, compressive sampling and $QU$-fitting. The frequency set is similar to POSSUM/GALFACTS with a 300 MHz bandwidth from 1.1 to 1.4 GHz. We define three figures of merit motivated by the underlying science: a) an average RM weighted by polarized intensity, RMwtd, b) the separation $\Delta\phi$ of two Faraday components and c) the reduced chi-squared. Based on the current test data of signal to noise ratio of about 32, we find that: (1) When only one Faraday thin component is present, most methods perform as expected, with occasional failures where two components are incorrectly found; (2) For two Faraday thin components, QU-fitting routines perform the best, with errors close to the theoretical ones for RMwtd, but with significantly higher errors for $\Delta\phi$. All other methods including standard Faraday synthesis frequently identify only one component when $\Delta\phi$ is below or near the width of the Faraday point spread function; (3) No methods, as currently implemented, work well for Faraday thick components due to the narrow bandwidth; (4) There exist combinations of two Faraday components which produce a large range of acceptable fits and hence large uncertainties in the derived single RMs; in these cases, different RMs lead to the same Q, U behavior, so no method can recover a unique input model., Comment: 15 pages, 8 figures, 5 tables, accepted by AJ. Some figures have been degraded, for a full resolution version, see http://physics.usyd.edu.au/~xhsun/datachallengeI.pdf

Two Chandra observations of the R Aqr symbiotic binary system taken 3.3 years apart show dramatic changes in the X-ray morphology and spectral characteristics in the inner 500 AU of this system. The morphology of the soft X-ray emission has evolved from a nearly circular region centered on the binary system to an hourglass shape that indicates the formation of a new southwest jet. Synchrotron radiation from the new jet in contemporaneous VLA radio spectra implies the physical conditions in the early stages of jet development are different from those in the more extended outer thermal jets known to exist for decades in this system. The central binary source has two X-ray spectral components in each of the two epochs, a soft component and a highly absorbed hard component characterized by T ~ 10^8 K if fit with a thermal plasma model. The spectrum hardened considerably between 2000.7 and 2004.0, primarily due to increased flux above 5 keV, suggesting a change in the accretion activity of the white dwarf on a timescale of a few years or less. Point-source Fe K emission is detected at the position of the central binary system in both observations. While the earlier observation shows evidence of only a single emission peak near Fe K alpha at 6.4 keV, the later observation shows a more complex emission structure between 6 and 7 keV. Finally, we have discovered a modulation in the hard X-ray flux with a period of 1734 s at a 95% confidence level in the 2004 observation only. The modulation potentially arises from standing shocks in an accretion column and we have explored the possibility that the white dwarf in R Aqr is analogous to the magnetic white dwarfs in Intermediate Polar., Comment: 31 pages, 14 figures, 1 table To be published in ApJ, expected April 10 issue. AAS LaTex style