Detection of a spectral break in the extra hard component of GRB 090926A

TL;DR

This paper reports the first significant detection of a spectral break at around 1.4 GeV in the high-energy component of GRB 090926A, enabling estimation of the outflow's bulk Lorentz factor.

Contribution

It presents the first clear observation of a spectral cutoff in the high-energy emission of a GRB, providing new insights into the burst's emission mechanisms and source opacity.

Findings

Detection of a spectral break at 1.4 GeV in the GRB spectrum

Implication of source opacity to pair production

Estimation of the bulk Lorentz factor of the outflow

Abstract

We report on the observation of the bright, long gamma-ray burst, GRB 090926A, by the Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) instruments on board the \Fermi\ Gamma-ray Space Telescope. GRB 090926A shares several features with other bright LAT bursts. In particular, it clearly shows a short spike in the light curve that is present in all detectors that see the burst, and this in turn suggests that there is a common region of emission across the entire \Fermi\ energy range. In addition, while a separate high-energy power-law component has already been observed in other GRBs, here we report for the first time the detection with good significance of a high-energy spectral break (or cutoff) in this power-law component around 1.4 GeV in the time-integrated spectrum. If the spectral break is caused by opacity to electron-positron pair production within the source, then this observation allows us to compute the bulk Lorentz factor for the outflow, rather than a lower limit.