Gamma-ray bursts afterglows in magnetized stellar winds

TL;DR

This paper investigates gamma-ray burst afterglows in magnetized stellar winds, showing that magnetization causes early X-ray drop-outs and Fermi acceleration resumes at lower Lorentz factors, explaining observed early afterglow features.

Contribution

It provides a detailed analysis of how magnetization affects afterglow emission and the timing of Fermi acceleration in gamma-ray bursts, using first-principles microphysics.

Findings

Magnetized environments cause early X-ray drop-outs in afterglows.

Fermi acceleration resumes when the blast Lorentz factor drops below a critical value.

The early X-ray decay resembles observed fast decay in GRB afterglows.

Abstract

Recent analytical and numerical work argue that successful relativistic Fermi acceleration requires a weak magnetization of the unshocked plasma, all the more so at high Lorentz factors. The present paper tests this conclusion by computing the afterglow of a gamma-ray burst outflow propagating in a magnetized stellar wind using "ab initio" principles regarding the microphysics of relativistic Fermi acceleration. It is shown that in magnetized environments, one expects a drop-out in the X-ray band on sub-day scales as the synchrotron emission of the shock heated electrons exits the frequency band. At later times, Fermi acceleration becomes operative when the blast Lorentz factor drops below a certain critical value, leading to the recovery of the standard afterglow light curve. Interestingly, the observed drop-out bears resemblance with the fast decay found in gamma-ray bursts early X-ray afterglows.