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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 StokesIflux 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 ofL_ν∼ 10^14.8erg s⁻¹Hz⁻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 ofB> 0.71 kG on the dwarf’s magnetic field, and measured aP= 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.

 

Radio transient searches using traditional variability metrics struggle to recover sources whose evolution time-scale is significantly longer than the survey cadence. Motivated by the recent observations of slowly evolving radio afterglows at gigahertz frequency, we present the results of a search for radio variables and transients using an alternative matched-filter approach. We designed our matched-filter to recover sources with radio light curves that have a high-significance fit to power-law and smoothly broken power-law functions; light curves following these functions are characteristic of synchrotron transients, including ‘orphan’ gamma-ray burst afterglows, which were the primary targets of our search. Applying this matched-filter approach to data from Variables and Slow Transients Pilot Survey conducted using the Australian SKA Pathfinder, we produced five candidates in our search. Subsequent Australia Telescope Compact Array observations and analysis revealed that: one is likely a synchrotron transient; one is likely a flaring active galactic nucleus, exhibiting a flat-to-steep spectral transition over 4 months; one is associated with a starburst galaxy, with the radio emission originating from either star formation or an underlying slowly evolving transient; and the remaining two are likely extrinsic variables caused by interstellar scintillation. The synchrotron transient, VAST J175036.1–181454, has a multifrequency light curve, peak spectral luminosity, and volumetric rate that is consistent with both an off-axis afterglow and an off-axis tidal disruption event; interpreted as an off-axis afterglow would imply an average inverse beaming factor $\langle f^{-1}_{\text{b}} \rangle = 860^{+1980}_{-710}$, or equivalently, an average jet opening angle of $\langle \theta _{\textrm {j}} \rangle = 3^{+4}_{-1}\,$ deg.

 

We present the results of a radio transient and polarization survey towards the Galactic Centre, conducted as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients pilot survey. The survey region consisted of five fields covering $\sim 265\, {\rm deg}^2$ (350○ ≲ l ≲ 10○, |b| ≲ 10○). Each field was observed for 12 min, with between 7 and 9 repeats on cadences of between one day and four months. We detected eight highly variable sources and seven highly circularly polarized sources (14 unique sources in total). Seven of these sources are known pulsars including the rotating radio transient PSR J1739–2521 and the eclipsing pulsar PSR J1723–2837. One of them is a low-mass X-ray binary, 4U 1758–25. Three of them are coincident with optical or infrared sources and are likely to be stars. The remaining three may be related to the class of Galactic Centre Radio Transients (including a highly likely one, VAST J173608.2–321634, that has been reported previously), although this class is not yet understood. In the coming years, we expect to detect ∼40 bursts from this kind of source with the proposed 4-yr VAST survey if the distribution of the source is isotropic over the Galactic fields.

 

Solar radio emission at low frequencies (<1 GHz) can provide valuable information on processes driving flares and coronal mass ejections (CMEs). Radio emission has been detected from active M dwarf stars, suggestive of much higher levels of activity than previously thought. Observations of active M dwarfs at low frequencies can provide information on the emission mechanism for high energy flares and possible stellar CMEs. Here, we conducted two observations with the Australian Square Kilometre Array Pathfinder Telescope totalling 26 h and scheduled to overlap with the Transiting Exoplanet Survey Satellite Sector 36 field, utilizing the wide fields of view of both telescopes to search for multiple M dwarfs. We detected variable radio emission in Stokes I centred at 888 MHz from four known active M dwarfs. Two of these sources were also detected with Stokes V circular polarization. When examining the detected radio emission characteristics, we were not able to distinguish between the models for either electron cyclotron maser or gyrosynchrotron emission. These detections add to the growing number of M dwarfs observed with variable low-frequency emission.

 

We report the independent discovery of PSR J0026-1955 with the Murchison Widefield Array (MWA) in the ongoing Southern-sky MWA Rapid Two-metre pulsar survey. J0026-1955 has a period of ∼1.306 s, a dispersion measure of ∼20.869 pc cm⁻³, and a nulling fraction of ∼77%. This pulsar highlights the advantages of the survey's long dwell times (∼80 minutes), which, when fully searched, will be sensitive to the expected population of similarly bright, intermittent pulsars with long nulls. A single-pulse analysis in the MWA's 140–170 MHz band also reveals a complex subpulse drifting behavior, including both rapid changes of the drift rate characteristic of mode switching pulsars, as well as a slow, consistent evolution of the drift rate within modes. In some longer drift sequences, interruptions in the otherwise smooth drift rate evolution occur preferentially at a particular phase, typically lasting a few pulses. These properties make this pulsar an ideal test bed for prevailing models of drifting behavior such as the carousel model.

 

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.

 

ABSTRACT We present results from a radio survey for variable and transient sources on 15-min time-scales, using the Australian SKA Pathfinder (ASKAP) pilot surveys. The pilot surveys consist of 505 h of observations conducted at around 1 GHz observing frequency, with a total sky coverage of 1476 deg2. Each observation was tracked for approximately 8 – 10 h, with a typical rms sensitivity of ∼30 μJy beam−1 and an angular resolution of ∼12 arcsec. The variability search was conducted within each 8 – 10 h observation on a 15-min time-scale. We detected 38 variable and transient sources. Seven of them are known pulsars, including an eclipsing millisecond pulsar, PSR J2039−5617. Another eight sources are stars, only one of which has been previously identified as a radio star. For the remaining 23 objects, 22 are associated with active galactic nuclei or galaxies (including the five intra-hour variables that have been reported previously), and their variations are caused by discrete, local plasma screens. The remaining source has no multiwavelength counterparts and is therefore yet to be identified. This is the first large-scale radio survey for variables and transient sources on minute time-scales at a sub-mJy sensitivity level. We expect to discover ∼1 highly variable source per day using the same technique on the full ASKAP surveys. 

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 $0.4$ – $1.0\:\rm M_\odot$ . 

Abstract GW170817 is the first binary neutron star (NS) merger detected in gravitational waves (GWs) and photons, and so far remains the only GW event of its class with a definitive electromagnetic counterpart. Radio emission from the structured jet associated with GW170817 has faded below the sensitivity achievable via deep radio observations with the most sensitive radio arrays currently in operation. Hence, we now have the opportunity to probe the radio re-brightening that some models predict, which should emerge at late times from the interaction of the dynamically stripped merger ejecta with the interstellar medium. Here we present the latest results from our deep radio observations of the GW170817 field with the Karl G. Jansky Very Large Array (VLA), 4.5 yr after the merger. Our new data at 3 GHz do not show any compelling evidence for emission in excess to the tail of the jet afterglow (<3.3 μ Jy), confirming our previous results. We thus set new constraints on the dynamical ejecta afterglow models. These constraints favor single-speed ejecta with energies ≲10 50 erg (for an ejecta speed of β 0 = 0.5), or steeper energy–speed distributions of the kilonova ejecta. Our results also suggest larger values of the cold, nonrotating maximum NS mass in equal-mass scenarios. However, without a detection of the dynamical ejecta afterglow, obtaining precise constraints on the NS equation of state remains challenging. 

ABSTRACT We present a low-frequency (170–200 MHz) search for prompt radio emission associated with the long GRB 210419A using the rapid-response mode of the Murchison Widefield Array (MWA), triggering observations with the Voltage Capture System for the first time. The MWA began observing GRB 210419A within 89 s of its detection by Swift, enabling us to capture any dispersion delayed signal emitted by this gamma-ray burst (GRB) for a typical range of redshifts. We conducted a standard single pulse search with a temporal and spectral resolution of $100\, \mu$s and 10 kHz over a broad range of dispersion measures from 1 to $5000\, \text{pc}\, \text{cm}^{-3}$, but none were detected. However, fluence upper limits of 77–224 Jy ms derived over a pulse width of 0.5–10 ms and a redshift of 0.6 < z < 4 are some of the most stringent at low radio frequencies. We compared these fluence limits to the GRB jet–interstellar medium interaction model, placing constraints on the fraction of magnetic energy (ϵB ≲ [0.05–0.1]). We also searched for signals during the X-ray flaring activity of GRB 210419A on minute time-scales in the image domain and found no emission, resulting in an intensity upper limit of $0.57\, \text{Jy}\, \text{beam}^{-1}$, corresponding to a constraint of ϵB ≲ 10−3. Our non-detection could imply that GRB 210419A was at a high redshift, there was not enough magnetic energy for low-frequency emission, or the radio waves did not escape from the GRB environment.