Prevalence of Extra Power-Law Spectral Components in Short Gamma-Ray Bursts

TL;DR

This study systematically analyzes 13 short gamma-ray bursts and finds that an extra power-law spectral component is a common feature, likely sharing a physical origin with the main emission, and possibly caused by electromagnetic cascades.

Contribution

It provides the first systematic evidence that the extra power-law component is ubiquitous in short GRBs and suggests a potential physical mechanism involving electromagnetic cascades.

Findings

Extra power-law component is present in all 13 short GRBs analyzed.

The power-law indices are mostly harder than -2.0.

The flux of the extra component correlates with the main spectral component.

Abstract

Prompt extra power-law (PL) spectral component is discovered in some bright gamma-ray bursts (GRBs), which usually dominates the spectral energy distribution below tens of keV or above about 10 MeV. However, its origin is still unclear. In this paper, we present a systematic analysis 13 Fermi short GRBs as of August 2020, with the contemporaneous keV-MeV and GeV detections at the prompt emission phase. We find that the extra PL component is a ubiquitous spectral feature for short GRBs, showing up in all 13 analyzed GRBs. The PL indices are mostly harder than -2.0, which may be well reproduced by considering the electromagnetic cascade induced by ultra-relativistic protons or electrons accelerated in the prompt emission phase. The average flux of these extra PL components positively correlates with that of the main spectral components, which implies they may share the same physical origin.