Revisiting the Long/Soft-Short/Hard Classification of Gamma-Ray Bursts in the Fermi Era

TL;DR

This study reaffirms the long/short and soft/hard classification of gamma-ray bursts using Fermi data, revealing spectral and energy correlations that depend on spectral shape and instrument energy bands.

Contribution

It provides a comprehensive statistical analysis of Fermi GRBs, clarifying how spectral properties influence classification and energy correlations, and compares these with previous instruments.

Findings

Bimodal duration and energy ratio support traditional classification.

Spectral hardness and duration anti-correlation depends on spectral shape.

Short and long GRBs follow different intrinsic energy correlations.

Abstract

We perform a statistical analysis of the temporal and spectral properties of the latest Fermi gamma-ray bursts (GRBs) to revisit the classification of GRBs. We find that the bimodalities of duration and the energy ratio ($E_{\mathrm{peak}}$/Fluence) and the anti-correlation between spectral hardness (hardness ratio ($HR$), peak energy and spectral index) and duration ($T_{90}$) support the long/soft $-$ short/hard classification scheme for Fermi GRBs. The $HR - T_{90}$ anti-correlation strongly depends upon the spectral shape of GRBs and energy bands, and the bursts with the curved spectra in the typical BATSE energy bands show a tighter anti-correlation than those with the power-law spectra in the typical BAT energy bands. This might explain why the $HR - T_{90}$ correlation is not evident for those GRB samples detected by instruments like {\it Swift} with a narrower/softer energy bandpass. We also analyze the intrinsic energy correlation for the GRBs with measured redshifts and well defined peak energies. The current sample suggests $E_{\mathrm{p,rest}}=2455\times (E_{\mathrm{iso}}/10^{52})^{0.59}$ for short GRBs, significantly different from that for long GRBs. However, both the long and short GRBs comply with the same $E_{\mathrm{p,rest}}-L_{\mathrm{iso}}$ correlation.