2011MNRAS.416.3089B

The Large Area Telescope (LAT) on-board the Fermi satellite detected emission above 20 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–40 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 T₉₀ and an average upper limit ratio of 0.45 for GeV fluence during the first 600 s after the trigger. These ratios strongly constrain various emission models and in particular rule out synchrotron self-Compton 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.