arXiv subjects

Gal Birenbaum

Publications and source records attributed to Gal Birenbaum.

5 recordsLinked to original sources

arXiv2026

The Gamma-ray Burst Jet Energy Distribution Suggests A Quasi-universal, Weakly Magnetized Jet Evolving Over Cosmic Time

We present distributions of gamma-ray burst observed and inferred properties for those GRBs with redshifts. We show that the isotropic energy distribution, which spans over four orders of magnitude, can be reproduced reasonably well under a simplistic assumption that every observed GRB originates from a quasi-universal jet with roughly the same decreasing power-law profile of energy as a function of angle, with a power-law index of $ 3 \lesssim \zeta \lesssim 4$. The spread in the observed distribution can be explained by the variation in observer viewing angle alone. Furthermore, this power-law jet structure provides an even better fit to both the isotropic energy and luminosity distributions if the isotropic energy normalization evolves as a function of redshift in a manner that has been suggested by a range of previously published studies. The relatively steep power-law index of this jet is consistent with the structure predicted by simulations of weakly magnetized jets in collapsars, whereas simulations of hydrodynamic jets predict a structure shallower than what we find here. The predicted afterglow light curves within this model framework show steepening behavior at times commensurate with observed jet break times.

astro-ph.HE↗

arXiv2026

Polarization of impulsive relativistic jets propagating in a stratified medium

Gamma-ray bursts (GRBs) and at least some tidal disruption events (TDEs) are highly energetic transients involving impulsive relativistic jets. As these jets propagate into their surrounding media, they drive a relativistic forward shock into it, which radiates multi-wavelength polarized synchrotron emission, known as an afterglow. Analyzing the afterglow light curve along with its polarization curve can shed light on the geometrical properties of the GRB or TDE system such as the magnetic field structure behind the shock, the jet angular structure and even distinguish between proposed scenarios of delayed radio emission seen in some TDEs. While most afterglow polarization measurements are for long GRBs, whose massive star progenitor winds are expected to form a stratified external medium, the polarization from such environments is largely unexplored. Similarly, stratified media surround jetted TDEs. In this work we explore how shock propagation into a stratified medium affects the observed afterglow polarization from impulsive relativistic jets. We find that for both top hat and structured jets, in most cases the polarization levels are reduced and its temporal signature evolves on longer time scales compared to a uniform external medium. The polarization peaks at times close to a geometrical break in the light curve, indicating measurements during this time probe the maximal levels of polarization possible for the system. In addition, observationally capturing the polarization evolution time scales can assist in constraining our viewing angle and the external medium stratification. Such composite models, that account for more realistic systems that are motivated by light curve fittings, will allow us to complement afterglow light curves better and promote joint modeling of these observables.

astro-ph.HE↗

arXiv2025

Afterglow Linear Polarization Signatures from Steep GRB Jets: Implications for Orphan Afterglows

Gamma-Ray Bursts (GRBs) are the strongest explosions in the Universe, and are powered by initially ultra-relativistic jets. The angular profile of GRB jets encodes important information about their launching and propagation near the central source, and can be probed through their afterglow emission. Detailed analysis of the multi-wavelength afterglow light curves of recent GRBs indeed shows evidence for an extended angular structure beyond the jet's narrow core. The afterglow emission is determined by the jet angular structure, our viewing angle, and the magnetic field structure behind the shock, often leading to degeneracies when considering the light curves and broad-band spectrum alone. Such degeneracies can be lifted with joint modeling of the afterglow light curves and polarization. In this work we study the evolution of the afterglow linear polarization and flux density from steep, core-dominated GRB jets, where most of their energy resides within a narrow core. We explore the dependence of the light and polarization curves on the viewing angle, jet angular energy structure and magnetic field configuration, and provide an analytical approximation for the peak polarization level, which occurs at a time close to that of a break in the light curve. Finally, we demonstrate how our results can be used to determine the nature of orphan GRB afterglows, distinguishing between a quasi-spherical "dirty fireball" and a steep jets viewed far off-axis and apply them on the Zwicky Transient Facility (ZTF) detected orphan afterglow candidate AT2021lfa.

astro-ph.HE↗

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↗

arXiv2021

Modeling the Linear Polarization of GRB Afterglows Across the Electromagnetic Spectrum

Linear polarization measurements in the optical band show polarization degrees of a few percent at late times. Recently, polarization at sub-percent level was also detected in radio by ALMA, opening the window for multi-wavelength polarimetry and stressing the importance of properly modeling polarization in GRB afterglows across the EM spectrum. We introduce a numerical tool that can calculate the polarization from relativistically moving surfaces by discretizing them to small patches of uniform magnetic field, calculating the polarized emission from each cell assuming synchrotron radiation and summing it to obtain the total degree of polarization. We apply this tool to afterglow shocks with random magnetic fields confined to the shock plane, considering electron radiative cooling. We analyze the observed polarization curves in several wavelengths above the cooling frequency and below the minimal synchrotron frequency and point to the characteristic differences between them. We present a method to constrain the jet opening angle and the viewing angle within the context of our model. Applying it to GRB 021004 we obtain angles of 10 and 8 degrees respectively and conclude that a non-negligible component of radial magnetic field is required to explain the 1% polarization level observed 3.5 days after the burst.

astro-ph.HE↗