Limits on the GeV Emission from Gamma-Ray Bursts

TL;DR

This paper investigates the high-energy gamma-ray emission limits from gamma-ray bursts, constraining emission models and suggesting a spectral cutoff possibly due to pair creation.

Contribution

It provides upper limits on GeV emission from bright GRBs without LAT detection, challenging certain emission models and indicating spectral features.

Findings

Average LAT/GBM fluence ratio of 0.13 during T90

Average ratio of 0.45 in the first 600 seconds

Extrapolated MeV spectra often exceed observed GeV fluence

Abstract

The Large Area Telescope (LAT) on board of the Fermi satellite detected emission above 30 MeV only in a small fraction of the long gamma-ray bursts (GRBs) detected by the Fermi Gamma-ray Burst Monitor (GBM) at 8 keV - 10 MeV. Those bursts that were detected by the LAT were among the brightest GBM bursts. We examine a sample of the most luminous GBM bursts with no LAT detection and obtain upper limits on their high energy fluence. We find an average upper limit of LAT/GBM fluence ratio of 0.13 for GeV fluence during $T_{90}$ and an average upper limit ratio of 0.45 for GeV fluence during the first 600 seconds after the trigger. These ratios strongly constrain various emission models and in particular rule out SSC models for the prompt emission. In about a third of both LAT detected and LAT non-detected bursts, we find that the extrapolation of the MeV range Band spectrum to the GeV range is larger than the observed GeV fluence (or its upper limit). While this excess is not highly significant for any specific burst, the overall excess in a large fraction of the bursts suggests a decline in the high energy spectral slope in at least some of these bursts. Possibly an evidence for the long sought after pair creation limit.