in many bursts, we investigate how the time-averaged [$E_{\rm peak}$]{} emerges from the spectral evolution within a single pulse, how this average naturally correlates with the peak flux derived [$E_{\rm peak}$]{} in a burst and how the distribution in [$E_{\rm peak}$]{} values from many bursts derives its surprisingly narrow width.'
author:
- Robert Preece
- Adam Goldstein
- Narayana Bhat and Matthew Stanbro
- Jon Hakkila
- Dylan Blalock
title: 'Which [$E_{\rm peak}$]{}? – The Characteristic Energy of Gamma-Ray Burst Spectra'
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Introduction
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Unquestionably, a characteristic energy can be an important diagnostic feature of the spectra of cosmological sources. If the sources are standard in some way, with identifiable atomic or nuclear lines, for example, the energy measurement could then directly correspond with distance via the cosmological redshift. From early on, analyses of the prompt emission of gamma-ray bursts (GRBs) revealed that a spectral break typically occurred within the instrumental pass band of scintillator-based detectors – between approximately 20 keV to 1 MeV [@Mazets; @Metzger; @Harris]. The spectral break can take on the form of an exponentially-attenuated power-law; in which case, it had been associated with an optically-thin thermal bremsstrahlung temperature. However, many burst spectra are typically harder than 