GAMMA-RAY BURSTS FROM STELLAR REMNANTS - PROBING THE UNIVERSE AT HIGH-REDSHIFT

A gamma-ray burst (GRB) releases an amount of energy similar to that o f a supernova explosion, which combined with its rapid variability sug gests an origin related to neutron stars or black holes. Since these c ompact stellar remnants form from the most massive stars not long afte r their birth, GRBs should trace the star formation rate in the Univer se; we show that the GRB flux distribution is consistent with this. Be cause of the strong evolution of the star formation rate with redshift , it follows that the dimmest known bursts have z similar to 6, much a bove the value usually quoted and beyond the most distant quasars. Thi s explains the absence of bright galaxies in well-studied GRB error bo xes. The increased distances imply a peak luminosity of 8.3 x 10(51) e rg s(-1) and a rate density of 0.025 per million years per galaxy. The se values are 20 times higher and 150 times lower, respectively, than are implied by fits with nonevolving GRB rates. This means either that GRBs are caused by a much rarer phenomenon than mergers of binary neu tron stars, or that their gamma-ray emission is often invisible to us due to beaming. Precise burst locations from optical transients will d iscriminate between the various models for GRBs from stellar deaths, b ecause the distance between progenitor birth place and burst varies gr eatly among them. The dimmest GRBs are then the most distant known obj ects, and may probe the Universe at an age when the first stars were f orming.