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The population of radio-loud stars has to date been studied primarily through either targeted observations of a small number of highly active stars or wide-field, single-epoch surveys that cannot easily distinguish stellar emission from background extragalactic sources. As a result it has been difficult to constrain population statistics such as the surface density and fraction of the population producing radio emission in a particular variable or spectral class. In this paper, we present a sample of 36 radio stars detected in a circular polarization search of the multi-epoch Variables and Slow Transients (VAST) pilot survey with ASKAP at 887.5 MHz. Through repeat sampling of the VAST pilot survey footprint we find an upper limit to the duty cycle of M-dwarf radio bursts of $8.5 \,\rm {per\,cent}$, and that at least 10 ± 3 $\rm {per\,cent}$ of the population should produce radio bursts more luminous than $10^{15} \,\rm {erg}\mathrm{s}^{-1} \,\mathrm{Hz}^{-1}$. We infer a lower limit on the long-term surface density of such bursts in a shallow $1.25 \,\mathrm{m}\rm {Jy}\rm\ {PSF}^{-1}$ sensitivity survey of ${9}^{\, +{11}}_{-{7}}\times 10^{-3}$  $\,\deg ^{-2}$ and an instantaneous radio star surface density of 1.7 ± 0.2 × 10−3  $\,\deg ^{-2}$ on 12 min time-scales. Based on these rates we anticipate ∼200 ± 50 new radio star detections per year over the full VAST survey and ${41\, 000}^{\, +{10\, 000}}_{-{9\, 000}}$ in next-generation all-sky surveys with the Square Kilometre Array.

 

We present an intensive multiwavelength monitoring campaign of the quasar PG 1302−102 with Swift and the Las Cumbres Observatory network telescopes. Atz∼ 0.3, it tests the limits of the reverberation mapping (RM) technique in probing the accretion disk around a supermassive black hole (SMBH) and extends the parameter space to high masses and high accretion rates. This is also the first time the RM technique has been applied to test disk structures predicted in the SMBH binary model that has been suggested for this source. PG 1302−102 was observed at a ∼daily cadence for ∼9 months in 14 bands spanning from X-ray to UV and optical wavelengths, and it shows moderate to significant levels of variability correlated between wavelengths. We measure the interband time lags, which are consistent with aτ∝λ^4/3relation as expected from standard disk reprocessing, albeit with large uncertainties. The disk size implied by the lag spectrum is consistent with the expected disk size for its black hole mass within uncertainties. While the source resembles other reverberation-mapped active galactic nuclei in many respects, and we do not find evidence supporting the prevalent hypothesis that it hosts an SMBH binary, we demonstrate the feasibility of studying SMBH binaries from this novel angle and suggest possibilities for the LSST Deep Drilling Fields.

 

In this paper, we present an optimized version of the detection pipeline for the ASKAP Variables and Slow Transients (VAST) survey, offering significant performance improvement. The key to this optimization is the replacement of the original w-projection algorithm integrated in the Common Astronomy Software Applications package with the w-stacking algorithm implemented in the WSClean software. Our experiments demonstrate that this optimization improves the overall processing efficiency of the pipeline by approximately a factor of 3. Moreover, the residual images generated by the optimized pipeline exhibit lower noise levels and fewer artefact sources, suggesting that our optimized pipeline not only enhances detection accuracy but also improves imaging fidelity. This optimized VAST detection pipeline is integrated into the Data Activated Liu Graph Engine (DALiuGE) execution framework, specifically designed for SKA-scale big data processing. Experimental results show that the performance and scalability advantages of the pipeline using DALiuGE over traditional MPI or BASH techniques increase with the data size. In summary, the optimized transient detection pipeline significantly reduces runtime, increases operational efficiency, and decreases implementation costs, offering a practical optimization solution for other ASKAP imaging pipelines as well.

 

We report the discovery of a young, highly scattered pulsar in a search for highly circularly polarized radio sources as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients survey. In follow-up observations with the Parkes radio telescope, Murriyang, we identified PSR J1032−5804 and measured a period of 78.7 ms, a dispersion measure of 819 ± 4 pc cm⁻³, a rotation measure of −2000 ± 1 rad m⁻², and a characteristic age of 34.6 kyr. We found a pulse scattering timescale at 3 GHz of ∼22 ms, implying a timescale at 1 GHz of ∼3845 ms, which is the third most scattered pulsar known and explains its nondetection in previous pulsar surveys. We discuss the identification of a possible pulsar wind nebula and supernova remnant in the pulsar’s local environment by analyzing the pulsar spectral energy distribution and the surrounding extended emission from multiwavelength images. Our result highlights the possibility of identifying extremely scattered pulsars from radio continuum images. Ongoing and future large-scale radio continuum surveys will offer us an unprecedented opportunity to find more extreme pulsars (e.g., highly scattered, highly intermittent, and highly accelerated), which will enhance our understanding of the characteristics of pulsars and the interstellar medium.

 

Although neutron star–black hole binaries have been identified through mergers detected in gravitational waves, a pulsar–black hole binary has yet to be detected. While short-period binaries are detectable due to a clear signal in the pulsar’s timing residuals, effects from a long-period binary could be masked by other timing effects, allowing them to go undetected. In particular, a long-period binary measured over a small subset of its orbital period could manifest via time derivatives of the spin frequency incompatible with isolated pulsar properties. We assess the possibility of pulsars having unknown companions in long-period binaries and put constraints on the range of binary properties that may remain undetected in current data, but that may be detectable with further observations. We find that for 35% of canonical pulsars with published higher-order derivatives, the precision of measurements is not enough to confidently reject binarity (period ≳2 kyr), and that a black hole binary companion could not be ruled out for a sample of pulsars without published constraints if the period is >1 kyr. While we find no convincing cases in the literature, we put more stringent limits on orbital period and longitude of periastron for the few pulsars with published higher-order frequency derivatives (n≥ 3). We discuss the detectability of candidates and find that a sample pulsar in a 100 yr orbit could be detectable within 5–10 yr.

 

Large widefield surveys make possible the serendipitous discovery of rare subclasses of pulsars. One such class are ‘spider’-type pulsar binaries, comprised of a pulsar in a compact orbit with a low-mass (sub)stellar companion. In a search for circularly polarized radio sources in Australian Square Kilometre Array Pathfinder (ASKAP) Pilot Survey observations, we discovered highly variable and circularly polarized emission from a radio source within the error region of the γ-ray source 4FGL J1646.5−4406. The variability is consistent with the eclipse of a compact, steep-spectrum source behind ablated material from a companion in an ∼5.3 h binary orbit. Based on the eclipse properties and spatial coincidence with 4FGL J1646.5−4406, we argue that the source is likely a recycled pulsar in a ‘redback’ binary system. Using properties of the eclipses from ASKAP and Murchison Widefield Array observations, we provide broad constraints on the properties of the eclipse medium. We identified a potential optical/infrared counterpart in archival data consistent with a variable low-mass star. Using the Parkes radio telescope ‘Murriyang’ and the Meer Karoo Array Telescope (MeerKAT) , we searched extensively for radio pulsations but yielded no viable detections of pulsed emission. We suggest that the non-detection of pulses is due to scattering in the intra-binary material, but scattering from the interstellar medium can also plausibly explain the pulse non-detections if the interstellar dispersion measure exceeds ∼600 pc cm−3. Orbital constraints derived from optical observations of the counterpart would be highly valuable for future γ-ray pulsation searches, which may confirm the source nature as a pulsar.

 

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.

 

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. 

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.