Gamma-Ray Burst Sequences in Hardness Ratio-Peak Energy Plane

TL;DR

This study analyzes the spectral properties of gamma-ray bursts in the hardness ratio-peak energy plane, revealing distinct sequences for long, short, and X-ray flashes, and suggesting different origins or mechanisms.

Contribution

It uncovers that long GRBs and XRFs follow similar spectral sequences, while short GRBs form a separate sequence, indicating different physical origins.

Findings

Long GRBs and XRFs follow a similar spectral sequence.

Short GRBs have higher hardness ratios at given peak energies.

Short GRBs are a distinct subclass, not an extension of long GRBs.

Abstract

The narrowness of the distribution of the peak energy of $\nu F_{\nu}$ spectrum of gamma-ray bursts (GRBs) and the unification of GRB population are great puzzles yet to be solved. We investigate the two puzzles based on the global spectral behaviors of different GRB population in the $HR-E_{\rm{p}}$ plane (HR the spectral hardness ratio) with BATSE and HETE-2 observations. It is found that long GRBs and XRFs observed by HETE-2 seem to follow the same sequence in the $HR-E_{\rm{p}}$ plane, with the XRFs at the low end of this sequence. The long and short GRBs observed by BATSE follow significantly different sequences in the $HR-E_{\rm p}$ plane, with most of the short GRBs having a larger hardness ratio than the long GRBs at a given $E_{\rm{p}}$. These results indicate that the global spectral behaviors of the long GRB sample and the XRF sample are similar, while that of short GRBs is different. The short GRBs seem to be a unique subclass of GRBs, and they are not the higher energy extension of the long GRBs (abridged).