Where are Swift Gamma-ray bursts beyond the "synchrotron deathline"?

TL;DR

This study investigates the spectral properties of Swift GRBs to determine if they exhibit the same hard low-energy photon indices as previously claimed by BATSE, finding consistency when accounting for instrumental differences.

Contribution

The paper systematically compares Swift and BATSE GRB spectra, clarifying the impact of instrument differences on observed spectral hardness.

Findings

Swift data are consistent with BATSE when considering instrument differences.

Lower detection of very hard spectra in Swift is due to its narrower energy band.

Instrumental factors explain the apparent discrepancy in spectral hardness.

Abstract

We study time-resolved spectra of the prompt emission of Swift Gamma-ray bursts (GRB). Our goal is to see if previous BATSE claims of the existence of a large amount of spectra with the low energy photon indices harder than 2/3 are consistent with Swift data. We perform a systematic search of the episodes of the spectral hardening down to the photon indices below 2/3 in the prompt emission spectra of Swift GRBs. We show that the data of the BAT instrument on board of Swift are consistent with BATSE data, if one takes into account differences between the two instruments. Much lower statistics of the very hard spectra in Swift GRBs is explained by the smaller field of view and narrower energy band of the BAT telescope.