arXiv subjects

Narjes Shahamat Dehsorkh

Publications and source records attributed to Narjes Shahamat Dehsorkh.

3 recordsLinked to original sources

arXiv2026

An extremely bright slow-rising afterglow from an off-axis jet in GRB 260310A

We present a multi-wavelength study of GRB 260310A, a nearby long-duration gamma-ray burst at $z\simeq0.153$ associated with a broad-lined Type Ic supernova. Despite its modest prompt gamma-ray output, $E_{γ,\rm iso}\simeq3.5\times10^{50}$ erg, GRB 260310A exhibits one of the brightest afterglows ever observed in the X-ray, optical, and radio bands. Its apparent brightness is not its only remarkable feature. The optical afterglow displays a delayed onset, characterized by a slow rising phase, with slope $α\approx-1$, and a late peak at $\approx$0.1 d. We argue that the combination of weak prompt emission, hard peak energy, and late afterglow onset is naturally explained by a GRB jet viewed off-axis. The radio spectral energy distributions are consistent with synchrotron radiation and indicate the presence of both reverse- and forward-shock components, thus providing a first test of reverse-shock models in an off-axis geometry. The X-ray afterglow displays a prominent rebrightening, monitored for up to $\approx$68 d with no evidence of spectral evolution. A low level of linear polarization, $Π\approx1.7\%$, is measured at 15 GHz at $T_0+55$ d and suggests that, at these late times, the forward-shock is the dominant emission component from radio to X-rays. This late-time rebrightening represents a critical test for the two-component jet model. If interpreted as the emergence of a narrow jet core viewed further off-axis, it would imply extreme luminosities and energetics for an on-axis observer.

astro-ph.HE↗

arXiv2026

A systematic search for long GRBs without supernovae: global properties and rate of events

Some long duration gamma-ray bursts (LGRBs) are not associated with bright supernovae (SNe), indicating that their explosion mechanism differs from the traditional collapsar channel. SN-less LGRBs are rare, thus it remains unclear whether they are sporadic outliers or a substantial component of the GRB population. In this work, we use two decades of Swift observations to conduct a systematic search for nearby ($z\lesssim$0.35) LGRBs without a confirmed SN association. Our analysis reveals a heterogeneous population linked, in part, to star-forming or dusty environments and, in part, to quiescent galaxies. Through the analysis of their high-energy properties, afterglows, and environments, we identify a sample of LGRBs with robust SN limits and distance scales, more than doubling the number of candidate SN-less events. From our fiducial sample, we derive a volumetric rate, not corrected for beaming, of $R = (0.5 \pm 0.2) {\rm~Gpc^{-3}\ yr^{-1}}$, comparable to the apparent rate of short duration GRBs (SGRBs) and standard LGRBs with SNe with luminosities $L \gtrsim\,10^{50}\,{\rm erg\,s^{-1}}$. For plausible beaming factors, the inferred rate of SN-less LGRBs is consistent with several compact binary merger channels, although the allowed parameter space is already narrow if the entire population is attributed to a single progenitor channel. Owing to the small number of candidate events and the uncertainties in their distance scale, our sample leaves the low-luminosity tail unconstrained. A larger sample will be needed to determine whether they are the faint tail of known progenitor channels or evidence of a distinct route to relativistic explosions.

astro-ph.HE↗

arXiv2023

Self-gravitating collapsing star and black hole spin-up in long gamma ray bursts

Long Gamma Ray Bursts (GRBs) originate from the collapse of massive, rotating stars. We aim to model the process of stellar collapse in the scenario of a self-gravitating collapsing star. We account for the changes in Kerr metric induced by the growth of the black hole, accretion of angular momentum, as well as the self-gravity effect due to a large mass of the collapsing stellar core falling onto black hole in a very short time. We also investigate the existence of accretion shocks in the collapsar, and role of magnetic field in their propagation. We compute the time-dependent axially-symmetric General Relativistic magnetohydrodynamic model of a collapsing stellar core in the dynamical Kerr metric. We explore the influence of self-gravity in such star, where the newly formed black hole is increasing the mass, and changing its spin. The Kerr metric evolves according to the mass and angular momentum changes during the collapse. We parameterize the rotation inside the star, and account for the presence of large-scale poloidal magnetic field. For the set of the global parameters, such as the initial black hole spin, and initial content of specific angular momentum in the stellar envelope, we determine the evolution of black hole parameters (mass and spin) and we quantify the strength of the gravitational instability, variability timescales and amplitudes. We find that the role of the gravitational instability measured by the value of the Toomre parameter is relatively important in the innermost regions of the collapsing star. The character of accretion rate variability strongly depends on the assumption of self-gravity in the model, and is also affected by the magnetic field. Additional factors are initial spin and rotation of the stellar core.

astro-ph.HE↗