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

TL;DR

This paper investigates the linear polarization signatures of steep GRB jets' afterglows, providing analytical tools to interpret polarization data and distinguish between different jet structures, especially in orphan afterglow observations.

Contribution

It offers an analytical approximation for polarization peaks in steep GRB jets and demonstrates how polarization can differentiate jet structures in orphan afterglows.

Findings

Polarization peaks occur near light curve breaks.

Steep jets produce distinct polarization signatures.

Application to ZTF orphan candidate AT2021lfa.

Abstract

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.