Depolarization by jet precession in early optical afterglows of gamma-ray bursts

TL;DR

This paper explores how jet precession can cause depolarization in early optical afterglows of gamma-ray bursts, potentially explaining the low polarization degrees observed, by modeling the effects in different magnetic field configurations.

Contribution

It introduces a model for depolarization caused by jet precession in GRB afterglows, considering magnetic field structures and observational angles, offering an alternative explanation for low polarization measurements.

Findings

PDs are slightly affected by magnetic field configurations.

PDs increase with longer precession periods.

Low observed PDs suggest small viewing angles.

Abstract

Polarization observations provide a unique way to probe the nature of jet magnetic fields in gamma-ray bursts (GRBs). Currently, some GRBs have been detected to be polarized in their early optical afterglows. However, the measured polarization degrees (PDs) of these GRBs are much lower than those predicted by theoretical models. In this work, we investigate the depolarization induced by jet precession in combination with the measured PDs of the GRB early optical afterglows in the reverse shock (RS) dominated phase ($\sim 10^2-10^3 \,{\rm s}$). We calculate the PDs of RS emission with and without jet precession in both magnetic field configurations, i.e., aligned and toroidal magnetic fields, and meanwhile explore the effect of different parameters on the PDs. We find that the PDs are slightly affected by the configurations of the ordered magnetic fields and are positively related to the precession period. Moreover, the PDs are sensitive to the observed angle and the measured low PDs favor a small one. Thus, as one of the plausible origins of the structured jets, jet precession could be considered as an alternative mechanism for the low PDs observed in GRB early optical afterglows.