  After almost 4 years of operation, the two instruments onboard the Fermi
Gamma-ray Space Telescope have shown that the number of gamma-ray bursts with
high energy photon emission above 100 MeV cannot exceed roughly 9% of the total
number of all such events, at least at the present detection limits. In a
recent paper (Zheng et al. 2012c), we found that GRBs with photons detected in
the Large Area Telescope (LAT) have a surprisingly broad distribution with
respect to the photon number above background. Extrapolation of our empirical
fit to numbers of photons below our quoted detection limit suggests that the
overall rate of such events could be determined by standard image co-adding
techniques. In this case, we have taken advantage of the excellent angular
resolution of the Swift mission to provide accurate reference points for 79 GRB
events which have eluded any previous correlations with high energy photons. We
find a small but significant signal. Guided by the power law fit obtained
previously for the number distribution of GRBs, the data suggests that only a
small fraction of GRBs are sources of high energy photons.
