Constraining the High-Energy Emission from Gamma-ray Bursts with Fermi
  We examine 288 GRBs detected by the Fermi Gamma-ray Space Telescope's
Gamma-ray Burst Monitor (GBM) that fell within the field-of-view of Fermi's
Large Area Telescope (LAT) during the first 2.5 years of observations, which
showed no evidence for emission above 100 MeV. We report the photon flux upper
limits in the 0.1-10 GeV range during the prompt emission phase as well as for
fixed 30 s and 100 s integrations starting from the trigger time for each
burst. We compare these limits with the fluxes that would be expected from
extrapolations of spectral fits presented in the first GBM spectral catalog and
infer that roughly half of the GBM-detected bursts either require spectral
breaks between the GBM and LAT energy bands or have intrinsically steeper
spectra above the peak of the {\nu}F{\nu} spectra (Epk). In order to
distinguish between these two scenarios, we perform joint GBM and LAT spectral
fits to the 30 brightest GBM-detected bursts and find that a majority of these
bursts are indeed softer above Epk than would be inferred from fitting the GBM
data alone. Approximately 20% of this spectroscopic subsample show
statistically significant evidence for a cut-off in their high-energy spectra,
which if assumed to be due to {\gamma}{\gamma} attenuation, places limits on
the maximum Lorentz factor associated with the relativistic outflow producing
this emission. All of these latter bursts have maximum Lorentz factor estimates
that are well below the minimum Lorentz factors calculated for LAT- detected
GRBs, revealing a wide distribution in the bulk Lorentz factor of GRB outflows
and indicating that LAT-detected bursts may represent the high end of this
distribution.
