arXiv subjects

David McConnell

Publications and source records attributed to David McConnell.

17 recordsLinked to original sources

arXiv2025

Revealing the magnetization of the intracluster medium of Abell 3581 using background Faraday rotation measures from the POSSUM survey

The line-of-sight magnetic field of galaxy clusters can be probed using Faraday rotation measure (RM) data. However, our understanding of cluster magnetism is limited due to the scarcity of polarized background radio sources, with most previous studies being constrained to $\sim 10$ sources per cluster. Leveraging the increased source density of the POlarisation Sky Survey of the Universe's Magnetism (POSSUM), we probe the magnetic field properties of the galaxy cluster Abell 3581 {(A3581)} with 111 RMs. We find that the standard deviation in the RM declines monotonically with increasing radius up to 0.75 Mpc{, agreeing with a radially declining magnetic field and electron density profile modeled as Gaussian and lognormal random fields, respectively. {We compare our observations of the inner 0.75 Mpc of A3581 to various semi-analytic models of the magnetic field and electron density, and obtain several best-fit models.} For the first time, we compare the observed RMs in a cluster to full magnetohydrodynamic simulated clusters from TNG-Cluster and find that the non-monotonic trend in RM standard deviation past 0.75 Mpc in A3581 is likely caused by past or present merger activity. We identify a possible candidate for a merger to be the galaxy group [DZ2015b] 276, which would be the first group detected in RMs that is not strongly emitting in X-rays. We find a possible merger axis of A3581 with this group at a position angle of $\theta = 52\pm 4$ deg.

astro-ph.GA↗

arXiv2024

Radio afterglows from tidal disruption events: An unbiased sample from ASKAP RACS

Late-time ($\sim$ year) radio follow-up of optically-discovered tidal disruption events (TDEs) is increasingly resulting in detections at radio wavelengths, and there is growing evidence for this late-time radio activity to be common to the broad class of sub-relativistic TDEs. Detailed studies of some of these TDEs at radio wavelengths are also challenging the existing models for radio emission. Using all-sky multi-epoch data from the Australian Square Kilometre Array Pathfinder (ASKAP), taken as a part of the Rapid ASKAP Continuum Survey (RACS), we searched for radio counterparts to a sample of optically-discovered TDEs. We detected late-time emission at RACS frequencies (742-1032\,MHz) in five TDEs, reporting the independent discovery of radio emission from TDE AT2019ahk and extending the time baseline out to almost 3000\,days for some events. Overall, we find that at least $22^{+15}_{-11}$\% of the population of optically-discovered TDEs has detectable radio emission in the RACS survey, while also noting that the true fraction can be higher given the limited cadence (2 epochs separated by $\sim 3\,$ years) of the survey. Finally, we project that the ongoing higher-cadence ($\sim 2$\,months) ASKAP Variable and Slow Transients (VAST) survey can detect $\sim 20$ TDEs in its operational span (4\,yrs), given the current rate from optical surveys.

astro-ph.HE↗

arXiv2023

The Rapid ASKAP Continuum Survey V: cataloguing the sky at 1367.5 MHz and the second data release of RACS-mid

The Australian SKA Pathfinder (ASKAP) has surveyed the sky at multiple frequencies as part of the Rapid ASKAP Continuum Survey (RACS). The first two RACS observing epochs, at 887.5 (RACS-low) and 1367.5 (RACS-mid) MHz, have been released (McConnell et al., 2020; Duchesne et al., 2023). A catalogue of radio sources from RACS-low has also been released, covering the sky south of declination +30$^\circ$ (Hale et al., 2021). With this paper, we describe and release the first set of catalogues from RACS-mid, covering the sky below declination +49$^\circ$. The catalogues are created in a similar manner to the RACS-low catalogue, and we discuss this process and highlight additional changes. The general purpose primary catalogue covering 36 200 deg$^2$ features a variable angular resolution to maximise sensitivity and sky coverage across the catalogued area, with a median angular resolution of 11.2" times 9.3". The primary catalogue comprises 3 105 668 radio sources, including those in the Galactic Plane (2 861 923 excluding Galactic latitudes of $|b|<5^\circ$) and we estimate the catalogue to be 95% complete for sources above 1.6 mJy. With the primary catalogue, we also provide two auxiliary catalogues. The first is a fixed-resolution, 25-arcsec catalogue approximately matching the sky coverage of the RACS-low catalogue. This 25-arcsec catalogue is constructed identically to the primary catalogue, except images are convolved to a less-sensitive 25-arcsec angular resolution. The second auxiliary catalogue is designed for time-domain science, and is the concatenation of source-lists from the original RACS-mid images with no additional convolution, mosaicking, or de-duplication of source entries to avoid losing time-variable signals. All three RACS-mid catalogues, and all RACS data products, are available through the CSIRO ASKAP Science Data Archive (CASDA).

astro-ph.GA↗

arXiv2023

Characterizing Pulsars Detected in the Rapid ASKAP Continuum Survey

We present the detection of 661 known pulsars observed with the Australian SKA Pathfinder (ASKAP) telescope at 888 MHz as a part of the Rapid ASKAP Continuum Survey (RACS). Detections were made through astrometric coincidence and we estimate the false alarm rate of our sample to be ~0.5%. Using archival data at 400 and 1400 MHz, we estimate the power law spectral indices for the pulsars in our sample and find that the mean spectral index is -1.78 +/- 0.6. However, we also find that a single power law is inadequate to model all the observed spectra. With the addition of the flux densities between 150 MHz and 3 GHz from various imaging surveys, we find that up to 40% of our sample shows deviations from a simple power law model. Using Stokes V measurements from the RACS data, we measured the circular polarization fraction for 9% of our sample and find that the mean polarization fraction is ~10% (consistent between detections and upper limits). Using the dispersion measure (DM) derived distance we estimate the pseudo luminosity of the pulsars and do not find any strong evidence for a correlation with the pulsars' intrinsic properties.

astro-ph.HE↗

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↗

arXiv2022

Constraining the Radio Properties of the $z$=6.44 QSO VIK J2318$-$3113

The recent detection of the quasi-stellar object (QSO) VIKING J231818.3$-$311346 (hereafter VIK J2318$-$3113) at redshift $z=6.44$ in the Rapid ASKAP Continuum Survey (RACS) uncovered its radio-loud nature, making it one of the most distant known to date in this class. By using data from several radio surveys of the Galaxy and Mass Assembly 23$^\mathrm{h}$ field and from dedicated follow-up, we were able to constrain the radio spectrum of VIK J2318$-$3113 in the observed range $\sim$0.1--10 GHz. At high frequencies (0.888--5.5 GHz in the observed frame) the QSO presents a steep spectrum ($α_{\rm r}$=1.24, with $S_ν\propto ν^{-α_{\rm r}}$), while at lower frequencies (0.4--0.888 GHz in the observed frame) it is nearly flat. The overall spectrum can be modelled by either a curved function with a rest-frame turnover around 5 GHz, or with a smoothly varying double power law that is flat below a rest-frame break frequency of about 20 GHz and which significantly steepens above it. Based on the model adopted, we estimated that the radio jets of VIK J2318$-$3113 must be a few hundred years old, in the case of a turnover, or less than few$\times$10$^4$ years, in the case of a break in the spectrum. Having multiple observations at two frequencies (888 MHz and 5.5 GHz), we further investigated the radio variability previously reported for this source. We found that the marginally significant flux density variations are consistent with the expectations from refractive interstellar scintillation, even though relativistic effects related to the orientation of the source may still play a non-negligible role. Further radio and X-ray observations are required to conclusively discern the nature of this variation.

astro-ph.GA↗

arXiv2021

The ASKAP Variables and Slow Transients (VAST) Pilot Survey

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.

astro-ph.HE↗

arXiv2021

A search for radio afterglows from gamma-ray bursts with the Australian Square Kilometre Array Pathfinder

We present a search for radio afterglows from long gamma-ray bursts using the Australian Square Kilometre Array Pathfinder (ASKAP). Our search used the Rapid ASKAP Continuum Survey, covering the entire celestial sphere south of declination $+41^\circ$, and three epochs of the Variables and Slow Transients Pilot Survey (Phase 1), covering $\sim 5,000$ square degrees per epoch. The observations we used from these surveys spanned a nine-month period from 2019 April 21 to 2020 January 11. We crossmatched radio sources found in these surveys with 779 well-localised (to $\leq 15''$) long gamma-ray bursts occurring after 2004 and determined whether the associations were more likely afterglow- or host-related through the analysis of optical images. In our search, we detected one radio afterglow candidate associated with GRB 171205A, a local low-luminosity gamma-ray burst with a supernova counterpart SN 2017iuk, in an ASKAP observation 511 days post-burst. We confirmed this detection with further observations of the radio afterglow using the Australia Telescope Compact Array at 859 days and 884 days post-burst. Combining this data with archival data from early-time radio observations, we showed the evolution of the radio spectral energy distribution alone could reveal clear signatures of a wind-like circumburst medium for the burst. Finally, we derived semi-analytical estimates for the microphysical shock parameters of the burst: electron power-law index $p = 2.84$, normalised wind-density parameter $A_* = 3$, fractional energy in electrons $ε_{e} = 0.3$, and fractional energy in magnetic fields $ε_{B} = 0.0002$.

astro-ph.HE↗

arXiv2021

A circular polarisation survey for radio stars with the Australian SKA Pathfinder

We present results from a circular polarisation survey for radio stars in the Rapid ASKAP Continuum Survey (RACS). RACS is a survey of the entire sky south of $δ=+41^\circ$ 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 includes Stokes I and V polarisation products to an RMS sensitivity of 250 $μ$Jy PSF$^{-1}$. We searched RACS for sources with fractional circular polarisation above 6 per cent, and after excluding imaging artefacts, polarisation 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.

astro-ph.SR↗

arXiv2019

An ASKAP search for a radio counterpart to the first high-significance neutron star-black hole merger LIGO/Virgo S190814bv

We present results from a search for a radio transient associated with the LIGO/Virgo source S190814bv, a likely neutron star-black hole (NSBH) merger, with the Australian Square Kilometre Array Pathfinder. We imaged a $30\,{\rm deg}^2$ field at $ΔT$=2, 9 and 33 days post-merger at a frequency of 944\,MHz, comparing them to reference images from the Rapid ASKAP Continuum Survey observed 110 days prior to the event. Each epoch of our observations covers $89\%$ of the LIGO/Virgo localisation region. We conducted an untargeted search for radio transients in this field, resulting in 21 candidates. For one of these, \object[AT2019osy]{AT2019osy}, we performed multi-wavelength follow-up and ultimately ruled out the association with S190814bv. All other candidates are likely unrelated variables, but we cannot conclusively rule them out. We discuss our results in the context of model predictions for radio emission from neutron star-black hole mergers and place constrains on the circum-merger density and inclination angle of the merger. This survey is simultaneously the first large-scale radio follow-up of an NSBH merger, and the most sensitive widefield radio transients search to-date.

astro-ph.HE↗

arXiv2017

The detection of an extremely bright fast radio burst in a phased array feed survey

We report the detection of an ultra-bright fast radio burst (FRB) from a modest, 3.4-day pilot survey with the Australian Square Kilometre Array Pathfinder. The survey was conducted in a wide-field fly's-eye configuration using the phased-array-feed technology deployed on the array to instantaneously observe an effective area of $160$ deg$^2$, and achieve an exposure totaling $13200$ deg$^2$ hr. We constrain the position of FRB 170107 to a region $8'\times8'$ in size (90% containment) and its fluence to be $58\pm6$ Jy ms. The spectrum of the burst shows a sharp cutoff above $1400$ MHz, which could be either due to scintillation or an intrinsic feature of the burst. This confirms the existence of an ultra-bright ($>20$ Jy ms) population of FRBs.

astro-ph.HE↗

arXiv2016

A Stone-Čech Theorem for $C_0(X)$-algebras

For a $C_0(X)$-algebra $A$, we study $C(K)$-algebras $B$ that we regard as compactifications of $A$, generalising the notion of (the algebra of continuous functions on) a compactification of a completely regular space. We show that $A$ admits a Stone-Čech-type compactification $A^β$, a $C(βX)$-algebra with the property that every bounded continuous section of the C$^*$-bundle associated with $A$ has a unique extension to a continuous section of the bundle associated with $A^β$. Moreover, $A^β$ satisfies a maximality property amongst compactifications of $A$ (with respect to appropriately chosen morphisms) analogous to that of $βX$. We investigate the structure of the space of points of $βX$ for which the fibre algebras of $A^β$ are non-zero, and partially characterise those $C_0(X)$-algebras $A$ for which this space is precisely $βX$.

math.OA↗

arXiv2014

Exact C$^{\ast}$-algebras and $C_0(X)$-structure

We study tensor products of a $C_0 (X)$-algebra $A$ and a $C_0 (Y)$-algebra $B$, and analyse the structure of their minimal tensor product $A \otimes B$ as a $C_0 (X \times Y)$-algebra. We show that when $A$ and $B$ define continuous C$^{\ast}$-bundles, that continuity of the bundle arising from the $C_0 (X \times Y)$-algebra $A \otimes B$ is a strictly weaker property than continuity of the `fibrewise tensor products' studied by Kirchberg and Wassermann. For a fixed quasi-standard C$^{\ast}$-algebra $A$, we show that $A \otimes B$ is quasi-standard for all quasi-standard $B$ precisely when $A$ is exact, and exhibit some related equivalences.

math.OA↗

arXiv2013

Australia Telescope Compact Array observations of Fermi unassociated sources

We report results of the first phase of observations with the Australia Telescope Compact Array (ATCA) at 5 and 9 GHz of the fields around 411 gamma-ray sources with declinations < +10 deg detected by Fermi but marked as unassociated in the 2FGL catalogue. We have detected 424 sources with flux densities in a range of 2 mJy to 6 Jy that lie within the 99 per cent localisation uncertainty of 283 gamma-ray sources. Of these, 146 objects were detected in both the 5 and 9 GHz bands. We found 84 sources in our sample with a spectral index flatter than -0.5. The majority of detected sources are weaker than 100 mJy and for this reason were not found in previous surveys. Approximately 1/3 of our sample, 128 objects, have the probability of being associated by more than 10 times than the probability of being a background source found in the vicinity of a gamma-ray object by chance. We present the catalogue of positions of these sources, estimates of their flux densities and spectral indices where available.

astro-ph.CO↗

arXiv2012

The Glimm space of the minimal tensor product of C$^{\ast}$-algebras

We show that for C$^{\ast}$-algebras $A$ and $B$, there is a natural open bijection from $\mathrm{Glimm}(A) \times \mathrm{Glimm} (B)$ to $\mathrm{Glimm}(A \otimes_α B)$ (where $A \otimes_α B$ denotes the minimal C$^{\ast}$-tensor product), and identify a large class of C$^{\ast}$-algebras $A$ for which the map is continuous for arbitrary $B$. As a consequence we determine the structure space of the centre of the multiplier algebra $ZM(A \otimes_α B)$ in terms of $\mathrm{Glimm}(A)$ and $\mathrm{Glimm} (B)$, and give necessary and sufficient conditions for the inclusion $ZM(A) \otimes ZM(B) \subseteq ZM(A \otimes_α B)$ to be surjective. Further we show that when the Glimm spaces are considered as sets of ideals, the map $(G,H) \mapsto G \otimes_α B + A \otimes_α H$ implements the above bijection, extending a result of Kaniuth from a 1996 paper by eliminating the assumption of property (F).

math.OA↗

arXiv2004

The Vela pulsar, the key?

Of all pulsars known Vela has been one of the most productive in terms in understanding pulsars and their characteristics. We present the latest results derived from Australian telescopes. These include a more accurate pulsar distance, a more precise pulsar local space velocity, a new model of the spin up and the association of a radio nebula with the X-ray pulsar wind nebula.

astro-ph↗

arXiv2001

The Vela Pulsar Wind Nebula at 6cm

Observations using the Australia Telescope Compact Array at a wavelength of 6 cm have uncovered the radio counterpart to the compact X-ray nebula surrounding the Vela pulsar. Two lobes were found oriented about the spin axis of the pulsar, starting at the edge of X-ray emission, they extend to three times the size. The northern lobe has a bright, defined edge and an integrated flux of 0.14 Jy, while the southern lobe of 0.12 Jy is more diffuse.

astro-ph↗