James K. Leung

Context. X-ray flashes (XRFs) are a type of gamma-ray burst (GRB) with prompt emission predominantly below ∼30 keV and have been poorly detected by previous missions monitoring the high-energy sky. The advent of the Space-based multiband astronomical Variable Objects Monitor (SVOM), with its wide-field instrument ECLAIRs, provides a new way to detect soft X-ray transients such as XRFs, complemented by rapid onboard multiwavelength follow-up. Aims. We present photometric and spectroscopic observations of XRF 241001A detected by SVOM, a soft, subluminous, and low-energetic burst located in a poorly populated region of the Amati relation. We investigated the origin of its faint, soft high-energy emission to assess its connection to the long GRB population. Methods. We analyzed the SVOM/ECLAIRs prompt emission and modeled its afterglow emission from X-ray-to-radio. We present JWST /NIRSpec and SVOM/VT observations of the associated supernova (SN 2024aiiq), which we modeled with an Arnett radioactive decay component, and we compared its properties with previously detected GRB/SNe. Results. The event XRF 241001A is located at z = 0.573 and has a prompt emission dominated by photons below 20 keV, with a duration of T ₉₀ = 3.14 seconds. Its spectrum is consistent with both thermal and nonthermal models, each implying a low E _peak < 10 keV and E _iso ∼ 8 × 10 ⁴⁹ erg. The X-ray-to-radio afterglow modeling favors an origin from a relativistic jet viewed on-axis and supports a nonthermal origin of the prompt emission. In the optical, XRF 241001A exhibits an early blue emission, similar to that detected in some extragalactic fast X-ray transients (eFXTs) and inconsistent with the expected synchrotron emission from a relativistic jet. The JWST /NIRSpec observations firmly established its collapsar origin by revealing an SN Type Ic with broad lines, comparable to SN 1998bw and SN 2025kg-like events near maximum light and with similar properties. Conclusions. The event XRF 241001A is a soft low-luminosity collapsar event produced by a weak relativistic jet observed on-axis, supporting the view that at least part of the XRF population forms the low-energy soft tail of the long GRB population. Its observation demonstrates the potential of SVOM/ECLAIRs to probe the soft regime of the high-energy transient population that remains largely unexplored.

In 2026 March, the Einstein Probe (EP) discovered its nearest ( z = 0.0343) Fast X-ray Transient (FXT), EP 260321a, the first EP FXT to provide a strong match to expectations for X-ray “shock breakout” (SBO) emission. Here, we present our multiwavelength follow-up campaign of EP 260321a and its broad-line Type Ic (Ic-BL) supernova (SN) counterpart, SN 2026gzf. We show that our radio follow-up extending over 5.8–54.5 days post-FXT rules out an on-axis jet counterpart of isotropic-equivalent kinetic energy E _K ≳ 10 ⁴⁹ erg for circumburst densities n > 10 ⁻² cm ⁻³ and assuming microphysical parameters ϵ _e = ϵ _B = 0.1. Our radio data also constrain a median mass-loss rate of M ̇ ≲ 1.2 × 1 0 − 5 M ⊙ yr − 1 for a Wolf–Rayet progenitor. In addition, we derive SN 2026gzf’s properties, including ⁵⁶ Ni mass, diffusion timescale, and expansion velocities, from our ∼nightly cadence optical data and compare them with those of optically discovered Type Ic-BL SNe, finding that SN 2026gzf is well within the 90% confidence interval across all properties. We further fit SN 2026gzf’s light curve and determine that combined emission from both interaction with CSM and ⁵⁶ Ni radioactive decay provides the best fit with plausible model parameters. Finally, using the rate of Ic-BL SNe from the Zwicky Transient Facility Bright Transient Survey and assuming all Type Ic-BL SNe produce EP 260321a-like FXTs, we infer an expected rate of EP-detected SBOs of 4.4–16 yr ⁻¹ . This is inconsistent at the 90% confidence level with current EP detection rates, potentially indicating that most Type Ic-BL SNe produce less luminous X-ray SBO signals compared to EP 260321a.

Long-period radio transients (LPTs) are coherent bursts of polarized radio emission that repeat periodically on timescales of minutes to hours. Little is known about the physical origins of these systems. Astronomers have proposed magnetars that rotate slowly and white dwarfs that rapidly orbit with a companion star as potential explanations. While several recent examples appear to support the latter hypothesis, the mechanism generating these bright radio pulses remains poorly understood. Here we report our discovery and classification of the LPT ASKAP J174508.9-505149 as an accreting white dwarf binary. This object has an ~1.3 h spectroscopic orbital period and exhibits orbitally modulated X-ray emission and radio bursts. These elliptically polarized radio bursts drift in emission frequency, potentially due to a longer beat period, and turn off for several hours at a time. Some LPTs have been associated with non-interacting white dwarf binaries. We have spectroscopically confirmed this system as an accreting cataclysmic variable, identified through characteristic optical emission lines and an ongoing X-ray outburst. Our results strengthen the link between at least some LPTs and white dwarf binaries.

Three leading models have been put forth to justify the observed radio re-brightening associated with stripped-envelope supernovae (SESNe) years post-explosion: radiation from an emerging pulsar wind nebula (PWN), shock interaction with a dense circumstellar medium (CSM), or emission from off-axis, relativistic jets. SN 2012au is a particularly intriguing SESN in this regard, as observations obtained ≳6 yr post-explosion have shown both (i) optical emission features consistent with a young PWN and (ii) a radio re-brightening. We present the results of our very long baseline interferometric (VLBI) observations of SN 2012au performed between 8 and 13 yr post core-collapse. Our VLBI observations reveal a luminous, steadily fading radio source that remains compact (≤1.4 × 10 ¹⁷ cm) and stationary (≤0.36 c ) over the course of our campaign. Overall, we find that our VLBI measurements can be readily explained by a ∼decade-old PWN, potentially explained by shock interaction with specific CSM geometries, and are unlikely to be explained by emission from an off-axis, relativistic jet. Assuming a PWN origin, our observations require that the initial spindown luminosity of the central pulsar be between 1 0 36 erg s − 1 ≤ E ̇ 0 ≤ 4 × 10 42 erg s − 1 and radio efficiency factor be η _R ≥ 3 × 10 ⁻⁷ (both quoted at the 99.7% confidence interval). These results are consistent with independent inferences obtained using optical spectroscopy of SN 2012au, alongside inferences of known Galactic systems. If a PWN origin is confirmed, SN 2012au would represent the first extragalactic PWN emerging from a modern-day SN, providing a novel opportunity to study the formation properties of a decade-old pulsar.

We report the discovery of a slowly evolving, extragalactic radio transient, ASKAP J005512.2–255834 (hereafter ASKAP J0055–2558), identified using the Australian SKA Pathfinder in a search for orphan afterglows associated with archival gravitational-wave events. Although discovered in this context, there is no evidence that the transient is associated with any known gravitational-wave event. Nonetheless, this source exhibits a 20-fold increase in flux density over z = 0.116 ( d _L = 543 Mpc), placing its peak radio luminosity at νL _ν ∼ 10 ³⁹ erg s ⁻¹ . Analysis of its radio light curve, inferred blast-wave velocity, energetics, host-galaxy properties, and the absence of counterparts at other wavelengths suggests that ASKAP J0055–2558 is most consistent with either the late-time phase of an orphan long gamma-ray burst afterglow or a tidal disruption event involving an intermediate-mass black hole spatially offset from the galaxy nucleus. The radio discovery of either of these phenomena is extremely rare, with only a few or no confirmed examples to date.

Robotic telescope networks play an important role in capturing early and bright optical afterglows, providing critical insights into the energetics and emission mechanisms of GRBs. In this study, we analyze GRB 230204B, an exceptionally energetic and multipulsed long GRB, detected by the Fermi Gamma-ray Burst Monitor and MAXI detectors, with an isotropic equivalent gamma-ray energy exceeding 10 ⁵⁴ erg. Time-resolved spectral analysis reveals a transition in the prompt emission from hard (sub-photospheric-dominated) spectra during early pulses to softer (synchrotron-radiation-dominated) spectra in later pulses, indicative of a hybrid jet composition. We report the discovery and characterization of the optical afterglow using the Mobile Astronomical System of Telescope-Robots (MASTER) and Burst Observer and Optical Transient Exploring System (BOOTES) robotic telescope networks, which enabled rapid follow-up observations starting at ∼1.3 ks post-burst. The optical luminosity at this time was exceptionally high, surpassing that of many other optically bright GRBs, such as GRB 990123 and GRB 080319B. This places the burst among the most luminous optical GRBs observed to date. Long-term radio observations extending to 335 days post-burst were conducted with the Australia Telescope Compact Array. Multiwavelength modeling, incorporating data from MASTER, BOOTES, Devasthal Optical Telescope, Swift/XRT, and radio observations, was conducted using an external interstellar medium (ISM) forward-shock top-hat jet model with afterglowpy . The results reveal a narrow and highly collimated jet with a circumburst density of n ₀ ∼ 28.12 cm ⁻³ , kinetic energy E _K ∼ 4.18 × 10 ⁵⁵ erg, and a relatively low value of ϵ _B = 2.14 × 10 ⁻⁶ , indicating shock-compression of the magnetic field in the surrounding ISM. We constrained a low radiative efficiency of ∼4.3%. This study highlights the indispensable contribution of robotic networks to early afterglow observations and advances our understanding of GRB 230204B unique characteristics and underlying jet physics.

We introduce the PanRadio Gamma-ray Burst (GRB) program carried out on the Australia Telescope Compact Array: a systematic, multiyear, radio survey of all southern Swift GRB events, comprehensively following the multifrequency evolution of their afterglows from within an hour to years postburst. We present the results of the 400-day observing campaign following the afterglow of the long-duration (collapsar) GRB 230815A—the first one detected through this program. Typically, GRB 230815A would not otherwise receive traditional radio follow-up, given it has no known redshift and lacks comprehensive multiwavelength follow-up, due to its high line-of-sight extinction with A _V = 2.3. We found its early X-ray jet break at ∼0.1 days postburst to be at odds with the evolution of the multifrequency radio light curves that were traced over an unusually long duration of 400 days. The radio light curves approximately evolved (with minor deviations) according to the standard self-similar expansion for a relativistic blast wave in a homogeneous environment prior to the jet break, showing no evidence for evolutions of the microphysical parameters describing the electron acceleration processes. We reconcile these features by proposing a two-component jet: the early X-ray break originates from a narrow component with a half-opening angle of ∼2 . ° 1, while the evolution of the radio afterglow stems from a wider component with a half-opening angle of ⪆35°. The PanRadio GRB program will establish a sample of comprehensively followed GRBs, where a rigorous inspection of their microphysical and dynamical parameters can be performed, thereby revealing the diversity of features in their outflows and environments.

We present the discovery of the radio afterglow of the most distant ultralong gamma-ray burst (GRB) detected to date, GRB 220627A at redshift z = 3.084. Its prompt gamma-ray light curve shows a double-pulse profile, with the pulses separated by a period of quiescence lasting ∼15 minutes, leading to early speculation it could be a strongly gravitationally lensed GRB. However, our analysis of the Fermi Gamma-ray Burst Monitor spectra taken during the time intervals of both pulses show clear differences in their spectral energy distributions, disfavouring the lensing scenario. We observed the radio afterglow from 7 to 456 days postburst: an initial, steep decay ( F _ν ∝ t ⁻² ) is followed by a shallower decline ( F _ν ∝ t ^−1/2 ) after ∼20 days. There are three scenarios that could explain these radio properties: (i) energy injection from an additional, slower ejecta component catching up to the external shock; (ii) a stratified density profile going as n ∝ r ^−8/3 ; or alternatively, (iii) the presence of a slow, wide ejecta component in addition to a fast, narrow ejecta component. We also conducted an independent test of the lensing hypothesis via very long baseline interferometry (VLBI) observations at ∼12 days postburst by searching, for the first time, for multiple images of the candidate lensed GRB afterglow. Our experiment highlighted the growing need for developments in real-time correlation capabilities for time-critical VLBI experiments, particularly as we advance towards the SKA and ngVLA era of radio astronomy.