Stellar radio emission is highly variable with stellar flares lasting from milliseconds to hours. For some stars, their flares or bursts can repeat once every couple of hours, while other stars may flare only once in hundreds of hours of observing. Some stars and stellar systems vary slowly over months to years. In this chapter we present the current methods for identifying radio stars and the important role that variability plays in these detection methods. We highlight that radio stars are the second most common radio variable object in image-plane searches for radio variable sources. We also present our predictions for the number of stars both the SKA-mid and SKA-low AA^* arrays could detect based on searches using SKA pathfinders and precursors.

This chapter outlines the key, state of the art, techniques required to conduct commensal image plane transient searches. Using significant experience and expertise from conducting transient searches with the SKA pathfinders, we have developed efficient fast imaging strategies, automated transient detection pipelines, determined careful filtering techniques to avoid artifacts, and identified the most useful triggered reprocessing tools. Using these strategies, we will be ideally placed to optimally and reliably detect transient sources from commensal image plane transient surveys. The tools and methods presented in this chapter can be used for all SKA-Mid and SKA-Low array deployments from AA* inclusive.

We present the discovery of radio emission from the Be/X-ray binary A0538-66 with the Australian Square Kilometre Array Pathfinder, and results from a subsequent weekly monitoring campaign with the MeerKAT radio telescope. A0538-66, located in the Large Magellanic Cloud, hosts a neutron star with a short spin period (P approximate to 69ms) in a highly eccentric approximate to 16.6-d orbit . Its rare episodes of super-Eddington accretion, rapid optical and X-ray flares, and other peculiar properties make it an interesting system among high-mass X-ray binaries. Our MeerKAT data reveal that it is also one of the most radio-luminous neutron star X-ray binaries observed to date, reaching approximate to 3 & times;10(22)ergs(-1)Hz(-1)(at 1.28 GHz), with radio emission that appears to be orbitally modulated. We consider several possible mechanisms for the radio emission, and place A0538-66 in context by comparing it to similar systems.

We report the discovery and characterisation of MKT J032848.4-271904.6, a new radio transient identified in the LADUMA field using SARAO Science Data Processor (SDP) UHF-band images from approximately one year of MeerKAT observations. Using the Transient Pipeline (TraP) and advanced filtering techniques, we identified a number of candidate variable radio sources in the field. All but one show variability consistent with refractive interstellar scintillation, leaving MKT J032848.4-271904.6 as the sole source exhibiting intrinsic variability. In addition, MKT J032848.4-271904.6 shows intrinsic variability at 0.816 GHz, with 13 radio detections across 41 epochs and a peak flux density of 1.041+/-0.043 mJy. We associate this emission with a low-mass M-dwarf star LP 888-63, located 23 pc from the Sun and identified as a companion to the white dwarf binary system LAWD 14 / WD 0326-273. LP 888-63 displays active flaring behaviour across the electromagnetic spectrum, including multi-band optical data from MeerLICHT. TESS photometry reveals a periodic modulation in the blended light curve of 5.780+/-0.507 days, consistent with rotational variability of a mid-M dwarf. Archival ESO spectra reveal Halpha emission, confirming magnetic activity. These findings highlight the capability of MeerKAT's SDP imaging and facilities like MeerLICHT for real-time detection and characterisation of stellar transients.

Stellar and planetary magnetic fields play a crucial role in the habitability of a planet and the integrity of its atmosphere. The detection of methane and carbon dioxide, along with a tentative identification of the potential biosignature dimethyl sulfide/disulfide, in the atmosphere of K2-18 b, a sub-Neptune orbiting an M dwarf star, presents an intriguing question regarding the stellar magnetic environment and the resistance of the planet's magnetosphere (if it exists) to erosion by magnetic activity from the host. To probe for radio emission from the system, we have conducted observations using the Karl G. Jansky Very Large Array at S, C, and X bands (2-4, 5.5-7.5, and 8-10 GHz, respectively) to search for coherent and incoherent radio emission. We detect no radio emission associated with incoherent emission mechanisms. We report 3 sigma Stokes I upper limits of 49.8 mu Jy beam(-1 )at S band, 17.7 mu Jy beam(-1) at C band, and 18.0 mu Jy beam(-1) at X band and an upper limit of the ratio of the radio to the total bolometric luminosity of logL(R)/ logL(bol) < -8.8. We have also searched for short duration bursts associated with coherent emission mechanisms at C and X bands. No signals above a 3 sigma significance threshold are detected. Although no signals are detected, our radio observations offer constraints, albeit limited, on the stellar magnetic environment supporting recent X-ray observations indicating that K2-18 is a very faint emitter. Our results also contextualize any planetary transmission spectra by providing constraints on the activity level of the host.

During the hard and quiescent spectral states in X-ray binaries, a non-linear correlation is observed between radio and X-ray luminosities, providing a valuable tool to probe the connection between accretion and jet production. This relation was originally thought to define a single 'standard' correlation spanning several orders of magnitude in X-ray luminosity, and was extended to active galactic nuclei by including a mass term. However, subsequent studies revealed a more complex picture, with some sources deviating from the standard correlation and instead populating distinct tracks. To date, all large studies of the radio:X-ray plane have combined data from multiple telescopes, introducing uncertainties due to differing instrument systematics and flux conversions between observing frequencies, thereby complicating comparisons and limiting constraints. ThunderKAT was a five-year programme on the MeerKAT radio telescope that monitored X-ray binaries in outburst, and ran alongside SwiftKAT which provided quasi-simultaneous Swift/XRT X-ray coverage. We present the full set of light curves from these programmes, comprising 948 radio and 1029 X-ray data points. An important finding is the frequent detection of unresolved radio emission during the soft state, likely dominated by previously launched jet ejecta. Using these data, we construct the largest, observationally homogeneous X-ray binary radio:X-ray plane to date. We relate these results to the physical mechanisms proposed to drive inter-source diversity, and outline directions for future observational and theoretical work. This paper is accompanied by a public data release of the ThunderKAT and SwiftKAT measurements and a compiled radio:X-ray plane, available through an interactive website.

The search for technosignatures (also known as the Search for Extraterrestrial Intelligence or SETI) depends critically on our ability to distinguish artificial signals from the rich complexity of natural astrophysical phenomena and radio frequency interference from anthropogenic emissions. As the search for technosignatures increasingly aligns with mainstream astrophysics, complementing the search for biosignatures, it demands not only sophisticated statistical and computational approaches, but also deep domain knowledge across the electromagnetic spectrum. The SKA will play a pivotal role in the next-generation of technosignature searches, providing an unprecedented combination of sensitivity, field of view, and spatial resolution over its wavelength range. Integrating wide-field, high-resolution observations with machine learning and multi-wavelength diagnostics will represent key steps forward. The SKA's singular capabilities will render it an indispensable instrument for the rapid identification and follow-up characterisation of promising technosignature candidates. In this chapter, we discuss the request for high temporal and spectral resolution data products with a main focus on frequency-domain SETI. Nevertheless, multiple SKA observing modes have the potential to substantially advance technosignature research.

We present the detection of the binary system 2MASS J02132062 + 3648506 AB using the Karl G. Jansky Very Large Array archive data observed at 4-8 GHz. The system is a triple consisting of a tight binary (similar to 0.2 arcsec) of two M dwarfs of spectral types M4.5 and M6.5 and a wide T3 brown dwarf companion (similar to 16.4 arcsec). The binary displays coronal and chromospheric activity as traced by previously measured X-ray flux and H alpha emission. We detect the unresolved binary at a peak flux density of similar to 356 mu Jybeam(-1) at a signal-to-noise ratio (SNR) of similar to 36 and determine a radio luminosity of logL(R) /logL(bol) similar to -7.76. The radio emission is quiescent, polarized at a mean circular polarization fraction f(c) = 45.20 +/- 1.58% and exhibits a spectral index alpha = -0.44 +/- 0 . 07. We probe the binary using the Enhanced Multi-Element Remotely Linked Interferometer Network (e-MERLIN) with an angular resolution of similar to 40 mas at 5 GHz and detect a component at a peak flux density of similar to 90 mu Jy beam-1 at a SNR similar to 5. We propose a gyrosynchrotron origin for the radio emission and estimate a magnetic field strength B < 174.86 G, an emitting region of size L < 1 . 54 times the radius of the M4.5 primary and a plasma number density n(e) < 2.91 x 10(5) cm(-3). The brown dwarf companion is not detected. Additionally, we have analysed observations of 2MASS J04183483 + 213127, a chromospherically active L5 brown dwarf which is also not detected and can only place 3 sigma flux density upper limits at 36.9 mu Jy beam(-1) and 42.3 mu Jy beam(-1) for Stokes I and V, respectively.

The search for radio technosignatures is an anomaly detection problem: Candidate signals represent needles of interest in the proverbial haystack of radio-frequency interference (RFI). Current search frameworks find an enormity of false-positive signals, especially in large surveys, requiring manual follow-up to a sometimes prohibitive degree. Unsupervised learning provides an algorithmic way to winnow the most anomalous signals from the chaff, as well as group together RFI signals that bear morphological similarities. We present Grouping Low-frequency Observations By Unsupervised Learning After Reduction (GLOBULAR) clustering, a signal processing method that uses hierarchical density-based spatial clustering of applications with noise (or HDBSCAN) to reduce the false-positive rate and isolate outlier signals for further analysis. When combined with a standard narrowband signal detection and spatial filtering pipeline, such as turboSETI , GLOBULAR clustering offers significant improvements in the false-positive rate over the standard pipeline alone, suggesting dramatic potential for the amelioration of manual follow-up requirements for future large surveys. By removing RFI signals in regions of high spectral occupancy, GLOBULAR clustering may also enable the detection of signals missed by the standard pipeline. We benchmark our method against the C. Choza et al. turboSETI -only search of 97 nearby galaxies at the L band, demonstrating a false-positive hit reduction rate of 93.1% and a false-positive event reduction rate of 99.3%.

The Breakthrough Listen program is, to date, the most extensive search for technological life beyond Earth. Over the past 9 yr, it has surveyed thousands of nearby stars and close to 100 nearby galaxies with telescopes around the world, including the Robert C. Byrd Green Bank Telescope (GBT) in West Virginia. The goal is to find evidence of technosignatures of other civilizations, such as narrowband Doppler-drifting radio signals. Despite the GBT’s location in a radio-quiet zone, the primary challenge of this search continues to be the ability to pick out genuine candidates from the high quantities of human-generated radio-frequency interference (RFI). Here we present a novel search method aimed at finding these “needle-in-a-haystack”-type signals, applied to 9684 observation cadences of 3077 stars (each observed with one or more of the L- , S -, C -, and X -band receivers) from the GBT archive. We implement a low-complexity statistical process to vet out RFI and highlight signals that, upon visual inspection, are less evidently RFI than those from previous analyses. Our work returns candidate signals found previously using both traditional and machine learning algorithms, as well as many not previously identified. This analysis represents the largest data set searched for technosignatures to date, and highlights the efficacy that traditional algorithms continue to have in these types of technosignature searches. We find that less than 1% of stars host transmitters brighter than ∼0.3 Arecibo radar equivalents broadcasting in our direction over the frequency band covered.

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.

In this work, we explore the applicability of unsupervised machine learning algorithms to finding radio transients. Facilities such as the Square Kilometre Array (SKA) will provide huge volumes of data in which to detect rare transients; the challenge for astronomers is how to find them. We demonstrate the effectiveness of anomaly detection algorithms using 1.3 GHz light curves from the SKA precursor MeerKAT. We make use of three sets of descriptive parameters ('feature sets') as applied to two anomaly detection techniques in the astronomaly package and analyse our performance by comparison with citizen science labels on the same data set. Using transients found by volunteers as our ground truth, we demonstrate that anomaly detection techniques can recall over half of the radio transients in the 10 per cent of the data with the highest anomaly scores. We find that the choice of anomaly detection algorithm makes a minor difference, but that feature set choice is crucial, especially when considering available resources for human inspection and/or follow-up. Active learning, where human labels are given for just 2 per cent of the data, improves recall by up to 20 percentage points, depending on the combination of features and model used. The best-performing results produce a factor of 5 times fewer sources requiring vetting by experts. This is the first effort to apply anomaly detection techniques to finding radio transients and shows great promise for application to other data sets, and as a real-time transient detection system for upcoming large surveys.

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.

Fast X-ray Transients (FXTs) are extragalactic bursts of soft X-rays first identified >10 years ago. Since then, nearly 40 events have been discovered, although almost all of these have been recovered from archival Chandra and XMM-Newton data. To date, optical sky surveys and follow-up searches have not revealed any multi-wavelength counterparts. The Einstein Probe, launched in January 2024, has started surveying the sky in the soft X-ray regime (0.5-4 keV) and will rapidly increase the sample of FXTs discovered in real time. Here, we report the first discovery of both an optical and radio counterpart to an FXT, the fourth source publicly released by the Einstein Probe. We discovered a fast-fading optical transient within the 3 arcmin localisation radius of EP240315a with the all-sky optical survey ATLAS, and our follow-up Gemini spectrum provides a redshift, z=4.859+/-0.002. Furthermore, we uncovered a radio counterpart in the S-band (3.0 GHz) with the MeerKAT radio interferometer. The optical (rest-frame UV) and radio luminosities indicate the FXT most likely originates from either a long gamma-ray burst or a relativistic tidal disruption event. This may be a fortuitous early mission detection by the Einstein Probe or may signpost a mode of discovery for high-redshift, high-energy transients through soft X-ray surveys, combined with locating multi-wavelength counterparts.

GW190425 is the second of only two binary neutron star (BNS) merger events to be significantly detected by the LIGO-Virgo- Kagra gravitational wave detectors. With a detection only in LIGO Livingston, the skymap containing the source was large and no plausible electromagnetic counterpart was found in real time searching in 2019. Here we summarise our ATLAS and Pan-STARRS wide-field optical coverage of the skymap beginning within 1 hour and 3 hours respectively of the GW190425 merger time. More recently, a potential coincidence between GW190425 and a fast radio burst FRB 190425 has been suggested, given their spatial and temporal coincidence. The smaller sky localisation area of FRB 190425 and its dispersion measure have led to the identification of a likely host galaxy, UGC 10667 at a distance of 141 +/- 10 Mpc. Our optical imaging covered the galaxy 6.0 hrs after GW190425 was detected and 3.5 hrs after the FRB 190425. No optical emission was detected and further imaging at +1.2 and +13.2 days also revealed no emission. If the FRB 190425 and GW190425 association were real, we highlight our limits on kilonova emission from a BNS merger in UGC 10667. The model for producing FRB 190425 from a BNS merger involves a supramassive magnetised neutron star spinning down by dipole emission on the timescale of hours. We show that magnetar enhanced kilonova emission is ruled out by optical upper limits. The lack of detected optical emission from a kilonova in UGC 10667 disfavours, but does not disprove, the FRB-GW link for this source.

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.

As more Fast Radio Bursts (FRBs) are being localized, we are learning that some fraction have persistent radio sources (PRSs). Such a discovery motivates an improvement in our understanding of the nature of those counterparts, the relation to the bursts themselves and why only some FRBs have PRSs. We report on observations made of FRB 20121102A with the MeerKAT radio telescope. Across five epochs, we detect the PRS associated with FRB 20121102A. Our observations are split into a cluster of four epochs (MJD 58732-58764) and a separate single epoch about 1000d later. The measured flux density is constant across the first four observations but then decays by more than one-third in the final observation. Our observations on MJD 58736 coincided with the detections of 11 bursts from FRB 20121102A by the MeerTRAP backend, seven of which we detected in the image plane. We discuss the importance of image plane detections when considering the commensal transient searches being performed with MeerKAT and other radio facilities. We find that MeerKAT is so sensitive that within a 2-s image, we can detect any FRB with a flux density above 2.4mJy at 1.3 GHz and so could localize every FRB that has been detected by CHIME to date.

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.

A new generation of radio telescopes with excellent sensitivity, instantaneous uv coverage, and large fields of view, are providing unprecedented opportunities for performing commensal transient searches. Here, we present such a commensal search in deep observations of short gamma-ray burst fields carried out with the MeerKAT radio telescope in South Africa at 1.3GHz. These four hour observations of eight different fields span survey lengths of weeks to months. We also carry out transient searches in time slices of the full observations, at time-scales of 15min and 8s. We find 122 variable sources on the long time-scales, of which 52 are likely active galactic nuclei, but there are likely also some radio flaring stars. While the variability is intrinsic in at least two cases, most of it is consistent with interstellar scintillation. In this study, we also place constraints on transient rates based on state-of-the-art transient simulations codes. We place an upper limit of 2 x 10(-4) transients per day per square degree for transients with peak flux of 5mJy, and an upper limit of 2.5 x 10(-2) transients per day per square degree for transients with a fluence of 10 Jy ms, the minimum detectable fluence of our survey.

ABSTRACT Many transient and variable sources detected at multiple wavelengths are also observed to vary at radio frequencies. However, these samples are typically biased towards sources that are initially detected in wide-field optical, X-ray, or gamma-ray surveys. Many sources that are insufficiently bright at higher frequencies are therefore missed, leading to potential gaps in our knowledge of these sources and missing populations that are not detectable in optical, X-rays, or gamma-rays. Taking advantage of new state-of-the-art radio facilities that provide high-quality wide-field images with fast survey speeds, we can now conduct unbiased surveys for transient and variable sources at radio frequencies. In this paper, we present an unbiased survey using observations obtained by MeerKAT, a mid-frequency (∼GHz) radio array in South Africa’s Karoo Desert. The observations used were obtained as part of a weekly monitoring campaign for X-ray binaries (XRBs) and we focus on the field of MAXI J1820+070. We develop methods to efficiently filter transient and variable candidates that can be directly applied to other data sets. In addition to MAXI J1820+070, we identify four likely active galactic nuclei, one source that could be a Galactic source (pulsar or quiescent XRB) or an AGN, and one variable pulsar. No transient sources, defined as being undetected in deep images, were identified leading to a transient surface density of <3.7 × 10−2 deg−2 at a sensitivity of 1 mJy on time-scales of 1 week at 1.4 GHz.