Finding LoTSS of hosts for GRBs: a search for galaxy - gamma-ray burst coincidences at low frequencies with LOFAR

The LOFAR Two-Metre Sky Survey (LoTSS) is an invaluable new tool for investigating the properties of sources at low frequencies and has helped to open up the study of galaxy populations in this regime. In this work, we perform a search for host galaxies of gamma-ray bursts (GRBs). We use the relative density of sources in Data Release 2 of LoTSS to define the probability of a chance alignment, Pchance, and find 18 sources corresponding to 17 GRBs which meet a Pchance<1% criterion. We examine the nature and properties of these radio sources using both LOFAR data and broadband information, including their radio spectral index, star formation rate estimates and any contributions from active galactic nucleus emission. Assuming the radio emission is dominated by star formation, we find that our sources show high star formation rates (101–103M⊙ yr−1) compared with both a field galaxy sample and a sample of core-collapse supernova hosts, and the majority of putative hosts are consistent with ultraluminous infrared galaxy (ULIRG) classifications. As a result of our analyses, we define a final sample of eight likely GRB host candidates in the LoTSS DR2 survey.</p

Extragalactic transient candidates in the second Swift-XRT point source catalogue

The second Swift-XRT Point Source catalogue offers a combination of sky coverage and sensitivity and presents an invaluable opportunity for transient discovery. We search the catalogue at the positions of inactive and active galaxies, and identify transient candidates by comparison with XMM–Newton and ROSAT. We recover 167 previously known transients and find 19 sources consistent with being new sources, estimating a completeness of $\sim 65{{\ \rm per\ cent}}$. These 19 new sources are split approximately equally between inactive and active hosts and their peak X-ray luminosities span ∼1042–1047 erg s−1. We find eight are best fit with non-thermal spectral models and one with a blackbody. We also discuss our methodology and its application to the forthcoming Living Swift-XRT Point Source catalogue for the potential near real time serendipitous discovery of ∼few new X-ray transients per year.</p

Ultradeep Cover: An Exotic and Jetted Tidal Disruption Event Candidate Disguised as a Gamma-Ray Burst

Gamma-ray bursts (GRBs) are traditionally classified as either short GRBs with durations ≲2 s that are powered by compact object mergers or long GRBs with durations ≳2 s that are powered by the deaths of massive stars. Recent results, however, have challenged this dichotomy and suggest that there exists a population of merger-driven long bursts. One such example, GRB 191019A, has a t 90 ≈ 64 s, but many of its other properties—including its host galaxy, afterglow luminosity and lack of associated supernova—are more consistent with a short GRB. Here we propose an alternative interpretation: that GRB 191019A (which is located in the nucleus of its host) is an atypical jetted tidal disruption event (TDE). In particular, we suggest the short timescale and rapid decline, not expected for standard TDEs, are the result of an “ultradeep” encounter, in which the star came well within the tidal radius of the black hole and promptly self-intersected, circularized, accreted, and launched a relativistic outflow. This model reproduces the timescale and luminosity through a prompt super-Eddington accretion phase and accounts for the lack of late optical emission. This would make GRB 191019A only the fifth jetted TDE and the first discovered ultradeep TDE. The ultradeep TDE model can be distinguished from merger-driven long GRBs via the soft X-ray flash that results from prompt self-intersection of the debris stream; the detection of this flash will be possible with wide-field and soft-X-ray satellites such as Einstein Probe or SVOM.</p

Simulated optical light curves of super-Eddington tidal disruption events with ZEBRA flows

We present simulated optical light curves of super-Eddington tidal disruption events (TDEs) using the ZEro-BeRnoulli Accretion (ZEBRA) flow model, which proposes that during the super-Eddington phase, the disc is quasi-spherical, radiation-pressure dominated, and accompanied by the production of strong jets. We construct light curves for both on- and off-axis (with respect to the jet) observers to account for the anisotropic nature of the jetted emission. We find that at optical wavelengths, emission from the accretion flow is orders of magnitude brighter than that produced by the jet, even with boosting from synchrotron self-Compton. Comparing to the observed jetted TDE Swift J2058.4+0516, we find that the ZEBRA model accurately captures the time-scale for which accretion remains super-Eddington and reproduces the luminosity of the transient. However, we find the shape of the light curves deviate at early times and the radius and temperature of our modelled ZEBRA are ∼2.7–4.1 times smaller and ∼1.4–2.3 times larger, respectively, than observed. We suggest that this indicates the ZEBRA inflates more, and more rapidly, than currently predicted by the model, and we discuss possible extensions to the model to account for this. Such refinements, coupled with valuable new data from upcoming large-scale surveys, could help to resolve the nature of super-Eddington TDEs and how they are powered.</p

A real-time transient detector and the living Swift-XRT point source catalogue

We present the Living Swift-XRT Point Source (LSXPS) catalogue and real-time transient detector. This system allows us for the first time to carry out low-latency searches for new transient X-ray events fainter than those available to the current generation of wide-field imagers, and report their detection in near real time. Previously, such events could only be found in delayed searches, e.g. of archival data; our low-latency analysis now enables rapid and ongoing follow-up of these events, enabling the probing of time-scales previously inaccessible. The LSXPS is, uniquely among X-ray catalogues, updated in near real time, making this the first up-to-date record of the point sources detected by a sensitive X-ray telescope: the SwiftX-ray Telescope. The associated upper limit calculator likewise makes use of all available data allowing contemporary upper limits to be rapidly produced on demand. These facilities, which enable the low-latency transient system, are also fully available to the community, providing a powerful resource for time-domain and multimessenger astrophysics

Monthly quasi-periodic eruptions from repeated stellar disruption by a massive black hole

In recent years, searches of archival X-ray data have revealed galaxies exhibiting nuclear quasi-periodic eruptions with periods of several hours. These are reminiscent of the tidal disruption of a star by a supermassive black hole. The repeated, partial stripping of a white dwarf in an eccentric orbit around an ~105 M⊙ black hole provides an attractive model. A separate class of periodic nuclear transients, with much longer timescales, have recently been discovered optically and may arise from the partial stripping of a main-sequence star by an ~107 M⊙ black hole. No clear connection between these classes has been made. We present the discovery of an X-ray nuclear transient that shows quasi-periodic outbursts with a period of weeks. We discuss possible origins for the emission and propose that this system bridges the two existing classes outlined above. This discovery was made possible by the rapid identification, dissemination and follow-up of an X-ray transient found by the new live Swift-XRT transient detector, demonstrating the importance of low-latency, sensitive searches for X-ray transients.</p

A LOFAR prompt search for radio emission accompanying X-ray flares in GRB 210112A

The composition of relativistic gamma-ray burst (GRB) jets and their emission mechanisms are still debated, and they could be matter or magnetically dominated. One way to distinguish these mechanisms arises because a Poynting flux dominated jet may produce low-frequency radio emission during the energetic prompt phase, through magnetic reconnection at the shock front. We present a search for radio emission coincident with three GRB X-ray flares with the LOw Frequency ARray (LOFAR), in a rapid response mode follow-up of long GRB 210112A (at z ∼ 2) with a 2 h duration, where our observations began 511 s after the initial Swift-BAT trigger. Using time-sliced imaging at 120–168 MHz, we obtain upper limits at 3σ confidence of 42 mJy averaging over 320 s snapshot images, and 87 mJy averaging over 60 s snapshot images. LOFAR’s fast response time means that all three potential radio counterparts to X-ray flares are observable after accounting for dispersion at the estimated source redshift. Furthermore, the radio pulse in the magnetic wind model was expected to be detectable at our observing frequency and flux density limits which allows us to disfavour a region of parameter space for this GRB. However, we note that stricter constraints on redshift and the fraction of energy in the magnetic field are required to further test jet characteristics across the GRB population.</p

Machine learning for transient recognition in difference imaging with minimum sampling effort

The amount of observational data produced by time-domain astronomy is exponentially increasing. Human inspection alone is not an effective way to identify genuine transients from the data. An automatic real-bogus classifier is needed and machine learning techniques are commonly used to achieve this goal. Building a training set with a sufficiently large number of verified transients is challenging, due to the requirement of human verification. We present an approach for creating a training set by using all detections in the science images to be the sample of real detections and all detections in the difference images, which are generated by the process of difference imaging to detect transients, to be the samples of bogus detections. This strategy effectively minimizes the labour involved in the data labelling for supervised machine learning methods. We demonstrate the utility of the training set by using it to train several classifiers utilizing as the feature representation the normalized pixel values in 21 × 21 pixel stamps centred at the detection position, observed with the Gravitational-wave Optical Transient Observer (GOTO) prototype. The real-bogus classifier trained with this strategy can provide up to 95 per cent prediction accuracy on the real detections at a false alarm rate of 1 per cent⁠

GRB 221009A: Discovery of an Exceptionally Rare Nearby and Energetic Gamma-Ray Burst

We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB 221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image, and Neutron Star Interior Composition Explorer Mission. This energetic GRB was located relatively nearby (z = 0.151), allowing for sustained observations of the afterglow. The large X-ray luminosity and low Galactic latitude (b = 4.°3) make GRB 221009A a powerful probe of dust in the Milky Way. Using echo tomography, we map the line-of-sight dust distribution and find evidence for significant column densities at large distances (≳10 kpc). We present analysis of the light curves and spectra at X-ray and UV-optical wavelengths, and find that the X-ray afterglow of GRB 221009A is more than an order of magnitude brighter at T 0 + 4.5 ks than that from any previous GRB observed by Swift. In its rest frame, GRB 221009A is at the high end of the afterglow luminosity distribution, but not uniquely so. In a simulation of randomly generated bursts, only 1 in 104 long GRBs were as energetic as GRB 221009A; such a large E γ,iso implies a narrow jet structure, but the afterglow light curve is inconsistent with simple top-hat jet models. Using the sample of Swift GRBs with redshifts, we estimate that GRBs as energetic and nearby as GRB 221009A occur at a rate of ≲1 per 1000 yr—making this a truly remarkable opportunity unlikely to be repeated in our lifetime