Ramandeep Gill

We report the detection of linear polarization in the radio afterglow of GRB 260310A, representing the first centimeter-wavelength polarization detection of a gamma-ray burst (GRB) afterglow and the first measurement of Faraday rotation in a GRB environment. We detect linearly polarized emission across 11–25 GHz, with a polarization fraction decreasing monotonically from (3.18 ± 0.18)% at 25 GHz to (0.69 ± 0.22)% at 11 GHz. We model the multiwavelength data as emission from a refreshed forward shock (FS) that dominates in the optical and X-rays and a reverse shock (RS) in a structured, relativistic jet that dominates at radio wavelengths. The observed depolarization toward the lower radio frequencies is consistent with suppression by RS synchrotron self-absorption, while the low observed polarization at high frequencies relative to the theoretical maximum suggests a patchy magnetic field in the jet with a coherence scale, θ_B ≈ 10⁻² rad. We identify a frequency-dependent rotation of the polarization angle consistent with Faraday rotation, with a rotation measure (RM) of RM = −(8300 ± 90) rad m⁻² at the GRB redshift. The magnitude of the RM is consistent with propagation through a dense, magnetized environment, such as a progenitor H II region. These findings demonstrate that GRB afterglows exhibit measurable linear polarization at centimeter wavelengths, and that their polarimetric properties probe both intrinsic jet magnetization and the surrounding medium. Future multifrequency polarimetric monitoring over timescales of days to weeks will enable detailed studies of the evolution of magnetic field structure and provide new constraints on the role of magnetic fields in GRB afterglows.

Context. Gamma-ray burst GRB 241030A ( z = 1.411) exhibited a particularly bright afterglow (similar to the ‘BOAT’, GRB 221009A), detected across gamma-ray, X-ray, UV, and optical bands. The extensive, multi-wavelength observations of this remarkable event provide a valuable opportunity to advance our understanding of GRB afterglow physics. Aims. We aim to constrain the physical properties of the jet, its microphysics, and the characteristics of the circumburst environment in the context of forward-shock emission. Methods. We compiled multi-wavelength observations spanning from a minute to a week after the prompt emission, processing the data through a unified photometry pipeline. Leveraging this comprehensive dataset, we analysed the observations both analytically and using Bayesian inference with two independent models. Our models assume that the afterglow emission arises from the strong forward shock of a laterally structured jet, with possible contributions from synchrotron self-Compton (SSC) scatterings. Results. We find that our models do reproduce the afterglow observations accurately, from the X-rays to the optical, favouring a jet propagating into a constant-density interstellar medium, with a viewing angle within the jet core. However, both analyses – with and without the inclusion of SSC scatterings – require parameter values that are extreme compared to expectations from standard theory. In particular, our results imply extremely energetic jets despite regular prompt energy, leading to a very inefficient prompt emission. Furthermore, the jets are particularly inefficient at accelerating particles, with low ϵ _e and ϵ _B , leading to significant SSC emission. Finally, our analyses indicate that the jets have large opening angles and propagate in high-density media. Conclusions. If the afterglow is indeed powered by radiation emitted behind a strong forward shock, our results place GRB 241030A within a sub-class of GRBs characterised by extreme kinetic energies, large jet opening angles, and very low prompt emission efficiencies, below 10 ⁻³ , with strong SSC radiation. These predictions are difficult to reconcile with typical expectations from other GRBs. We therefore suggest that the afterglow of GRB 241030A is not solely powered by forward shock emission, and we discuss other options such as a long-lasting reverse-shock contribution.

Context. Most long gamma-ray bursts (GRBs) exhibit afterglows that are broadly consistent with external forward-shock emission, which is typically described by smooth broken power-law decays in the multiband light curve. However, a minority of well-sampled GRBs deviates from this behavior, including the GRB we investigated, GRB 250129A. This source shows multiple late-time rebrightenings at X-ray and optical wavelengths. Rebrightenings are often attributed to energy injection from prolonged activity of the central engine, refreshed shocks from delayed shell collisions, density jumps in the ambient medium, or to the angular jet structure and viewing-angle effects. Aims After a comprehensive analysis of the prompt emission of the GRB observed in gamma rays and in the near-infrared, we investigated the physical origin of the multiple X-ray and optical flaring episodes observed in GRB 250129A. Methods. We conducted comprehensive temporal and spectral multiband analyses of GRB 250129A. The physical processes at play in the afterglow light curves were investigated using several methods, ranging from empirical fitting to Bayesian inference. The high-quality monitoring of the flare episodes, together with the connection between the prompt emission and the afterglow, enabled us to test the consistency of the fireball model and to constrain, reject, or confirm alternative scenarios. Results. GRB 250129A is an interesting GRB with multiwavelength prompt and afterglow emission. By conducting the prompt and time-resolved analyses, we obtained an isotropic-equivalent energy of E _{iso,  γ} = (1.35 ± 0.12)×10 ⁵³ erg. By modeling the afterglow using an agnostic Bayesian framework ( NMMA ), we ruled out a single external-shock evolution and a one-time energy-injection scenario. By further performing numerical calculations, we demonstrated that the rebrightening episodes are consistent with refreshed shocks arising from delayed collisions between relativistic shells. This agrees with evolving outflow dynamics. Conclusions. Based on the consistency between our analyses of the prompt and afterglow GRB 250129A data, together with prior general knowledge on microphysical properties, we find that two statistically significant rebrightening episodes occurred within 1.1 days post trigger and can be explained by a sequence of refreshed shocks. We stress that the availability of temporally and spectrally rich GRB datasets, such as the one we present here, provides a powerful means to test current modeling frameworks.

We present ongoing radio observations of the tidal disruption event (TDE) AT2018hyz, which was first detected in the radio at 972 days after disruption, following multiple nondetections from earlier searches. The new observations presented here span ≈1370–2160 days and 0.88–240 GHz. We find that the light curves continue to rise at all frequencies during this time period, following a power law of about F _ν ∝ t ³ (compared to F _ν ∝ t ^5.7 at 972–1400 days) and reaching a peak luminosity of L ≈ 10 ⁴⁰ erg s ⁻¹ , comparable to the luminosity of the relativistic TDE Sw 1644+57 on the same timescale. The multifrequency data indicate that the peak frequency does not significantly evolve over the 1030 days span of our observations, while the peak flux density increases by an order of magnitude. The observed behavior is consistent with two possible scenarios: (i) a delayed spherical outflow launched about 620 days postdisruption with a velocity of ≈0.3 c and an energy of ∼10 ⁵⁰ erg; or (ii) a highly off-axis (≈80°–90°) relativistic jet with a Lorentz factor of Γ ∼ 8 and E _K ≈ 10 ⁵² erg. Continued radio observations to capture the light-curve peak, as well as very long baseline interferometry observations, could distinguish between these scenarios.