arXiv subjects

Ranadeep Ghosh Dastidar

Publications and source records attributed to Ranadeep Ghosh Dastidar.

5 recordsLinked to original sources

arXiv2026

AT2018hyz: Predictions for a Sky Projection from a Delayed Off-Axis Jet

Initially discovered optically, Tidal Disruption Event (TDE) AT2018hyz was first detected in radio 972 days later. The continued monitoring of the event revealed a late-time (1370-2160 days) radio brightening. The source for this radio brightening has been associated with an afterglow originating from an off-axis jet or a delayed mildly relativistic outflow. In this work, we explored the conditions required for a delayed off-axis jet to explain the observed radio brightening. We find that the combined effect of different delay times (t_delay) and observer angles (theta_obs) can produce radio afterglow, as observed for AT2018hyz. The further off-axis from the jet core an observer is, the shorter is the delay inferred between the optical disruption and launching of the jet. This presents a degeneracy between different theta_obs and tdelay. We estimate the sky projection of the jet for these different combinations to break this degeneracy. The spread in sky images ranges from 0.6 mas for a 30 deg off-axis observer to 0.3 mas for an observer perpendicular to the jet core. Future VLBI observations should be able resolve these structures. Thus a combined observation of the light curve and sky images for AT2018hyz (and other jetted TDEs) can be used to narrow down the observer angle and any delay in launching of the underlying jet.

astro-ph.HE↗

arXiv2026

A Novel Relationship Between Gamma Ray Burst Duration And Photospheric Radius

Long Gamma Ray Bursts (lGRBs) are associated with jets in Type Ic broadline supernovae. The Collapsar model provides a theoretical framework for the jet formation from the core collapse of a massive star in such supernovae. The GRB can only be produced after a successful jet break out from the star. Under this formalism the GRB duration ($t_{\rm{90}}$) has been hypothesized to be the difference between the central engine activity duration ($t_{\rm{eng}}$) and the jet breakout time ($t_{\rm{bo}}$), that is $t_{\rm{90}} = t_{\rm{eng}} - t_{\rm{bo}}$. This disallows $t_{\rm{90}} > t_{\rm{eng}}$ and puts a lower bound on successful lGRB jet central engine duration ($t_{\rm{eng}} > t_{\rm{bo}}$), various numerical simulations have shown otherwise. This study considers a photospheric GRB emission from a relativistic jet punching out of a Wolf-Rayet-like star. We use the bolometric lightcurve generated to calculate the lGRB duration ($t_{\rm{90}}$) for varying engine duration. We find for longer engine duration the lGRB lightcurve reflects the jet profile and $t_{\rm{90}} \approx t_{\rm{eng}}$. While for shorter engine duration, the $t_{\rm{90}}$ has photospheric radius ($R_{\rm{ph}}$) dependence. This can be modeled by a relation, $t_{\rm{90}} = t^{\rm{90}}_{\rm{eng}} + 0.03\left(\frac{R_{\rm{ph}}}{c}\right)$, where c is the speed of light, with a lower bound on $t_{\rm{90}}$ for a successful lGRB. This relation should be most relevant for possible low-luminous lGRBs originating from a collapsar with central engine duration comparable to the jet breakout time.

astro-ph.HE↗

arXiv2026

High-Resolution Numerical Calculations of GRB Afterglow Plateaus arising from Stratified Ejecta

Since the discovery of plateaus in GRB afterglows by Swift, they have been modeled predominantly by late-time energy injection. However, many studies have suggested that the plateau may be modeled by an early phase before reverse shock crossing (either coasting with constant Lorentz factor or decelerating very slowly as the reverse shock crosses the ejecta). The slope of the early plateau provides some constraints the stratification of the fastest-moving ejecta, which could be the ejecta responsible for the prompt emission. However, numerical studies typically do not model the jet as a stratified outflow; the reason being the extremely high resolution required in order to accurately evolve this tiny amount of highly relativistic material. In this study, we perform high-resolution numerical calculations ($Δr/r \lesssim 10^{-5}$) verifying that a stratified ejecta structure can indeed produce an afterglow plateau in both wind ($k=2$) and ISM ($k=0$) environments, and computing break times explicitly. We evolve the relativistic hydrodynamics using the JET code, and post-process this to compute the afterglow using the Firefly code. Our results show that a stratified ejecta structure (which should generically be present in GRB jets) is sufficient to explain GRB afterglows, and the plateau slopes can be used to constrain the ejecta stratification. We additionally provide precise measured scalings for the plateau duration as a function of the characteristic Lorentz factor of the ejecta. The long duration of typical plateaus requires a very modest characteristic Lorentz factor for the ejecta ($γ_0 \sim 10-50$), in agreement with other afterglow plateau models.

astro-ph.HE↗

arXiv2025

EP250108a/SN 2025kg: A Jet-Driven Stellar Explosion Interacting With Circumstellar Material

We present optical, radio, and X-ray observations of EP250108a/SN 2025kg, a broad-line Type Ic supernova (SN Ic-BL) accompanying an Einstein Probe (EP) fast X-ray transient (FXT) at $z=0.176$. EP250108a/SN 2025kg possesses a double-peaked optical light curve and its spectrum transitions from a blue underlying continuum to a typical SN Ic-BL spectrum over time. We fit a radioactive decay model to the second peak of the optical light curve and find SN parameters that are consistent with the SNe Ic-BL population, while its X-ray and radio properties are consistent with those of low-luminosity GRB (LLGRB) 060218/SN 2006aj. We explore three scenarios to understand the system's multi-wavelength emission -- (a) SN ejecta interacting with an extended circumstellar medium (CSM), (b) the shocked cocoon of a collapsar-driven jet choked in its stellar envelope, and (c) the shocked cocoon of a collapsar-driven jet choked in an extended CSM. Models (b) and (c) can explain the optical light curve and are also consistent with the radio and X-ray observations. We favor model (c) because it can self-consistently explain both the X-ray prompt emission and first optical peak, but we do not rule out model (b). From the properties of the first peak in model (c), we find evidence that EP250108a/SN 2025kg interacts with an extended CSM, and infer an envelope mass $M_{\rm e} \sim 0.1\,\rm M_\odot$ and radius $R_{\rm e} \sim 4 \times 10^{13}$ cm. EP250108a/SN 2025kg's multi-wavelength properties make it a close analog to LLGRB 060218/SN 2006aj, and highlight the power of early follow-up observations in mapping the environments of massive stars prior to core collapse.

astro-ph.HE↗

arXiv2024

Could the recent rebrightening of the GW170817A afterglow be caused by a counter jet?

GRB170817A (also GW170817) became the first binary neutron star (BNS) merger event detected via gravitational waves and electromagnetic signals. Over the next 4 years, various multiband observations have led to re-imagine the various short Gamma Ray Burts (sGRB) and interstellar medium interaction models. While these models successfully explain the observed afterglow until ~900 days, a re-brightening or excess flux was observed in the 1keV X-Ray band after ~1000 days. In this study, we re-evaluate the jet parameters using the new observations (until ~1234 days) with a boosted fireball jet model. We study the observable effects of the counter-jet for GRB170817A, using our new afterglow code, Firefly. Our results show that it is indeed possible for the observed excess to coincide with the emissions from the counter-jet (~800 days). We also computed an empirical scaling law between the jet and counter-jet peak emission timescales and the observer angle. The Firefly code can also track the simulated object through the observers' sky and numerically model the apparent motion. The calculated apparent motion (~2.6c) does not match the observed apparent motion (7.5c to 5.2c). Hence we conclude, the excess flux of GRB170817A may not be associated with the counter jet; however, it is not enough to reject this hypothesis from the traditional counter jet visibility time scales, which predicts > 5000 days. The apparent motion, combined with the multi-band lightcurves, is needed to break degeneracy between geometrical parameters and the microphysical parameters of the afterglow.

astro-ph.HE↗