The unusual afterglow of GRB 980326: evidence for the gamma-ray burst/supernova connection

摘要

Cosmic gamma-ray bursts (GRBs) have been firmly established as one of themost powerful phenomena in the Universe, releasing electromagnetic energyapproaching the rest-mass energy of a neutron star in a few seconds. The twocurrently popular models for GRB progenitors are the coalescence of two compactobjects (such as neutron stars or black holes) or collapse of a massive star.An unavoidable consequence of the latter model is that a bright supernovaeshould accompany the GRB. The emission from this supernova competes with themuch brighter afterglow produced by the relativistic shock that gives rise tothe GRB itself. Here we present evidence for an unusual light curve for GRB980326 based on new optical observations. The transient brightened ~3 weeksafter the burst to a flux sixty times larger than that extrapolated from therapid decay seen at early time. Furthermore, the spectrum changed dramaticallyand became extremely red. We argue that the new source is the underlyingsupernova. If our hypothesis is true then this would be the first evidence fora supernova connection with GRBs at cosmological distances. We suggest thatGRBs with long durations are associated with death of massive stars.