The Synchrotron Polarization in Decaying Magnetic Field in Gamma-Ray Bursts

TL;DR

This study investigates how decaying magnetic fields influence synchrotron polarization in gamma-ray bursts, revealing higher polarization degrees and PA rotations that align with observed burst behaviors, thus providing new insights into magnetic field dynamics.

Contribution

It introduces a model of synchrotron polarization in GRBs with decaying magnetic fields, explaining observed polarization evolution and PA rotations.

Findings

On-axis polarization degree is higher in decaying MF model (~0.6) than in constant MF (~0.5).

Off-axis instantaneous polarization can turn negative, causing a 90° PA change.

Results align with observed PA evolution in some GRB pulses.

Abstract

Polarization can serve as a probe of the radiation mechanism and magnetic field (MF) configuration in gamma-ray bursts (GRBs). In the case of constant MF, the synchrotron polarization in the prompt phase of GRBs has been widely studied. In this paper, we consider the case of the decaying MF. We calculate the time-averaged and instantaneous synchrotron polarizations in a pulse for different viewing angles and for the large-scale decaying MF model, which can explain the so-called Band spectrum. We find that the on-axis time-averaged polarization degree (PD) in the energy band of 50-500 keV for the decaying large-scale MF model ($\sim 0.6$ for typical parameters) is higher than that in the constant MF model ($\sim 0.5$). An interesting result is the instantaneous PD in the off-axis case will experience a turnover, i.e., the PD will evolve from a positive value to a negative one. This suggests the polarization angle (PA) change by an angle of $90^\circ$. Such a result is roughly consistent with the discovery of the PA evolution within a pulse in some bursts, such as GRB 170114A and GRB 160821A. Our result implies at least a part of bursts (off-axis bursts) should have the PA evolution in a pulse.