30 - X-ray polarization of gamma-ray bursts

Summary

The degree and the temporal evolution of linear polarization in the prompt and afterglow emission of gamma-ray bursts is a very robust diagnostic of some key features of gamma-ray-burst jets and their micro and macro physics. In this contribution, I review the current status of the theory of polarized emission from GRB jets during the prompt, optical flash, and afterglow emission. I compare the theoretical predictions to the available observations and discuss the future prospects from both the theoretical and observational standpoints.

Introduction

Gamma-ray bursts (GRBs) are the brightest explosions in the present day Universe. Unfortunately, our understanding of their physics is still incomplete, probably due to the fact that they are short-lived, point-like sources.

Polarization is a formidable tool to improve our understanding of GRB jets: their geometry, magnetization, and radiation mechanism could in principle be pinned down with a comprehensive and time-resolved analysis of linear polarization. Observationally speaking, however, polarization is not easy to measure. So far, only the optical afterglow has robust polarization measurements but the diverse features and the sensitivity of the models to detail has made their interpretation, at best, controversial.

In this review, I describe the theory underlying the production of polarized radiation in GRBs in their three main phases. I will focus on X-ray polarization but the discussion will be general, since the frequency dependence of GRB polarization is very weak, especially at frequencies where Faraday rotation is not relevant.