I would be surprise if someone saw a gamma ray burst (possibly superman excluded) unless there was a visual spectrum burst accompanying it.
Visible light frequencies are in the range of 10^15 hertz (400-700 nm) whereas Gamma rays start at 10^19 hertz (0.01 nm) , way beyond xrays.
Also it is unlikely to be a cosmic ray as these are in fact the naked nuclei of atoms moving with the energy of 10^20 eV. When a cosmic ray strikes the earths inert gaseous atmosphere the end resultant particle is a muon (travelling at near relativistic velocities
) . Because of their weekly interacting nature these muons have a high potential of reaching the surface where they are detected with bubble chambers or scintillation detectors. (I actually learnt something at Uni this year
) and other ionizing radiation detectors. (eg photographic film and ccd detectors
, these guys will also detect cosmic rays if the particles don't hit any atoms in the atmoshere first )