On 23 January 1999 BeppoSAX detected GRB 990123, one of the strongest recorded gamma-ray bursts (GRBs) (Heise 1999; Piro 1999). The optical transient (OT) associated with this GRB reached a peak magnitude of (Akerlof & McKay 1999) only 47 seconds after the initial detection. Spectroscopic observations with the Nordic Optical Telescope (Hjorth et al. 1999; Andersen et al. 1999), and the Keck Telescope (Kelson et al. 1999; Kulkarni et al. 1999), showed strong metallic absorption lines with redshifts of , suggesting that the OT is located either in, or behind, an absorbing system at . Andersen et al. (1999) placed an upper limit of on the redshift of based on ultraviolet photometry and the absence of Lyman- forest lines. For a Friedman cosmology with km s-1, , and a redshift of corresponds to Mpc and a distance modulus of . This means that the OT reached a peak absolute V-band magnitude of 1, which gives an intrinsic luminosity of , approximately times brighter than a Type I supernova. For an isotropic burst the implied energy release approached 3- erg, using BATSE fluence (Kippen 1999).
There are two broad families of models for GRBs, both of which are related to the end stages of the lives of massive stars, and both of which predict that GRBs should trace the star-formation rate in the Universe (e.g. Wijers et al. 1998). The first family is the exploding object family, which includes the "failed supernova" model of Woosley (1993) and the "hypernova" model of Paczynski (1998). These models predict that the progenitors of GRBs are short-lived objects with low space velocities, so GRBs should be found within kpc of the star-forming regions where the progenitors formed. The second family of models is the binary progenitor family, which includes the binary neutron star (NS-NS) model of Narayan et al. (1992) and the merger of a black hole and a neutron star (BS-NS) model of Paczynski (1991). These models predict that GRB progenitors will have high space velocities, due to two supernova explosions having occurred in the progenitor's binary system, so the GRB could be found a significant distance from the star-forming region where the progenitor was formed. OTs have been associated with several GRBs, and most of these OTs have been located within of a faint galaxy. Redshifts have been measured for four of these candidate host galaxies and all are located at cosmological distances.
© European Southern Observatory (ESO) 1999
Online publication: March 29, 1999