DETECTION OF A GAMMA-RAY BURST OF VERY LONG-DURATION AND VERY-HIGH-ENERGY

ALTHOUGH gamma-ray bursts (GRBs) have been known for more than 20 year s, their origin remains mysterious(1). Suggestions have ranged from ne arby colliding comets(2) to merging neutron stars at cosmological dist ances(3). The lack of any counterpart at wavelengths other than X-rays and gamma-rays has posed a major problem in identifying the source of GRBs(4). Although in principle the distribution of energies among the burst photons, as well as their light curves, may be used to constrai n the potential sources, this has proved difficult in practice(5). Her e we present the observation of a particularly energetic burst with a duration of 90 minutes, which includes the detection of an 18-GeV phot on. For comparison, typical GRBs emit photons in the energy range betw een a few keV and a few tens of MeV, and last a few tens of seconds(6, 7). The extended nature of this burst holds out the hope that there wi ll be opportunities for telescopes operating at other wavelengths to d etect a GRB source white it is still active, thus providing further co nstraints on the source's identity and properties.