Bimodal distribution of short gamma-ray bursts: evidence for two distinct types of short gamma-ray bursts

TL;DR

This study analyzes 51 short gamma-ray bursts, revealing a bimodal distribution in their observational features, supporting the idea of two distinct progenitor types: neutron star mergers and neutron star-black hole mergers.

Contribution

It provides statistical evidence for two separate classes of short gamma-ray bursts based on their flux, fluence, and duration, linked to different progenitor mechanisms.

Findings

Bimodal distribution in peak fluxes and fluences.

Short bursts with extended emission cluster in a specific region.

Distinct observational features suggest two progenitor types.

Abstract

Recently, GRB 170817A was confirmed to be associated with GW 170817, which was produced by a neutron star - neutron star merger. It indicates that at least some short gamma-ray bursts come from binary neutron star mergers. Theoretically, it is widely accepted that short gamma-ray bursts can be produced by two distinctly different mechanisms, binary neutron star mergers and neutron star - black hole mergers. These two kinds of bursts should be different observationally due to their different trigger mechanisms. Motivated by this idea, we collect a universal data set constituted of 51 short gamma-ray bursts observed by $Swift$/BAT, among which 14 events have extended emission component. We study the observational features of these 51 events statistically. It is found that our samples are consisted of two distinct groups. They clearly show a bimodal distribution when their peak photon fluxes at 15-150 keV band are plotted against the corresponding fluences. Most interestingly, all the 14 short bursts with extended emission lie in a particular region. When the fluences are plotted against the burst durations, short bursts with extended emission again tend to concentrate in the long duration segment. These features strongly indicate that short gamma-ray bursts really may come from two distinct types of progenitors. We argue that those short gamma-ray bursts with extended emission come from the coalescence of neutron stars, while the short gamma-ray bursts without extended emission come from neutron star - black hole mergers.