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.