  Since the launch of the Fermi Gamma-ray Space Telescope on June 11, 2008, 55
gamma-ray bursts (GRBs) have been observed at coordinates that fall within
66^\circ of the Fermi Large Area Telescope (LAT) boresight with precise
localizations provided by the NASA Swift mission or other satellites. Imposing
selection cuts to exclude low Galactic latitudes and high zenith angles reduces
the sample size to 41. Using matched filter techniques, the Fermi/LAT photon
data for these fields have been examined for evidence of bursts that have so
far evaded detection at energies above 100 MeV. Following comparisons with
similar random background fields, two events, GRB 080905A and GRB 091208B,
stand out as excellent candidates for such an identification. After excluding
the six bright bursts previously reported by the LAT team, the remaining 35
events exhibit an excess of LAT "diffuse" photons with a statistical
significance greater than 2 sigma, independent of the matched filter analysis.
After accounting for the total number of photons in the well-localized fields
and including estimates of detection efficiency, one concludes that somewhere
in the range of 11% to 19% of all GRBs within the LAT field of view illuminate
the detector with two or more energetic photons. These are the most stringent
estimates of the high energy photon content of GRBs to date. The two new events
associated with high energy photon emission have similar ratios of high to low
energy fluences as observed previously. This separates them from bursts with
similar low energy fluences by a factor of ten, suggesting a distinct class of
events rather than a smooth continuum.
