On the Polarized Absorption Lines in Gamma-ray Burst Optical Afterglows

TL;DR

This paper models the polarization of absorption lines in GRB optical afterglows caused by magnetic fields, providing a method to measure magnetic field strength through spectropolarimetric observations.

Contribution

It develops a polarization radiative transfer model for absorption lines in strong magnetic fields, linking polarization degree to optical depth and magnetic field strength.

Findings

Polarization degree depends on optical depth and magnetic field strength.

A magnetic field of at least 10^3 G is needed to measure polarization effectively.

Encourages spectropolarimetric observations to detect Zeeman triplet structures.

Abstract

Spectropolarimetric measurements of gamma-ray burst (GRB) optical afterglows contain polarization information for both continuum and absorption lines. Based on the Zeeman effect, an absorption line in a strong magnetic field is polarized and split into a triplet. In this paper, we solve the polarization radiative transfer equations of the absorption lines, and obtain the degree of linear polarization of the absorption lines as a function of the optical depth. In order to effectively measure the degree of linear polarization for the absorption lines, a magnetic field strength of at least $10^3$ G is required. The metal elements that produce the polarized absorption lines should be sufficiently abundant and have large oscillation strengths or Einstein absorption coefficients. We encourage both polarization measurements and high-dispersion observations of the absorption lines in order to detect the triplet structure in early GRB optical afterglows.