arXiv subjects

Ramandeep Gill

Publications and source records attributed to Ramandeep Gill.

At least 19 recordsLinked to original sources

arXiv2026

GRB 260310A / SN 2026fgk: A Multi-Wavelength Study of a Nearby Underluminous Long GRB and SN with a Complex Afterglow

We present a comprehensive multi-wavelength study of GRB 260310A / SN 2026fgk, a nearby ($z=0.153$), long-duration gamma-ray burst (GRB) with an exceptionally underluminous prompt $γ$-ray emission and a Comptonized spectrum. The burst occurred at the edge of a blue host galaxy at a projected distance of 15 kpc, which is one of the largest offsets reported for a long GRB. The bright optical afterglow, with dense coverage from COLIBRÍ, likely peaked at a few to several hours post-burst, followed by a shallow decay not expected from canonical afterglow models. Both the optical and X-ray light curves show a brief chromatic plateau from $4-7$ days. We show that the subsequent rebrightening observed at $\sim20$ days is best explained by the combined contribution of the associated Type Ic-BL supernova, identified in GTC spectra, and a late-time refreshed shock. The broadband optical to X-ray spectral energy distribution is well described by synchrotron emission from the forward shock, while the radio observations demand an additional emission component. We model the afterglow using (a) an on-axis uniform jet from a dirty fireball with late-time energy injection and (b) a misaligned jet with power-law angular structure, both having material emitting along our line-of-sight (LOS) moving with an initial Lorentz factor of $Γ_0\sim20-35$. We conclude that at more typical GRB distances ($z\gtrsim0.5$) the prompt $γ$-ray emission from this source would likely have escaped detection, whereas its optical afterglow would have remained observable, making the event appear as an orphan afterglow or a gamma-ray quiet fast X-ray transient.

astro-ph.HE↗

arXiv2026

Revealing the Structure and Magnetization of GRB Jets with ALMA Polarization Observations

We present a systematic study of the currently available ALMA millimeter polarimetric sample of gamma-ray burst (GRB) afterglows. Our sample comprises 24 observations (20 new) of 11 long-duration GRBs spanning $\approx0.1$-87 days after the burst. We detect significant linear polarization in 8 observations across 6 events, with polarization degrees ranging from $Π_L\approx0.6\%$ to $2.4\%$. For the remaining observations, we place deep upper limits (median $Π_L\lesssim1\%$). Multi-epoch observations reveal diverse polarization evolution. GRB 190114C yields the best-sampled $Π_L$ for a radio afterglow to date, with early evolution favoring patchy magnetic fields in the reverse shock (RS) and later polarization broadly consistent with forward-shock (FS) models. GRB 220921A exhibits a rapid rise in polarization from $Π_L\lesssim0.4\%$ to $2.4\%$ over 1.9-6.8 days, inconsistent with toroidal RS magnetic-field models but broadly consistent with several FS random-field models. GRB 221009A yields the highest-significance polarization detections in the sample ($Π_L\approx1.4$-$1.6\%$), yet neither existing FS nor RS polarization models reproduce both the observed polarization evolution and the viewing geometry inferred from broadband afterglow modeling, with the exception of patchy fields in the RS. Deep upper limits for GRB 171205A rule out RS toroidal-field models for the published off-axis geometry, while a strong, single-epoch detection of GRB 190829A, if RS-dominated, requires a nearly on-axis geometry for toroidal-field configurations. Interpreting the observations within a patchy-field framework implies magnetic-field coherence scales of order $θ_B\sim10^{-3}$-$10^{-2}$ rad. These observations demonstrate the diagnostic power of radio/mm polarimetry for probing the magnetic-field structure, emission region, and viewing geometry of relativistic GRB jets.

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arXiv2026

Early Optical Follow-up of Gamma-Ray Bursts: The Critical Role of Robotic Telescopes

Gamma-ray bursts (GRBs) are the most luminous electromagnetic explosions in the Universe, and offer unique laboratories for studying relativistic jets, compact-object formation, particle acceleration, and the high-redshift Universe. The early optical emission of GRBs, particularly within seconds to minutes after the burst, carries crucial information about the central engine, jet magnetization, bulk Lorentz factor, and circumburst environment. We present a comprehensive review of the early optical phenomenology of GRBs and the essential role played by ground-based robotic optical telescopes to observe the fleeting early-time phenomena through rapid, automated responses to real-time GRB alerts and high-cadence photometry. We examine the key early optical features of GRBs, including prompt optical emission coincident with the $γ$-ray phase, bright reverse shock optical flashes, the onset of external forward shock afterglow, and superimposed optical flares, plateaus, and discuss the diagnostic power of each in constraining jet physics. We discuss the physical mechanisms underlying these phenomena and their implications for GRB physics (e.g., estimating the initial Lorentz factor $Γ_0$, magnetization, and the density profile). Early optical observations have constrained the initial bulk Lorentz factor $Γ_0 \sim 100$--$1000$, weak-to-moderate ejecta magnetization for events with prominent reverse shocks, the circumburst density profile, and the geometry of the magnetic field in the ejecta through polarimetry. We also provide the technical capabilities and landmark contributions of major robotic facilities, and discuss future prospects in the era of SVOM, Einstein Probe, Rubin/LSST, ULTRASAT, TeV observatories, and multi-messenger alerts.

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arXiv2026

Failed jet breakout in the metal-poor broad-lined type Ic supernova 2026gzf

A long-standing question in the death of massive stars is the role of relativistic jets. While many gamma-ray bursts and some fast X-ray transients seem to be associated with broad-lined type Ic supernovae, the opposite is not true. The lack of observable jet emission in those Ic-BL SNe can be explained by invoking off-axis jets, choked jets that inject all their energy into the stellar envelope, baryon-loaded jets for which the prompt high-energy emission is strongly suppressed, or non-jetted SNe. The lack of exact explosion time in the majority of SNe presents an obstacle to distinguish between these scenarios. Here we report the properties of SN 2026gzf associated with the X-ray thermal Einstein Probe shock-breakout EP260321a at z=0.0343. The absence of compelling shocked cocoon and radio emission up to 54 days, combined with initial expansion velocities of ~30,000 km/s and a circumstellar shell of ~0.07 M$_\odot$, favour a scenario for SN 2026gzf in which a jet was choked in the circumstellar shell. Our high-spatial resolution images of the SN environment show that the progenitor was located between two highly star-forming regions with a metallicity lower than any previously known Ic-BL SN. As the first case of a Ic-BL SN associated with high-energy prompt emission without the signature of a jet, SN 2026gzf provides a unique perspective to understand the successful launch of relativistic jets during the deaths of massive stars.

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arXiv2026

First Detection of Faraday Rotation in a Gamma-Ray Burst Afterglow: Low Polarization and High Rotation Measure in GRB 260310A Reveal Jet Magnetic Structure and Environment

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 $(3.18 \pm 0.18)\%$ at 25 GHz to $(0.69 \pm 0.22)\%$ at 11 GHz. Interpreting the radio data as emission from a reverse shock in a structured, relativistic jet, the observed depolarization toward lower frequencies is consistent with suppression by 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, $θ_{\rm B}\approx10^{-2}$ rad. We identify a frequency-dependent rotation of the polarization angle consistent with Faraday rotation, with a rotation measure of ${\rm RM} = -(8300 \pm 90)~\rm{rad/m^2}$ at the GRB redshift. The magnitude of the rotation measure is consistent with propagation through a dense, magnetized environment, such as a progenitor HII 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 multi-frequency 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.

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arXiv2026

Design and preliminary performance study of the broad-band spectrometer detector for POLAR-2

POLAR-2, the successor of the POLAR experiment aboard China's Tiangong-2 space lab, is set to be deployed on the China Space Station. The POLAR-2 mission aims to conducting high-precision polarization measurements of high-energy transients with a primary focus on Gamma-Ray Bursts (GRBs), following POLAR's pioneering accurate polarization measurements of GRB prompt emission. One of the key advancements in POLAR-2 is the inclusion of a dedicated Broad-band Spectrometer Detector (BSD) instrument, designed to provide precise measurements of GRB location and spectral parameters, which are critical inputs for accurate polarization analysis of POLAR-2's dedicated High-energy Polarimetry Detector (HPD), which is made of plastic scintillator bars array. BSD employs a coded-aperture mask imaging technique and pixelated GAGG scintillation crystals, offering a wide half-coded field of view of ~132° x 125° and an operational energy range of 10-1000 keV. Simulation results indicate that the instrument can achieve a localization accuracy of approximately 1.5° for faint GRBs similar to GRB 170817A, satisfying the core requirements of GRB polarimetry with HPD. BSD also has moderate capability for GRB polarimetry, particularly at several hundred keV energy. This paper outlines the preliminary design of BSD and presents an overall evaluation of its expected scientific performance, based on extensive Monte Carlo simulations and preliminary ground-based calibration tests.

astro-ph.IM↗

arXiv2026

Prospects of Prompt Gamma-Ray Burst Polarimetry with POLAR-2

The dominant radiation mechanism that powers the prompt $γ$-ray emission in gamma-ray bursts (GRBs) remains poorly understood. High quality, time- and energy-resolved linear polarization measurements of prompt $γ$-ray photons can distinguish between synchrotron and inverse-Compton processes and provide crucial constraints on the outflow properties. This will be achieved by POLAR-2 that is proposed as a dedicated GRB polarimeter and successor to POLAR. The High-energy Polarimetry Detector (HPD) is one of the three instruments of POLAR-2 that features significantly improved sensitivity in the $(40-1000)$\,keV energy range and a detection area four times larger than that of POLAR. Here we demonstrate the capabilities of the HPD to constrain key physical model parameters by creating and fitting to synthetic sources using a time-resolved spectro-polarimetric theoretical model of prompt GRB emission. The time-resolved spectral and polarization fits are performed using a novel technique featuring maximum likelihood over an unbinned (in time and energy) list of detected events. The constrained model parameters directly relate to the underlying source physics that would reveal an accelerating, coasting or decelerating emission region. For a pulse fluence of $\mathcal{F}=10^{-5}\mathcal{F}_{-5}\,{\rm erg\,cm^{-2}}$ and higher we can constrain the time-integrated polarization degree to an absolute accuracy ($1\,σ$) of about $2.2\mathcal{F}_{-5}^{\,-1/2}$ per cent, as long as source photons dominate over the background. In bright GRBs, such unprecedented accuracy at these energies will allow to distinguish between different models for the prompt GRB emission mechanism and constrain the magnetic field geometry, jet angular structure and outflow composition.

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arXiv2026

Evidence of Energy Injection in the Short and Distant GRB 250221A in a High Density Environment

We present the photometric and spectroscopic analysis of the short-duration GRB 250221A ($T_{90}=1.80\pm0.32$ s), using a data set from the optical facilities COLIBRÍ, the Harlingten 50~cm Telescope, and the Very Large Telescope. We complement these observations with data from the Neil Gehrels Swift Observatory and the Einstein Probe, as well as radio observations from the Very Large Array. GRB 250221A is among the few short GRBs with direct afterglow spectroscopy, which gives a secure redshift determination of $z=0.768$ and allows the unambiguous identification of the host as a galaxy with a star-formation rate of $\sim3\,M_\odot\,{\rm yr}^{-1}$. The X-ray and optical light curves up to $T_0+3\times 10^4$ s (where $T_0$ refers to the GRB trigger time) are well described by forward-shock synchrotron emission in the slow-cooling regime within the standard fireball framework. However, at $T_0 \sim 5\times 10^4$ s, both the X-ray and optical bands exhibit an excess over the same interval, which we interpret as evidence of energy injection into a jet with a half-opening angle of $θ_j=11.5^{\circ}$ through a refreshed shock powered by late central engine activity or a radially stratified ejecta. The burst properties (duration, spectral hardness, peak energy, and location in the Amati plane) all favour a compact binary merger origin. However, our modelling of the afterglow suggests a dense circumburst medium ($n\sim80$ cm$^{-3}$), which is more typical of a collapsar environment.

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arXiv2025

A milli-Tidal Disruption Event Model for GRB$\;$250702B: Main Sequence Star Disrupted by an IMBH

GRB$\;$250702B is the longest GRB recorded so far, with multiple gamma-ray emission episodes spread over a duration exceeding $25\;$ks and a weaker soft X-ray pre-peak $\sim1\;$day gradually rising emission. It is offset from its host galaxy center by $\sim5.7\;$kpc, and displays a long-lived afterglow emission in radio to X-ray. Its true nature is unclear, with the two leading candidate classes of objects being a peculiar type of ultra-long GRB and a tidal disruption event (TDE) by an intermediate mass black hole (IMBH). Here, we consider the latter, mTDE origin. We model the afterglow data, finding a stratified external density profile $\propto r^{-k}$ with $k=1.60\pm0.17$, consistent with Bondi accretion of the interstellar medium (of initial number density $n_{\rm ISM}=n_0\;{\rm cm^{-3}}$ and sound speed $c_s=c_{s,6}10^6\;{\rm cm\,s^{-1}}$) for which $n(r)\approx n_{\rm ISM}(r/R_{\rm B})^{-3/2}$ within the Bondi radius $R_{\rm B}$. Moreover, we use the implied density normalization to infer the IMBH mass within this model, finding $M_\bullet\approx\left(6.55^{+3.51}_{-2.29}\right)\times10^3\,n_0^{-2/3}\,c_{s,6}^{2}(1+\mathcal{M}^2)\;M_\odot$ where $\mathcal{M}\equiv v_{\rm BH}/c_s$ is the IMBH's Mach number relative to the ISM. Together with an upper limit on $M_\bullet<\frac{c^3}{G}\frac{t_{\rm MV}}{1+z} \lesssim5\times10^4\,M_\odot$ from the source-frame minimum variability time $t_{\rm MV,src}\!=\!\frac{t_{\rm MV}}{1+z}\!\approx\!0.5\;$s this implies $v_{\rm BH}\lesssim 28\,n_0^{1/3}\;{\rm km\;s^{-1}}$. We show that a mTDE of a main-sequence star (but not of a white dwarf) can explain the duration and energetics of GRB$\;$250702B. The gradual rise to the peak may be caused by gradual circularization and accretion disk buildup, leading to an increase in the jet's power and Lorentz factor.

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arXiv2025

Synchrotron Self-Compton Model of TeV Afterglows in Gamma-Ray Bursts

The detection of a very-high-energy TeV spectral component in the afterglow emission of gamma-ray bursts (GRBs) has opened a new probe into the energetics of ultra-relativistic blast waves and the nature of the circumburst environment in which they propagate. The afterglow emission is well understood as the synchrotron radiation from the shock-accelerated electrons in the medium swept up by the blast wave. The same distribution of electrons also inverse-Compton upscatters the softer synchrotron photons to produce the synchrotron self-Compton (SSC) TeV emission. Accurate modeling of this component generally requires a computationally expensive numerical treatment, which makes it impractical when fitting to observations using Markov Chain Monte Carlo (MCMC) methods. Simpler analytical formalisms are often limited to broken power-law solutions and some predict an artificially high Compton-Y parameter. Here we present a semi-analytic framework for a spherical blast wave that accounts for adiabatic cooling and expansion, photon escape, and equal-arrival-time-surface integration, in addition to Klein-Nishina effects. Our treatment produces the broadband afterglow spectrum and its temporal evolution at par with results obtained from more sophisticated kinetic calculations. We fit our model to the afterglow observations of the TeV bright GRB\,190114C using MCMC, and find an energetic blast wave with kinetic energy $E_{k, \rm iso} = 9.1^{+7.41}_{-3.13} \times 10^{54} \, \rm erg$ propagating inside a radially stratified external medium with number density $n(r)\propto r^{-k}$ and $k=1.67^{+0.09}_{-0.10}$. A shallower external medium density profile ($k<2$) departs from the canonical approximation of a steady wind ($k=2$) from the progenitor star and may indicate a non-steady wind or a transition to an interstellar medium.

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arXiv2025

On the Efficiency of Producing Gamma-Ray Bursts from Isolated Population III Stars

The rate of long-duration gamma-ray bursts (GRBs) from isolated Pop III stars is not well known, as it depends on our poor understanding of their initial mass function (IMF), rotation rates, stellar evolution, and mass loss. Some massive ($M_{\rm ZAMS}\gtrsim20M_\odot$) Pop III stars are expected to suffer core-collapse and launch a relativistic jet that would power a GRB. In the collapsar scenario, a key requirement is that the pre-supernova star imparts sufficient angular momentum to the remnant black hole to form an accretion disc and launch a relativistic jet, which demands rapid initial rotation of the progenitor star and suppression of line-driven mass loss during its chemically homogeneous evolution. Here we explore a grid of stellar evolution models of Pop III stars with masses $20\leq M_{\rm ZAMS}/M_\odot \leq 100$, which are initially rotating with surface angular velocities $0.6\leq Ω_0/Ω_{\rm crit}\leq 0.9$, where centrifugally-driven mass loss ensues for $Ω>Ω_{\rm crit}$. Realistic accretion and jet propagation models are used to derive the initial black hole masses and spins, and jet breakout times for these stars. The GRB production efficiency is obtained over a phase space comprising progenitor initial mass, rotation, and wind efficiency. For modest wind efficiency of $η_{\rm wind}=0.45-0.35$, the Pop III GRB production efficiency is $η_{\rm GRB}\sim10^{-5}-3\times10^{-4}\,M_\odot^{-1}$, respectively, for a top-heavy IMF. This yields an observable all-sky equivalent rate of $\sim2-40\,{\rm yr}^{-1}$ by \textit{Swift}, with 75\% of the GRBs located at $z\lesssim8$. If the actual observed rate is much lower, then this would imply $η_{\rm wind}>0.45$, which leads to significant loss of mass and angular momentum that renders isolated Pop III stars incapable of producing GRBs and favors a binary scenario instead.

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arXiv2025

Optical/infrared observations of the extraordinary GRB 250702B: a highly obscured afterglow in a massive galaxy consistent with multiple possible progenitors

GRB 250702B was the longest gamma-ray burst ever detected, with a duration that challenges standard collapsar models and suggests an exotic progenitor. We collected a rich set of optical and infrared follow-up observations of its rapidly fading afterglow using a suite of telescopes including the W. M. Keck Observatory, the Gemini telescopes, the Magellan Baade Telescope, the Victor M. Blanco 4-meter telescope, and the Fraunhofer Telescope at Wendelstein Observatory. Our analysis reveals that the afterglow emission is well described by forward shock emission from a highly obscured relativistic jet. Deep photometric observations of the host galaxy reveal a massive (10^10.66 solar masses), dusty, and extremely asymmetric system that is consistent with two galaxies undergoing a major merger. The galactocentric offset, host galaxy properties, and jet characteristics disfavor a jetted TDE around a supermassive black hole but do not definitively distinguish between competing progenitor scenarios. We find that the afterglow and host are consistent with a range of progenitors including an atypical collapsar, a merger between a helium star and a stellar mass black hole, the disruption of a star by a stellar mass compact object (micro-TDE), and the tidal disruption of a star by an off-nuclear intermediate mass black hole.

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arXiv2025

Comprehensive X-ray Observations of the Exceptional Ultra-long X-ray and Gamma-ray Transient GRB 250702B with Swift, NuSTAR, and Chandra: Insights from the X-ray Afterglow Properties

GRB 250702B is an exceptional transient that produced multiple episodes of luminous gamma-ray radiation lasting for $>25$ ks, placing it among the class of ultra-long gamma-ray bursts (GRBs). However, unlike any known GRB, the \textit{Einstein Probe} detected soft X-ray emission up to 24 hours before the gamma-ray triggers. We present comprehensive X-ray observations of the transient's afterglow obtained with the Neil Gehrels Swift Observatory, the Nuclear Spectroscopic Telescope Array, and the Chandra X-ray Observatory between 0.5 to 65 days (observer frame) after the initial high-energy trigger. The X-ray emission decays steeply as $\sim t^{-1.9}$, and shows short timescale X-ray variability ($ΔT/T < 0.03$) in both Swift and NuSTAR, consistent with flares superposed on an external shock continuum. Serendipitous detections by the Swift Burst Alert Telescope (BAT) out to $\sim$0.3 days and continued NuSTAR variability to $\sim$2 days imply sustained central engine activity; including the precursor, the required engine duration is $\gtrsim 3$ days. Afterglow modeling favors the combination of forward and reverse shock emission in a wind-like ($k \approx 2$) environment. These properties, especially the long-lived engine and early soft X-ray emission, are difficult to reconcile with a collapsar origin, and GRB 250702B does not fit neatly with canonical ultra-long GRBs or relativistic tidal disruption events (TDEs). A hybrid scenario in which a star is disrupted by a stellar-mass black hole (a micro-TDE) provides a plausible explanation, although a relativistic TDE from an intermediate-mass black hole remains viable.

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arXiv2025

The redshift distribution of Einstein Probe transients supports their relation to gamma-ray bursts

The launch of the \textit{Einstein Probe} unleashed a new era of high-energy transient discovery in the largely unexplored soft X-ray band. The \textit{Einstein Probe} has detected a significant number of fast X-ray transients that display no gamma-ray emission, complicating their robust association to more common gamma-ray bursts. To explore their possible connection, we analyzed the redshift distribution of both \textit{Einstein Probe} fast X-ray transients and long duration gamma-ray bursts. A comparative analysis of their cumulative redshift distributions using non-parametric two-sample tests, namely the Kolmogorov-Smirnov and Anderson-Darling tests, finds no statistically significant difference. These tests favor that their redshifts are drawn from the same underlying distribution. This empirical connection between \textit{Einstein Probe} transients and long gamma-ray bursts is further supported by their agreement with the so-called ``Amati relation'' between the spectral peak energy and the isotropic-equivalent energy. Together, these results indicate that most extragalactic \textit{Einstein Probe} fast X-ray transients are closely related to long gamma-ray bursts and originate from a massive star (collapsar) progenitor channel. Our findings highlight the role of the \textit{Einstein Probe} in uncovering the missing population of failed jets and dirty fireballs that emit primarily at soft X-ray wavelengths.

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arXiv2025

GRB 250702B: Discovery of a Gamma-Ray Burst from a Black Hole Falling into a Star

Gamma-ray bursts are the most luminous electromagnetic events in the universe. Their prompt gamma-ray emission has typical durations between a fraction of a second and several minutes. A rare subset of these events have durations in excess of a thousand seconds, referred to as ultra-long gamma-ray bursts. Here, we report the discovery of the longest gamma-ray burst ever seen with a ~25,000 s gamma-ray duration, GRB 250702B, and characterize this event using data from four instruments in the InterPlanetary Network and the Monitor of All-sky X-ray Image. We find a hard spectrum, subsecond variability, and high total energy, which are only known to arise from ultrarelativistic jets powered by a rapidly-spinning stellar-mass central engine. These properties and the extreme duration are together incompatible with all confirmed gamma-ray burst progenitors and nearly all models in the literature. This burst is naturally explained with the helium merger model, where a field binary ends when a black hole falls into a stripped star and proceeds to consume and explode it from within. Under this paradigm, GRB 250702B adds to the growing evidence that helium stars expand and that some ultra-long GRBs have similar evolutionary pathways as collapsars, stellar-mass gravitational wave sources, and potentially rare types of supernovae.

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arXiv2025

Continued Rapid Radio Brightening of the Tidal Disruption Event AT2018hyz

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 non-detections from earlier searches. The new observations presented here span approximately 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 ~ t^3 (compared to F_nu ~ t^5.7 at 972-1400 days), and reaching a peak luminosity of L~ 10^40 erg/s, comparable to the luminosity of the relativistic TDE Swift 1644+57 on the same timescale. The multi-frequency data indicate that the peak frequency does not significantly evolve over the 1030-day 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 post-disruption with a velocity of ~0.3c and an energy of ~10^50 erg, and (ii) a highly off-axis (~80-90 deg) relativistic jet with a Lorentz factor of Gamma ~8 and E_K ~ 10^52 erg. Continued radio observations to capture the light curve peak, as well as VLBI observations, could distinguish between these scenarios.

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arXiv2025

The curious case of EP241021a: Unraveling the mystery of its exceptional rebrightening

Fast X-ray Transients (FXTs) are a rare and poorly understood phenomenon with a variety of possible progenitors. The launch of the Einstein Probe (EP) mission has facilitated a rapid increase in the real-time discovery and follow-up of FXTs. We focus on the recent EP discovered transient EP241021a, which shows a peculiar panchromatic behavior. We obtained optical and near-infrared multi-band imaging and spectroscopy with the Fraunhofer Telescope at Wendelstein Observatory, the Hobby-Eberly Telescope, and the Very Large Telescope over the first 100 days of its evolution. EP241021a was discovered by EP as a soft X-ray trigger, but was not detected at gamma-ray frequencies. The observed soft X-ray prompt emission spectrum is consistent with non-thermal radiation, which requires at least a mildly relativistic outflow with bulk Lorentz factor $Γ\gtrsim 4$. The optical and near-infrared lightcurve has a two component behavior where an initially fading component $\sim t^{-1}$ turns to a rise steeper than $\sim t^{4}$ after a few days before peaking at $M_r\approx -21.8$ mag and quickly returning to the initial decay. The peak absolute magnitude is the most luminous optical emission associated to an FXT, superseding EP240414a. Standard supernova models are unable to reproduce either the absolute magnitude or rapid timescale ($<2$ d) of the rebrightening. The X-ray, optical and near-infrared spectral energy distributions display a red color $r-J\approx 0.8$ mag, and point to a non-thermal origin ($ν^{-1}$) for the broadband emission. By considering a gamma-ray burst as a plausible scenario, we favor a refreshed shock as the cause of the rebrightening. This is consistent with the inference of an at least mildly relativistic outflow based on the prompt trigger. Our results suggest a link between EP discovered FXTs and gamma-ray bursts.

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arXiv2024

Afterglow Linear Polarization Signatures from Shallow GRB Jets: Implications for Energetic GRBs

Gamma-ray bursts (GRBs) are powered by ultra-relativistic jets. The launching sites of these jets are surrounded by dense media, which the jets must cross before they can accelerate and release the high energy emission. Interaction with the medium leads to the formation of a mildly relativistic sheath around the jet resulting in an angular structures in the jet's asymptotic Lorentz factor and energy per solid angle, which modifies the afterglow emission. We build a semi-analytical tool to analyze the afterglow light curve and polarization signatures of jets observed from a wide range of viewing angles, and focus on ones with slowly declining energy profiles known as shallow jets. We find overall lower polarization compared to the classical top-hat jet model. We provide an analytical expression for the peak polarization degree as a function of the energy profile power-law index, magnetic field configuration and viewing angle, and show that it occurs near the light curve break time for all viewers. When applying our tool to GRB 221009A, suspected to originate from a shallow jet, we find that the suggested jet structures for this event agree with the upper limits placed on the afterglow polarization in the optical and X-ray bands. We also find that at early times the polarization levels may be significantly higher, allowing for a potential distinction between different jet structure models and possibly constraining the magnetization in both forward and reverse shocks at that stage.

astro-ph.HE↗