There has been significant progress recently in our understanding of
gamma-ray bursts.
The long-sought counterparts at other wavelengths have finally been
found for a few
bursts. This breakthrough is the result of coordinated observations
involving several
satellites and ground-based optical and radio observatories. In one
case, GRB970508,
red-shifted absorption lines have been detected, finally settling the
debate about the
distance scale. The consensus is that the burst sources lie at
cosmological distances,
requiring at least $\sim 10^{51}$ ergs to be emitted in gamma rays in
just a few seconds.
The gamma radiation is thought to be produced by shocks in a highly
relativistic fireball.
Many mysteries remain. There is no consensus on the nature of the
sources, although
coalescing neutron stars are the leading candidate. There is evidence
that the sources
of the faintest bursts may at redshifts above 2. If so, gamma-ray
bursts may ultimately
tell us something about the early Universe.