Relativistic jets from supermassive black holes in active galactic nuclei are amongst the most powerful phenomena in the universe. Similar jets from stellar-mass black holes offer a chance to study the phenomena on accessible observation time scales. However, such comparative studies across black hole masses and time scales remain hampered by the long-standing perception that stellar-mass black hole jets are in a less relativistic regime. Here, we show the detection of two distinct, relativistic jet ejections from the Galactic black hole X-ray binary 4U 1543-47 during a single outburst, with radio interferometry monitoring observations. Our measurements reveal a likely Lorentz factor of approximately 8 and a minimum of 4.6 at launch with 95% confidence, demonstrating that stellar-mass black holes in X-ray binaries can launch jets as relativistic as those seen in active galactic nuclei.

Correlated behaviours between the radio emission and the X-ray emission in Galactic black hole X-ray binaries (BH XRBs) in the X-ray hard state are crucial to the understanding of disc-jet coupling of accreting black holes. The BH transient 4U 1543-47 went into outburst in 2021 following similar to 19 yr of quiescence. We followed it up with similar to weekly cadence with MeerKAT for about one year and a half until it faded into quiescence. Multi-epoch quasi-simultaneous MeerKAT and X-ray observations allowed us to trace the compact jet emission and its X-ray emission. In its hard spectral state across three orders of magnitude of X-ray luminosities above similar to 10(34) ergs s(-1), we found the correlation between radio and X-ray emission had a power-law index of 0.82 +/- 0.09, steeper than the canonical value of similar to 0.6 for BH XRBs. In addition, the radio versus X-ray correlation show a large range of the power-law normalization, with the maximum significantly larger than that obtained for most BH XRBs, indicating it can be particularly radio-bright and variable in the X-ray binary sample. The radio emission is unlikely diluted by discrete jet components. The observed peculiar radio-bright and variable behaviours provide the evidence for the relativistic effects of a variable Lorentz factor in the range between 1 and similar to 2 of the compact jet.

A key science goal of large sky surveys such as those conducted by the Vera C. Rubin Observatory and precursors to the Square Kilometre Array is the identification of variable and transient objects. One approach is analyzing time series of the changing brightness of sources, namely, light curves. However, finding adequate statistical representations of light curves is challenging because of the sparsity of observations, irregular sampling, and nuisance factors inherent in astronomical data collection. The wide diversity of objects that a large-scale survey will observe also means that making parametric assumptions about the shape of light curves is problematic. We present a Gaussian process (GP) regression approach for characterizing light-curve variability that addresses these challenges. Our approach makes no assumptions about the shape of a light curve and, therefore, is general enough to detect a range of variable and transient source types. In particular, we propose using the joint distribution of GP amplitude hyperparameters to distinguish variable and transient candidates from nominally stable ones and apply this approach to 6394 radio light curves from the ThunderKAT survey. We compare our results with two variability metrics commonly used in radio astronomy, namely η _ν and V _ν , and show that our approach has better discriminatory power and interpretability. Finally, we conduct a rudimentary search for transient sources in the ThunderKAT data set to demonstrate how our approach might be used as an initial screening tool. Computational notebooks in Python and R are available to help deploy this framework to other surveys.

We report on optically selected stellar candidates of SARAO MeerKAT 1.3 GHz radio continuum survey sources of the Galactic plane. Stellar counterparts to radio sources are selected by cross-matching the MeerKAT source positions with Gaia DR3, using two approaches. The first approach evaluated the probability of chance alignments between the radio survey and Gaia sources and used AllWISE infrared colour-colour information to select potential stellar candidates. The second approach utilized a Monte Carlo method to evaluate the cross-matching reliability probability, based on populations of known radio-emitting stars. From the combined approaches, we found 629 potential stellar counterparts, of which 169 have existing SIMBAD classifications, making it the largest Galactic plane radio-optical crossmatch sample to date. A colour-magnitude analysis of the sample revealed a diverse population of stellar objects, ranging from massive OB stars, main-sequence stars, giants, young stellar objects, emission line stars, red dwarfs and white dwarfs. Some of the proposed optical counterparts include chromospherically/coronally active stars, for example RS CVn binaries, BY Dra systems, YSOs and flare stars, which typically exhibit radio emission. Based on Gaia's low-resolution spectroscopy, some of the stars show strong Hα emission, indicating they are magnetically active, consistent with them being radio emitters. While MeerKAT's sensitivity and survey speed make it ideal for detecting faint radio sources, its angular resolution limits accurate counterpart identification for crowded fields such as the Galactic Plane. Higher frequency, and, thereby, better spatial resolution, radio observations plus circular polarization would be required to strengthen the associations.

The sensitivity and field of view of the MeerKAT radio telescope provide excellent opportunities for commensal transient searches. We carry out a commensal transient search in supernova and short gamma-ray burst fields using methodologies established by S. I. Chastain et al. We search for transients in MeerKAT L-band images with integration times of 30 minutes, finding 13 variable sources. We compare these sources to the VLASS and RACS survey data, and examine possible explanations for the variability. Additionally, for one of these sources we examine archival Chandra ACIS data. We find that 12 of these sources are consistent with variability due to interstellar scintillation. The remaining source could possibly have some intrinsic variability. We also split the MeerKAT L band into upper and lower halves, and search for transients in images with an integration time of 8 s. We find a source with a duration of 8–16 s that is highly polarized at the lowest frequencies. This source is spatially coincident with a star detected by the Transiting Exoplanet Survey Satellite. We conclude that this source may be consistent with a stellar flare. Finally, we calculate accurate upper and lower limits on the transient rate using transient simulations.

We present the SARAO MeerKAT Galactic Plane Survey (SMGPS), a 1.3 GHz continuum survey of almost half of the Galactic Plane (251 degrees <= l <= 358 degrees and 2 degrees <= l <= 61. at vertical bar b vertical bar = 1 degrees.5). SMGPS is the largest, most sensitive, and highest angular resolution 1 GHz survey of the plane yet carried out, with an angular resolution of 8 arcsec and a broad-band root-mean-square sensitivity of similar to 10-20 mu Jy beam(-1). Here, we describe the first publicly available data release from SMGPS which comprises data cubes of frequency-resolved images over 908-1656 MHz, power-law fits to the images, and broad-band zeroth moment integrated intensity images. A thorough assessment of the data quality and guidance for future usage of the data products are given. Finally, we discuss the tremendous potential of SMGPS by showcasing highlights of the Galactic and extragalactic science that it permits. These highlights include the discovery of a new population of non-thermal radio filaments; identification of new candidate supernova remnants, pulsar wind nebulae and planetary nebulae; improved radio/mid-infrared classification of rare luminous blue variables and discovery of associated extended radio nebulae; new radio stars identified by Bayesian cross-matching techniques; the realization that many of the largest radio-quiet Wide-field Infrared Survey Explorer (WISE) H II region candidates are not true H II regions; and a large sample of previously undiscovered background H I galaxies in the Zone of Avoidance.

Context. The recurrent and symbiotic nova RS Ophiuchi (RS Oph) underwent a new outburst phase during August 2021, about 15 years after the last event that occurred in 2006. This outburst represents the first nova event ever detected at very high energies (VHE, E> 100 GeV), and a whole set of coordinated multiwavelength observations were triggered by this event. Aims. The main goals of this work are to characterize the evolving morphology of the expanding bipolar ejecta with high accuracy and to determine the physical conditions of the surrounding medium in which they propagate. Methods. By means of high-resolution very long baseline interferometry (VLBI) radio observations, we monitored RS Oph with the European VLBI Network (EVN) and e-MERLIN at 1.6 and 5 GHz during multiple epochs from 14 to 65 days after the explosion. Results. We reveal an evolving source structure consisting of a central and compact core and two elongated bipolar outflows expanding on opposite sides of the core in the east-west direction. The ejecta angular separation with time is consistent with a linear expansion with an average projected speed of 7000 km s(-1). We find clear evidence of a radial dependence of the density along the density enhancement on the orbital plane (DEOP), going from 1 x 10(7) cm(-3) close to the central binary to 9 x 10(5) cm(-3) at 400 AU. Conclusions. Thanks to the accurate source astrometric position provided by Gaia DR3, in this work we draw a detailed scenario of the geometry and physics of the RS Oph evolving source structure after the most recent nova event. We conclude that most of the mass lost by the red giant companion goes into the DEOP, for which we estimate a total mass of 6.4 x 10(-6) M-(R), and into the circumstellar region, while only a small fraction (about one-tenth) is accreted by the white dwarf.

Transient surveys are a vital tool in exploring the dynamic Universe, with radio transients acting as beacons for explosive and highly energetic astrophysical phenomena. However, performing commensal transient surveys using radio imaging can require a significant amount of computing power, data storage, and time. With the instrumentation available to us, and with new and exciting radio interferometers in development, it is essential that we develop efficient methods to probe the radio transient sky. In this paper, we present results from a commensal short -duration transient survey, on time -scales of 8 s, 128 s, and 1 h, using data from the MeerKAT radio telescope. The data set used was obtained as part of a galaxy observing campaign, and we focus on the field of NGC 5068. We present a quick, wide -field imaging strategy to enable fast imaging of large data sets, and develop methods to efficiently filter detected transient candidates. No transient candidates were identified on the time -scales of 8 s, 128 s, and 1 h, leading to competitive limits on the transient surface densities of 6 . 7 x 10 (-5,) 1 . 1 x 10 (-3), and 3 . 2 x 10( -2) deg(-1) at sensitivities of 56.4, 19.2, and 3.9 mJy following primary beam correction for the respective time -scales. We find one possible candidate that could be associated with a stellar flare, which was rejected due to strict image quality control. Further short time -scale radio observations of this candidate could give definite results about its origin.

We present the first sub-arcsecond localized Fast Radio Burst (FRB) detected using MeerKAT. FRB 20210405I was detected in the incoherent beam using the MeerTRAP pipeline on 2021 April 05 with a signal to noise ratio of 140.8 and a dispersion measure of 565.17 pc cm(-3). It was detected while MeerTRAP was observing commensally with the ThunderKAT large survey project, and was sufficiently bright that we could use the ThunderKAT 8s images to localize the FRB. Two different models of the dispersion measure in the Milky Way and halo suggest that the source is either right at the edge of the Galaxy, or outside. This highlights the uncertainty in the Milky Way dispersion measure models, particularly in the Galactic Plane, and the uncertainty of Milky Way halo models. Further investigation and modelling of these uncertainties will be facilitated by future detections and localizations of nearby FRBs. We use the combined localization, dispersion measure, scattering, specific luminosity, and chance coincidence probability information to find that the origin is most likely extra-galactic and identify the likely host galaxy of the FRB: 2MASS J1701249-4932475. Using SALT spectroscopy and archival observations of the field, we find that the host is a disc/spiral galaxy at a redshift of z = 0.066.

We report on a radiopolarimetric observation of the Saturn-Jupiter Great Conjunction of 2020 using the MeerKAT L-band system, initially carried out for science verification purposes, which yielded a serendipitous discovery of a pulsar. The radiation belts of Jupiter are very bright and time variable: coupled with the sensitivity of MeerKAT, this necessitated development of dynamic imaging techniques, reported on in this work. We present a deep radio "movie" revealing Jupiter's rotating magnetosphere, a radio detection of Callisto, and numerous background radio galaxies. We also detect a bright radio transient in close vicinity to Saturn, lasting approximately 45 minutes. Follow-up deep imaging observations confirmed this as a faint compact variable radio source, and yielded detections of pulsed emission by the commensal MeerTRAP search engine, establishing the object's nature as a radio emitting neutron star, designated PSR J2009-2026. A further observation combining deep imaging with the PTUSE pulsar backend measured detailed dynamic spectra for the object. While qualitatively consistent with scintillation, the magnitude of the magnification events and the characteristic timescales are odd. We are tentatively designating this object a pulsar with anomalous refraction recurring on odd timescales (PARROT). As part of this investigation, we present a pipeline for detection of variable sources in imaging data, with dynamic spectra and lightcurves as the products, and compare dynamic spectra obtained from visibility data with those yielded by PTUSE. We discuss MeerKAT's capabilities and prospects for detecting more of such transients and variables.

The MHONGOOSE (MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters) survey maps the distribution and kinematics of the neutral atomic hydrogen (HI) gas in and around 30 nearby star-forming spiral and dwarf galaxies to extremely low HI column densities. The HI column density sensitivity (3 sigma over 16 km/s) ranges from ~ 5 x 10^{17} cm^{-2} at 90'' resolution to ~4 x 10^{19} cm^{-2} at the highest resolution of 7''. The HI mass sensitivity (3 sigma over 50 km/s) is ~5.5 X 10^5 M_sun at a distance of 10 Mpc (the median distance of the sample galaxies). The velocity resolution of the data is 1.4 km/s. One of the main science goals of the survey is the detection of cold, accreting gas in the outskirts of the sample galaxies. The sample was selected to cover a range in HI masses, from 10^7 M_sun to almost 10^{11} M_sun, to optimally sample possible accretion scenarios and environments. The distance to the sample galaxies ranges from 3 to 23 Mpc. In this paper, we present the sample selection, survey design, and observation and reduction procedures. We compare the integrated HI fluxes based on the MeerKAT data with those derived from single-dish measurement and find good agreement, indicating that our MeerKAT observations are recovering all flux. We present HI moment maps of the entire sample based on the first ten percent of the survey data, and find that a comparison of the zeroth- and second-moment values shows a clear separation between the physical properties of the HI in areas with star formation and areas without, related to the formation of a cold neutral medium. Finally, we give an overview of the HI-detected companion and satellite galaxies in the 30 fields, five of which have not previously been catalogued. We find a clear relation between the number of companion galaxies and the mass of the main target galaxy.

Short gamma-ray bursts (GRBs) are explosive transients caused by binary mergers of compact objects containing at least one neutron star. Multiwavelength afterglow observations provide constraints on the physical parameters of the jet, its surrounding medium, and the microphysics of the enhanced magnetic fields and accelerated electrons in the blast wave at the front of the jet. The synchrotron radio emission can be tracked for much longer than in other spectral regimes, and it can pin down the evolution of the spectral peak. We present the results of a systematic observing campaign of eight short GRBs with the MeerKAT radio telescope. Additionally, we present observations of four of these short GRBs using the ATCA radio telescope and two of these short GRBs with the e-MERLIN radio telescope. Using these results we report one possible detection of a short GRB afterglow from GRB 230217A and deep upper limits for the rest of our short GRB observations. We use these observations to place constraints on some of the physical parameters, in particular those related to electron acceleration, the circumburst density, and gamma-ray energy efficiency. We discuss how deeper observations with new and upgraded telescopes should be able to determine if the gamma-ray efficiency differs between long and short GRBs. We also report detections of the likely host galaxies for four of the eight GRBs and upper limits for another GRB, increasing the number of detected host galaxies in the radio with implications for the star formation rate in these galaxies.

ABSTRACT XTE J1701−462 is a neutron star low-mass X-ray binary (NS LMXB) discovered in 2006 as the first system to demonstrate unambiguously that the ‘Atoll’ and ‘Z’ classes of accreting neutron stars are separated by accretion rate. Radio observations during the 2006/7 outburst provided evidence for the formation of a relativistic jet, as now expected for all accreting neutron star and black hole X-ray binaries at high-accretion rates. The source entered a new outburst in 2022, and we report 29 observations made with the MeerKAT radio telescope. The first radio detection was on the 16th September 2022, we continued detecting the source until mid-December 2022. Thereafter, establishing radio upper limits till 25 March 2023. We present the radio analysis alongside analysis of contemporaneous X-ray observations from MAXI. The radio light curve shows evidence for at least three flare-like events over the first hundred days, the most luminous of which has an associated minimum energy of $1\times 10^{38}$ erg. We provide a detailed comparison with the 2006/7 outburst, and demonstrate that we detected radio emission from the source for considerably longer in the more recent outburst, although this is probably a function of sampling. We further constrain the radio emission from the source to have a polarization of less than 9 per cent at the time of 2022 IXPE detection of X-ray polarization. Finally, we place the source in the radio–X-ray plane, demonstrating that when detected in radio it sits in a comparable region of parameter space to the other Z-sources.

We present the discovery of the radio afterglow of the short gamma-ray burst (GRB) 210726A, localized to a galaxy at a photometric redshift of z ∼ 2.4. While radio observations commenced ≲1 day after the burst, no radio emission was detected until ∼11 days. The radio afterglow subsequently brightened by a factor of ∼3 in the span of a week, followed by a rapid decay (a “radio flare”). We find that a forward shock afterglow model cannot self-consistently describe the multiwavelength X-ray and radio data, and underpredicts the flux of the radio flare by a factor of ≈5. We find that the addition of substantial energy injection, which increases the isotropic kinetic energy of the burst by a factor of ≈4, or a reverse shock from a shell collision are viable solutions to match the broadband behavior. At z ∼ 2.4, GRB 210726A is among the highest-redshift short GRBs discovered to date, as well as the most luminous in radio and X-rays. Combining and comparing all previous radio afterglow observations of short GRBs, we find that the majority of published radio searches conclude by ≲10 days after the burst, potentially missing these late-rising, luminous radio afterglows.

Fifteen years after its previous outburst, the symbiotic recurrent nova RS Oph exploded again on 2021 Aug 8th, its first outburst during the Fermi era. In symbiotic novae, the material ejected from the surface of the white dwarf (WD) after the thermonuclear runaway drives a strong shock through the dense circumstellar gas produced by the red giant (RG) wind. This nova is a perfect real-time laboratory for studying physical processes as diverse as accretion, thermonuclear explosions, shock dynamics and particle acceleration; in many ways it is like a supernova remnant on fast forward. The experience of its previous outburst and that of 2010 for V407 (the symbiotic nova that has been extensively observed during the Fermi era), indicates that a large sensitivity and a broad range of baseline lengths are necessary to follow its evolution over a period of several weeks. This would provide unique constraints on major outstanding problems, including the emission mechanisms, the physical processes at work, the presence and location of shock acceleration, the geometry of the system, and the density of the RG wind. We present preliminary results from the EVN+e-MERLIN observations carried out on weeks/months time scales after the August explosion.

We report the serendipitous discovery of an extremely intermittent radio pulsar, PSR J1710-3452, with a relatively long spin period of 10.4 s. The object was discovered through the detection of 97 bright radio pulses in only one out of 66 epochs of observations spanning almost three years. The bright pulses have allowed the source to be localised to a precision of 0.5" through radio imaging. We observed the source location with the Swift X-ray telescope but did not detect any significant X-ray emission. We did not identify any high-energy bursts or multi-frequency counterparts for this object. The solitary epoch of detection hinders the calculation of the surface magnetic field strength, but the long period and the microstructure in the single-pulses resembles the emission of radio-loud magnetars. If this is indeed a magnetar, it is located at a relatively high Galactic latitude (2.9 degree), making it potentially one of the oldest and the most intermittent magnetars known in the Galaxy. The very short activity window of this object is unique and may point towards a yet undetected population of long period, highly transient radio emitting neutron stars.

In this short paper we outline the case for a small radio telescope array in the southern hemisphere with operations dedicated to rapid follow-up and monitoring of astrophysical transients. We argue that the science harvest from such a facility would be very large, using AMI-LA as an outstanding example of how such a programme is already being operated in the north with an enormous track record of success. A southern radio transients facility would in turn take pressure off the Square Kilometre Array and the other world class larger arrays with 10-100 times more collecting area, which will never have the programme time available to comprehensively pursue this science. We discuss comparisons with the development of transient surveys and follow up in optical astronomy, and also how single millimetre dishes can contribute to radio transients science in the south. This paper is not a funding proposal aimed at any particular body, but rather a concept and discussion piece, and the authors welcome comments and feedback.

Here we present results from an in-depth search for pulsed emission from both close binary systems AE Aquarii (AE Aqr) and AR Scorpii (AR Sco) in radio and gammaray energies. Both systems were observed recently with the MeerKAT telescope, and combined with this, we utilized the combined 10 year Pass 8 Fermi-LAT dataset to search for pulsed gamma-ray emission from both white dwarfs in these systems. Pulsed emission was detected in MeerKAT data from both these close binary systems at a period that is at, or close to, the spin period of the white dwarf. The search for pulsed gammaray emission revealed pulsed emission at the spin period of the white dwarf of AE Aqr after selecting data sets with duration of 2 weeks that show excess emission above the 2 sigma significance level. Braking these two-week sets up in 10 minute intervals and stacking the power spectra revealed pulsed emission at both the spin (P-* = 33:08 s) and its associated first harmonic (P-1 = 16:54 s). A full 10 year analysis of the AR Sco data revealed pulsed emission at the spin period/beat period of the white dwarf, albeit at a lower significance level. Several control analyses were performed to verify the authenticity of the emission in both radio and gamma-rays, which will be discussed in the main text. The results of this study definitely reveal that both white dwarfs in these systems contain a particle accelerator that accelerates charged particles to high energies resulting in associated non-thermal radio and gamma-ray emission.

We present radio coverage of the 2019 outburst of the accreting millisecond X-ray pulsar SAX J1808.4-3658, obtained with MeerKAT. We compare these data to contemporaneous X-ray and optical measurements in order to investigate the coupling between accretion and jet formation in this system, while the optical lightcurve provides greater detail of the outburst. The reflaring activity following the main outburst peak was associated with a radio re-brightening, indicating a strengthening of the jet in this phase of the outburst. We place quasi-simultaneous radio and X-ray measurements on the global radio:X-ray plane for X-ray binaries, and show they reside in the same region of luminosity space as previous outburst measurements, but significantly refine the correlation for this source. We also present upper limits on the radio emission from the accreting millisecond X-ray pulsar MAXI J0911-655 and the transitional Z/Atoll-type transient XTE J1701-462. In the latter source we also confirm that nearby large-scale structures reported in previous radio observations of the source are persistent over a period of ~15 years, and so are almost certainly background radio galaxies and not associated with the X-ray transient.

We present radio observations of the symbiotic recurrent nova V3890 Sagitarii following the 2019 August eruption obtained with the MeerKAT radio telescope at 1.28 GHz and Karl G. Janksy Very Large Array (VLA) at 1.26 to 5 GHz. The radio light curves span from day 1 to 540 days after eruption and are dominated by synchrotron emission produced by the expanding nova ejecta interacting with the dense wind from an evolved companion in the binary system. The radio emission is detected early on (day 6) and increases rapidly to a peak on day 15. The radio luminosity increases due to a decrease in the opacity of the circumstellar material in front of the shocked material and fades as the density of the surrounding medium decreases and the velocity of the shock decelerates. Modelling the light curve provides an estimated mass-loss rate of $M_{\textrm {wind}} \approx 10^{-8} {\textrm {M}}_\odot~{\textrm {yr}}^{-1}$ from the red giant star and ejecta mass in the range of $M_{\textrm {ej}}=10^{-5}-10^{-6}~{\textrm {M}}_\odot$from the surface of the white dwarf. V3890 Sgr likely hosts a massive white dwarf similar to other symbiotic recurrent novae, thus considered a candidate for supernovae type Ia (SNe Ia) progenitor. However, its radio flux densities compared to upper limits for SNe Ia have ruled it out as a progenitor for SN 2011fe.