The width of gamma-ray burst spectra

TL;DR

This study introduces a new spectral width measure for gamma-ray bursts, revealing significant differences between long and short GRBs and challenging existing emission models like synchrotron radiation.

Contribution

The paper proposes a novel spectral width metric for GRB spectra and applies it to data, providing new insights into emission mechanisms and class distinctions.

Findings

Long and short GRBs have significantly different spectral widths.

Most GRB spectra are incompatible with standard synchrotron emission models.

Photospheric emission could explain broader spectra than a blackbody.

Abstract

The emission processes active in the highly relativistic jets of gamma-ray bursts (GRBs) remain unknown. In this paper we propose a new measure to describe spectra: the width of the $EF_E$ spectrum, a quantity dependent only on finding a good fit to the data. We apply this to the full sample of GRBs observed by Fermi/GBM and CGRO/BATSE. The results from the two instruments are fully consistent. We find that the median widths of spectra from long and short GRBs are significantly different (chance probability $<10^{-6}$). The width does not correlate with either duration or hardness, and this is thus a new, independent distinction between the two classes. Comparing the measured spectra with widths of spectra from fundamental emission processes -- synchrotron and blackbody radiation -- the results indicate that a large fraction of GRB spectra are too narrow to be explained by synchrotron radiation from a distribution of electron energies: for example, 78% of long GRBs and 85% of short GRBs are incompatible with the minimum width of standard slow cooling synchrotron emission from a Maxwellian distribution of electrons, with fast cooling spectra predicting even wider spectra. Photospheric emission can explain the spectra if mechanisms are invoked to give a spectrum much broader than a blackbody.