3.5. Gamma-Ray Bursts
While the physical origin of gamma-ray bursts (GRBs) remains enigmatic,
it is now clear that they have an extragalactic origin and are
associated with an immense, possibly beamed, energy release. This makes
them visible at large look-back times. Several GRBs have been credibly
identified with X-ray, radio, and optical transients. Most
spectacularly, GRB 990123 was identified with an optical transient
whose peak apparent brightness 47 s after the gamma-ray release was 9th
magnitude in the optical, dimming by 5 mag within 500 s
(Akerlof et al. 1999).
As of this writing, five GRBs have reliable spectroscopic redshifts,
with one event (GRB 971214) identified with a galaxy at z = 3.428
(Kulkarni et al. 1998).
Spectroscopy of the GRB 990123 optical transient reveals an absorption
system with zabs = 1.6004
(Kelson et al. 1999),
setting a minimum distance for that extremely bright source. Assuming
unlensed isotropic energy release, the implied energies are immense
(isotropic equivalent E
1052±1 ergs in the gamma-rays alone). Thus, at least
some fraction of GRBs are sufficiently powerful to be detected at great
distances. The optical transients typically are not associated with the
central nuclei of galaxies, implying that GRBs are not related to AGN or
the massive black holes which are thought to reside in the centers of
many galaxies
(Bloom et al. 1999).
The two leading theories suggest that GRBs are associated with the
creation of a stellar-mass black hole, either through coalescence of the
remnants of a massive stellar binary (e.g., neutron star-neutron star or
neutron star-black hole;
Paczynski 1986;
Goodman 1986)
or through direct collapse of a massive star
(Woosley 1993;
Pacznyski 1998).
Both models predict that GRBs should preferentially occur in
star-forming galaxies. Follow-up observations of the faint host galaxies
of GRBs may become a robust method of studying normal galaxies at the
highest accessible redshifts. In particular, spectroscopy during the
bright phase may yield redshifts which would be untenable or impossible
for the faint hosts during their quiescent phase.