arXiv subjects

Laura Driessen

Publications and source records attributed to Laura Driessen.

11 recordsLinked to original sources

arXiv2026

Radio Properties of RS Canum Venaticorum Variables in VLASS and RACS

We performed a systematic search for radio emission from RS Canum Venaticorum (RS CVn) binaries, selected from the International Variable Star Index (VSX) catalog, in the Very Large Array Sky Survey (VLASS; three epochs) and Rapid ASKAP Continuum Survey (RACS; two epochs) data. We detected 108 candidate radio-emitting RS CVn in at least one epoch. Several of these systems rank among the most radio-luminous RS CVn binaries reported to date. The radio and X-ray luminosities, obtained from cross-matching with the eROSITA and ROSAT X-ray catalogs, are consistent with the Guedel-Benz relation for magnetically active stars, but are also comparable to radio-luminous quiescent black hole X-ray binaries, indicating a potential for misidentification between these two classes. Analysis of optical, radio, and stellar properties indicates that optically bright RS CVn (i.e., those with at least one giant component) are radio-quieter and have periods that are consistent with lower coronal activity. However, two of these optically bright RS CVn systems show persistent and unusually high radio specific luminosities (>2e17 erg/s/Hz) across all observed epochs, showing that stellar activity can produce relatively persistent radio signals as bright as quiescent black hole binaries.

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arXiv2026

Coherent and Incoherent Emission from the Ordered Magnetospheres of Low-Mass Stars, UCDs, and Massive Stars

Massive early-type (B/A) stars and ultracool dwarfs (UCDs) represent two distinct regimes in which ordered, large-scale magnetospheres are observed. In rapidly rotating massive stars, incoherent radio emission is explained by the centrifugal breakout (CBO) mechanism: plasma confined within the rigidly rotating magnetosphere accumulates beyond the co-rotation radius, where centrifugal forces trigger breakout events and magnetic reconnection, generating non-thermal electrons that produce incoherent gyro-synchrotron emission. Empirically, the radio luminosity correlates with the power released by CBO events, establishing a clear link between stellar rotation, magnetic confinement, and radio output. In UCDs, persistent non-thermal radio emission exhibits similar luminosity trends to those of massive magnetic stars, despite the absence of strong stellar winds. This similarity suggests that a CBO-like process may also operate in these fully convective, low-mass objects, though the plasma source and acceleration mechanisms remain uncertain. In both classes, coherent electron cyclotron maser emission (ECME), characterized by strong polarization and rotational modulation, is observed, indicating common magnetospheric processes analogous to planetary auroral emission. The Square Kilometre Array (SKA) will be able to deeply observe about 70\% of the sky. We expect to observe $\sim 1000$ UCDs, enabling better statistical analysis of their emission and a test of the CBO hypothesis.

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arXiv2026

A sample of short-lived Galactic radio transients from ASKAP VAST

Galactic radio transients (GRTs) are mysterious short-lived (~days to months) radio transients that are quiet at all other wavelengths. Until now, roughly half a dozen such sources have been reported, predominantly towards the Galactic center. However, no unifying properties have been identified among these, leaving their nature, emission mechanism, and even classification poorly understood. Due to the lack of periodic and uniform radio observations over wide areas of the Galactic plane until now, the sample size of such transients remained limited. Here, we use radio observations from the Australian SKA Pathfinder's Variables and Slow Transients survey to discover six new radio transients along the Galactic plane that resemble GRTs. Detailed investigation of archival data suggests that these sources may be divided into two classes: sources that exhibit sporadic, pulse-like (minutes) radio emission, and sources that exhibit long-term (weeks) flaring-type radio emission. For the short-time variable sources, we draw similarities between optically bright long-period radio transients and our sample to propose wide-orbit (~days) white dwarf binaries as underlying sources. For sources that show long-term outbursts, we draw comparisons between dust-obscured outbursts from WD binaries and our sample. These results could imply that the ongoing wide-field radio surveys are uncovering radio emission from sub-populations of WD binaries that were previously unexplored.

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arXiv2025

Discovery of Two Highly Scattered Pulsars from Image-Based Circular Polarization Searches with the Australian SKA Pathfinder

We report the discovery and timing of two pulsars from a sample of four circularly polarized sources identified in radio continuum images taken as part of the Australian SKA Pathfinder (ASKAP) Variables and Slow Transients (VAST) survey. Observations with the Parkes (Murriyang) radio telescope confirmed both sources as normal pulsars with high dispersion measures. PSR J1646$-$4451 has a spin period of 217 ms and a dispersion measure (DM) of 928 $\rm cm^{-3} \, pc$, while PSR J1837$-$0616 exhibits a spin period of 118 ms and a DM of 793 $\rm cm^{-3} pc$. These pulsars show extreme pulse broadening due to scattering, with measured scattering timescales of 117 ms and 75 ms at observing frequencies of $\sim$ 1.8 GHz, respectively. These measurements imply extrapolated scattering timescales at 1 GHz of $\sim$1346 ms and 740 ms, placing them among the most heavily scattered known pulsars. Our findings underscore the potential of using circular polarization in radio continuum images as a tool for identifying highly scattered pulsars. Future wide-field radio continuum surveys are poised to uncover a broader population of extreme pulsars, particularly those that are heavily scattered at 1.4 GHz, intrinsically faint, or residing in binaries$-$offering valuable insights into both pulsar demographics and the complex structure of the ionized interstellar medium.

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arXiv2025

ASKAP J144834-685644: a newly discovered long period radio transient detected from radio to X-rays

Long-period radio transients (LPTs) are an emerging group of radio transients that show periodic polarized radio bursts with periods varying from a few minutes to a few hours. Fewer than a dozen LPTs have been detected so far, and their origin (source and emission mechanism) remains unclear. Here, we report the discovery of a 1.5 h LPT, ASKAP J144834-685644, adding to the current sample of sources. ASKAP J144834-685644 is one of the very few LPTs that has been detected from X-rays to radio. It shows a steep radio spectrum and polarized radio bursts, which resemble the radio emission in known LPTs. In addition, it also shows highly structured and periodic narrow-band radio emission. Multiwavelength properties suggest that the spectral energy distribution (SED) peaks at near ultraviolet wavelengths, indicating the presence of a hot magnetic source. Combining multiwavelength information, we infer that ASKAP J144834-685644 may be a near edge-on magnetic white dwarf binary (MWD), although we cannot fully rule out ASKAP J144834-685644 being an isolated white dwarf pulsar or even a transitional millisecond pulsar (despite the lack of radio pulsations). If ASKAP J144834-685644 is a MWD binary, the observed broad-band SED can be explained by emission from an accretion disc. This hints that some fraction of optically bright LPTs may be accreting binaries with the radio period being the orbital period. It might further suggest a connection between optically bright synchronized WD binaries, such as polars, and non-accreting asynchronous WD pulsars, such as AR Sco and J1912-4410.

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arXiv2024

Late-Time Supernovae Radio Re-brightening in the VAST Pilot Survey

We present our analysis of supernovae serendipitously found to be radio-bright several years after their optical discovery. We used recent observations from the Australian SKA Pathfinder taken as part of the pilot Variables and Slow Transients and Rapid ASKAP Continuum Survey programs. We identified 29 objects by cross-matching sources from these ASKAP observations with known core-collapse supernovae below a declination of $+40^{\circ}$ and with a redshift of $z\leq0.15$. Our results focus on eight cases that show potential late-time radio emission. These supernovae exhibit significantly greater amounts of radio emission than expected from the standard model of a single shockwave propagating through a spherical circumstellar medium, with a constant density structure produced by regular stellar mass-loss. We also discuss how we can learn from future ASKAP surveys about the circumstellar environments and emission mechanisms of supernovae that undergo late-time radio re-brightening. This pilot work tested and confirmed the potential of the Variables and Slow Transients survey to discover and study late-time supernova emission.

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arXiv2023

Periodic Radio Emission from the T8 Dwarf WISE J062309.94-045624.6

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.

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arXiv2023

Radio Variable and Transient Sources on Minute Timescales in the ASKAP Pilot Surveys

We present results from a radio survey for variable and transient sources on 15-min timescales, 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 deg$^2$. Each observation was tracked for approximately 8-10h, with a typical rms sensitivity of $\sim$30 $\mu$jy/beam and an angular resolution of $\sim$12 arcsec. The variability search was conducted within each 8-10h observation on a 15-min timescale. We detected 38 variable and transient sources. Seven of them are known pulsars, including an eclipsing millisecond pulsar, PSR J2039$-$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 multi-wavelength counterparts and is therefore yet to be identified. This is the first large-scale radio survey for variables and transient sources on minute timescales at a sub-mJy sensitivity level. We expect to discover $\sim$1 highly variable source per day using the same technique on the full ASKAP surveys.

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arXiv2023

Bursts from Space: MeerKAT - The first citizen science project dedicated to commensal radio transients

The newest generation of radio telescopes are able to survey large areas with high sensitivity and cadence, producing data volumes that require new methods to better understand the transient sky. Here we describe the results from the first citizen science project dedicated to commensal radio transients, using data from the MeerKAT telescope with weekly cadence. Bursts from Space: MeerKAT was launched late in 2021 and received ~89000 classifications from over 1000 volunteers in 3 months. Our volunteers discovered 142 new variable sources which, along with the known transients in our fields, allowed us to estimate that at least 2.1 per cent of radio sources are varying at 1.28 GHz at the sampled cadence and sensitivity, in line with previous work. We provide the full catalogue of these sources, the largest of candidate radio variables to date. Transient sources found with archival counterparts include a pulsar (B1845-01) and an OH maser star (OH 30.1-0.7), in addition to the recovery of known stellar flares and X-ray binary jets in our observations. Data from the MeerLICHT optical telescope, along with estimates of long time-scale variability induced by scintillation, imply that the majority of the new variables are active galactic nuclei. This tells us that citizen scientists can discover phenomena varying on time-scales from weeks to several years. The success both in terms of volunteer engagement and scientific merit warrants the continued development of the project, whilst we use the classifications from volunteers to develop machine learning techniques for finding transients.

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arXiv2022

Serendipitous discovery of radio flaring behaviour from a nearby M dwarf with MeerKAT

We report on the detection of MKT J174641.0$-$321404, a new radio transient found in untargeted searches of wide-field MeerKAT radio images centred on the black hole X-ray binary H1743$-$322. MKT J174641.0$-$321404 is highly variable at 1.3 GHz and was detected three times during 11 observations of the field in late 2018, reaching a maximum flux density of 590 $\pm$ 60 $\mu$Jy. We associate this radio transient with a high proper motion, M dwarf star SCR~1746$-$3214 12 pc away from the Sun. Multiwavelength observations of this M dwarf indicate flaring activity across the electromagnetic spectrum, consistent with emission expected from dMe stars, and providing upper limits on quiescent brightness in both the radio and X-ray regimes. \textit{TESS} photometry reveals a rotational period for SCR~1746$-$3214 of $0.2292 \pm 0.0025$ days, which at its estimated radius makes the star a rapid rotator, comparable to other low mass systems. Dedicated spectroscopic follow up confirms the star as a mid-late spectral M dwarf with clear magnetic activity indicated by strong H$\alpha$ emission. This transient's serendipitous discovery by MeerKAT, along with multiwavelength characterisation, make it a prime demonstration of both the capabilities of the current generation of radio interferometers and the value of simultaneous observations by optical facilities such as MeerLICHT. Our results build upon the literature of of M dwarfs' flaring behaviour, particularly relevant to the habitability of their planetary systems.

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arXiv2021

MeerTRAP in the era of multi-messenger astrophysics

Real-time detections of transients and rapid multi-wavelength follow-up are at the core of modern multi-messenger astrophysics. MeerTRAP is one such instrument that has been deployed on the MeerKAT radio telescope in South Africa to search for fast radio transients in real-time. This, coupled with the ability to rapidly localize the transient in combination with optical co-pointing by the MeerLICHT telescope gives the instrument the edge in finding and identifying the nature of the transient on short timescales. The commensal nature of the project means that MeerTRAP will keep looking for transients even if the telescope is not being used specifically for that purpose. Here, we present a brief overview of the MeerTRAP project. We describe the overall design, specifications and the software stack required to implement such an undertaking. We conclude with some science highlights that have been enabled by this venture over the last 10 months of operation.

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