What Gamma-Ray Bursts Explode Into

TL;DR

This paper reviews the environments into which gamma-ray bursts explode, discussing evidence for both stellar wind and constant density media, and explores implications for progenitor models.

Contribution

It synthesizes observational evidence to evaluate the nature of the circumburst medium and proposes possible explanations for the observed diversity in GRB environments.

Findings

Evidence for wind and constant density media in GRB afterglows

Radio observations challenge simple wind models for some long bursts

Short bursts support interaction with constant density medium

Abstract

The association of long gamma-ray bursts (GRBs) with Type Ib/c supernovae implies that they explode into the winds of their Wolf-Rayet progenitor stars. Although the evolution of some GRB afterglows is consistent with expansion into a free wind, there is also good evidence for expansion into a constant density medium. The evidence includes the evolution of X-ray afterglows (when X-rays are below the cooling frequency), the evolution of the pre-jet break optical and X-ray afterglow, and the sharp turn-on observed for some afterglows. Recent observations of short bursts, which are expected to be interacting with a constant density medium, provide a check on the standard afterglow model. Although radio observations do not support the constant density model for long bursts in some cases, the evidence for constant density interaction is strong. The most plausible way to produce such a medium around a massive star is to shock the progenitor wind. This requires a smaller termination shock than would be expected, possibly due to a high pressure surroundings, a high progenitor velocity, or the particular evolution leading to a GRB. However, the need for the termination shock near the deceleration radius cannot be plausibly accomodated and may indicate that some long bursts have compact binary progenitors and explode directly into the interstellar medium.