Testing Models for the Shallow Decay Phase of Gamma-Ray Burst Afterglows with Polarization Observations

TL;DR

This paper compares two models explaining the shallow decay phase of gamma-ray burst afterglows by analyzing their polarization signatures, proposing that polarization observations can distinguish between the models.

Contribution

It predicts distinct polarization evolution patterns for the two models, providing a method to test the origin of the shallow decay phase in GRB afterglows.

Findings

RWB model shows a polarization degree bump and 90° position angle change.

Structured ejecta model shows minimal polarization evolution.

Polarization observations can differentiate between the models.

Abstract

The X-ray afterglows of almost one half of gamma-ray bursts have been discovered to have a shallow decay phase by the {\em Swift} satellite, whose origin remains mysterious. Two main models have been proposed to explain this phase, relativistic wind bubbles (RWBs) and structured ejecta, which could originate from millisecond magnetars and rapidly-rotating black holes, respectively. Based on these models, we here investigate polarization evolution in the shallow decay phase of X-ray and optical afterglows. We find that in the RWB model, a significant bump of the polarization degree evolution curve appears during the shallow decay phase of both optical and X-ray afterglows, while the polarization position angle changes its direction by $90^\circ$ abruptly. In the structured ejecta model, however, the polarization degree does not evolve significantly during the shallow decay phase of afterglows, no matter whether the magnetic field configuration in the ejecta is random or globally large-scale. Therefore, we conclude that these two models for the shallow decay phase and relevant central engines would be testable with future polarization observations.