X-raying extended emission and rapid decay of short gamma-ray bursts

TL;DR

This paper classifies early X-ray emissions in short gamma-ray bursts into two types, revealing that the rapid decay emission likely originates from photospheric or patchy surface processes, indicating a stopping central engine.

Contribution

It provides a time-resolved spectral analysis of nine bright SGRBs, identifying two distinct early emission types and challenging existing high-latitude curvature emission models.

Findings

Two types of early X-ray emission identified in SGRBs.

Rapid decay emission suggests photospheric or patchy surface origin.

Correlations inconsistent with high-latitude curvature emission models.

Abstract

Extended emission is a mystery in short gamma-ray bursts (SGRBs). By making time resolved spectral analyses of brightest nine events observed by ${\it Swift}$ XRT, we obviously classify the early X-ray emission of SGRBs into two types. One is the extended emission with exponentially rapid decay, which shows significant spectral softening during hundreds seconds since the SGRB trigger and is also detected by ${\it Swift}$-BAT. The other is a dim afterglow only showing power-law decay over $10^4$ s. The correlations between the temporal decay and spectral indices of the extended emissions are inconsistent with the $\alpha$-$\beta$ correlation expected for the high-latitude curvature emission from a uniform jet. The observed too-rapid decay suggests the emission from a photosphere or a patchy surface, and manifests the stopping central engine via such as magnetic reconnection at the black hole.