Hubble Space Telescope and Palomar imaging of GRB 990123: Implications for the nature of gamma-ray bursts and their hosts

We report on Hubble Space Telescope and Palomar optical images of the field of GRB 990123, obtained in 1999 February 8 and 9. We find that the optical transient (OT) associated with GRB 990123 is located on an irregular galaxy, with a magnitude of V = 24.20 ± 0.15. The strong metal absorption lines seen in the spectrum of the OT, along with the low probability of a chance superposition, lead us to conclude that this galaxy is the host of the gamma-ray burst (GRB). The OT is projected within the ∼1″ visible stellar field of the host, nearer the edge than the center. We cannot, on this basis, rule out the galactic nucleus as the site of the GRB, since the unusual morphology of the host may be the result of an ongoing galactic merger, but our demonstration that this host galaxy has extremely blue optical-to-infrared colors more strongly supports an association between GRBs and star formation. We find that the OT magnitude in 1999 February 9.05, V = 25.45 ± 0.15, is about 1.5 mag fainter than expected from the extrapolation of the decay rate found in earlier observations. A detailed analysis of the OT light curve suggests that its fading has gone through three distinct phases: an early, rapid decline (f_v ∝ t^-1.6 for t < 0.1 days); a slower, intermediate decline power-law decay (f_v ∝ t^-1.1 for 0.1 < t < 2 days); and then a more rapid decay (at least as steep as f_v oc t^-1.8 for t > 2 days). The break to a steeper slope at late times may provide evidence that the optical emission from this GRB was highly beamed.