  Cosmic gamma-ray bursts (GRBs) have been firmly established as one of the
most powerful phenomena in the Universe, releasing electromagnetic energy
approaching the rest-mass energy of a neutron star in a few seconds. The two
currently popular models for GRB progenitors are the coalescence of two compact
objects (such as neutron stars or black holes) or collapse of a massive star.
An unavoidable consequence of the latter model is that a bright supernovae
should accompany the GRB. The emission from this supernova competes with the
much brighter afterglow produced by the relativistic shock that gives rise to
the GRB itself. Here we present evidence for an unusual light curve for GRB
980326 based on new optical observations. The transient brightened ~3 weeks
after the burst to a flux sixty times larger than that extrapolated from the
rapid decay seen at early time. Furthermore, the spectrum changed dramatically
and became extremely red. We argue that the new source is the underlying
supernova. If our hypothesis is true then this would be the first evidence for
a supernova connection with GRBs at cosmological distances. We suggest that
GRBs with long durations are associated with death of massive stars.

Astrophysics