Abstract
We present the detection of rotationally modulated, circularly polarized radio emission from the T8 brown dwarf WISE J062309.94−045624.6 between 0.9 and 2.0 GHz. We detected this ...high-proper-motion ultracool dwarf with the Australian SKA Pathfinder in 1.36 GHz imaging data from the Rapid ASKAP Continuum Survey. We observed WISE J062309.94−045624.6 to have a time and frequency averaged Stokes
I
flux density of 4.17 ± 0.41 mJy beam
−1
, with an absolute circular polarization fraction of 66.3% ± 9.0%, and calculated a specific radio luminosity of
L
ν
∼ 10
14.8
erg s
−1
Hz
−1
. In follow-up observations with the Australian Telescope Compact Array and MeerKAT we identified a multipeaked pulse structure, used dynamic spectra to place a lower limit of
B
> 0.71 kG on the dwarf’s magnetic field, and measured a
P
= 1.912 ± 0.005 hr periodicity, which we concluded to be due to rotational modulation. The luminosity and period we measured are comparable to those of other ultracool dwarfs observed at radio wavelengths. This implies that future megahertz to gigahertz surveys, with increased cadence and improved sensitivity, are likely to detect similar or later-type dwarfs. Our detection of WISE J062309.94−045624.6 makes this dwarf the coolest and latest-type star observed to produce radio emission.
ABSTRACT
The Square Kilometre Array (SKA) will be the first low-frequency instrument with the capability to directly image the structures of the epoch of reionization (EoR). Indeed, deep imaging of ...the EoR over five targeted fields of 20 sq deg each has been selected as the highest priority science objective for SKA1. Aiming at preparing for this highly challenging observation, we perform an extensive pre-selection of the ‘quietest’ and ‘cleanest’ candidate fields in the southern sky to be suited for deep imaging of the EoR using existing catalogues and observations over a broad frequency range. The candidate fields should meet a number of strict criteria to avoid contaminations from foreground structures and sources. The candidate fields should also exhibit both the lowest average surface brightness and smallest variance to ensure uniformity and high-quality deep imaging over the fields. Our selection eventually yields a sample of 7 ‘ideal’ fields of 20 sq deg in the southern sky that could be targeted for deep imaging of the EoR. Finally, these selected fields are convolved with the synthesized beam of SKA1-low stations to ensure that the effect of sidelobes from the far-field bright sources is also weak.
We present a method for identifying radio stellar sources using their proper-motion. We demonstrate this method using the FIRST, VLASS, RACS-low and RACS-mid radio surveys, and astrometric ...information from Gaia Data Release 3. We find eight stellar radio sources using this method, two of which have not previously been identified in the literature as radio stars. We determine that this method probes distances of
$\sim$
90pc when we use FIRST and RACS-mid, and
$\sim$
250pc when we use FIRST and VLASS. We investigate the time baselines required by current and future radio sky surveys to detect the eight sources we found, with the SKA (6.7 GHz) requiring
$<$
3 yr between observations to find all eight sources. We also identify nine previously known and 43 candidate variable radio stellar sources that are detected in FIRST (1.4 GHz) but are not detected in RACS-mid (1.37 GHz). This shows that many stellar radio sources are variable, and that surveys with multiple epochs can detect a more complete sample of stellar radio sources.
Classical novae in the ASKAP pilot surveys Gulati, Ashna; Murphy, Tara; Kaplan, David L. ...
Publications of the Astronomical Society of Australia,
05/2023, Letnik:
40
Journal Article
Recenzirano
Odprti dostop
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
$0.4$
–
$1.0\:\rm M_\odot$
.
The 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 Stokes I, Q, U at 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 Stokes Q and U images have an rms noise 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 Stokes I spectrum 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.
We present new observations of the large-scale radio emission surrounding the lenticular galaxy NGC~1534 with the Australia Telescope Compact Array and Murchison Widefield Array. We find no ...significant compact emission from the nucleus of NGC 1534 to suggest an active core, and instead find low-power radio emission tracing its star-formation history with a radio-derived star-formation rate of \(0.38\pm0.03\) M\(_\odot\) yr\(^{-1}\). The spectral energy distribution of the extended emission is well-fit by a continuous injection model with an 'off' component, consistent with dead radio galaxies. We find the spectral age of the emission to be 203 Myr, having been active for 44 Myr. Polarimetric analysis points to both a large-scale magneto-ionic Galactic foreground at \(+33\) rad m\(^{-2}\) and a component associated with the northern lobe of the radio emission at \(-153\) rad m\(^{-2}\). The magnetic field of the northern lobe shows an unusual circular pattern of unknown origin. While such remnant sources are rare, combined low- and high-frequency radio surveys with high surface-brightness sensitivities are expected to greatly increase their numbers in the coming decade, and combined with new optical and infrared surveys should provide a wealth of information on the hosts of the emission.
We present a method for identifying radio stellar sources using their proper-motion. We demonstrate this method using the FIRST, VLASS, RACS-low and RACS-mid radio surveys, and astrometric ...information from Gaia Data Release 3. We find eight stellar radio sources using this method, two of which have not previously been identified in the literature as radio stars. We determine that this method probes distances of ~90pc when we use FIRST and RACS-mid, and ~250pc when we use FIRST and VLASS. We investigate the time baselines required by current and future radio sky surveys to detect the eight sources we found, with the SKA (6.7 GHz) requiring <3 years between observations to find all eight sources. We also identify nine previously known and 43 candidate variable radio stellar sources that are detected in FIRST (1.4 GHz) but are not detected in RACS-mid (1.37 GHz). This shows that many stellar radio sources are variable, and that surveys with multiple epochs can detect a more complete sample of stellar radio sources.
We present the detection of rotationally modulated, circularly polarized radio emission from the T8 brown dwarf WISE J062309.94-045624.6 between 0.9 and 2.0 GHz. We detected this high proper motion ...ultracool dwarf with the Australian SKA Pathfinder in \(1.36\) GHz imaging data from the Rapid ASKAP Continuum Survey. We observed WISE J062309.94-045624.6 to have a time and frequency averaged Stokes I flux density of \(4.17\pm0.41\) mJy beam\(^{-1}\), with an absolute circular polarization fraction of \(66.3\pm9.0\%\), and calculated a specific radio luminosity of \(L_{\nu}\sim10^{14.8}\) erg s\(^{-1}\) Hz\(^{-1}\). In follow-up observations with the Australian Telescope Compact Array and MeerKAT we identified a multi-peaked pulse structure, used dynamic spectra to place a lower limit of \(B>0.71\) kG on the dwarf's magnetic field, and measured a \(P=1.912\pm0.005\) h periodicity which we concluded to be due to rotational modulation. The luminosity and period we measured are comparable to those of other ultracool dwarfs observed at radio wavelengths. This implies that future megahertz to gigahertz surveys, with increased cadence and improved sensitivity, are likely to detect similar or later-type dwarfs. Our detection of WISE J062309.94-045624.6 makes this dwarf the coolest and latest-type star observed to produce radio emission.
Among the bent tail radio galaxies common in galaxy clusters are some with long, collimated tails (so-called head-tail galaxies) shaped by their interactions with the intracluster medium (ICM). Here ...we report the discovery of intricate filamentary structure in and beyond the ~28' (570 kpc) long, helical radio tail of the Corkscrew Galaxy (1610-60.5, ESO137-G007), which resides in the X-ray bright cluster Abell 3627 (D = 70 Mpc). Deep radio continuum data were obtained with wide-field Phased Array Feeds on the Australian Square Kilometer Array Pathfinder (ASKAP) at 944 MHz and 1.4 GHz. The Corkscrew Galaxy is located 15' north of the prominent wide-angle tail (WAT) radio galaxy 1610-60.8 (ESO137-G006) near the cluster centre. While the bright (young) part of its radio tail is highly collimated, the faint (old) part shows increasing oscillation amplitudes, break-ups, and filaments. We find a stunning set of arc-shaped radio filaments beyond and mostly orthogonal to the collimated Corkscrew tail end, forming a partial bubble. This may be the first detection of a "proto-lobe" seen in 3D MHD simulations by Nolting et al. (2019), formed by the face-on impact of the Corkscrew Galaxy with a shock front in the cluster outskirts. Interactions of the radio galaxy tail with the ICM are likely responsible for the tail collimation and shear forces within the ICM for its increasingly filamentary structure. We also report the discovery of small (~20-30 kpc) ram-pressure stripped radio tails in four Abell 3627 cluster galaxies.
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