Search for sub-TeV gamma rays in coincidence with gamma ray bursts

Abstract

We report on a study of sub-TeV $(10\mathrm{GeV}<{E}_{\ensuremath{\gamma}}<1\mathrm{TeV})$ gamma-ray-induced muon secondaries in coincidence with BATSE gamma ray bursts (GRBs). Each TeV gamma ray striking the atmosphere produces $\ensuremath{\approx}0.2$ muons whose identity and angle can be measured by the Project GRAND array. Eight GRB candidates were studied; seven were selected based upon the optimum product of (detected BATSE fluence) $\ifmmode\times\else\texttimes\fi{}$ (GRAND's acceptance). One candidate was added because it was reported as a possible detection by the Milagrito Collaboration. Seven candidates show a positive, though not statistically significant, muon excess. The only significant possible coincidence shows an excess of $466\ifmmode\pm\else\textpm\fi{}171$ muons during the BATSE T90 time interval for GRB 971110. The chance probability of such an excess in GRAND's background at the time of this event is $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}.$ The chance probability of observing such an excess in one of the eight bursts studied here is 0.025. If this event is real, the implied fluence of energetic $(>10\mathrm{GeV})$ gamma rays necessary to account for the observed muon excess would require that most of the GRB fluence arrived in the form of energetic gamma rays.