Hubble Space Telescope and Palomar imaging of GRB 990123: Implications forthe 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 galax y, with a magnitude of V = 24.20 +/- 0.15. The strong metal absorption line s seen in the spectrum of the OT, along with the low probability of a chanc e superposition, lead us to conclude that this galaxy is the host of the ga mma-ray burst (GRB). The OT is projected within the similar to 1" visible s tellar field of the host, nearer the edge than the center. We cannot, on th is basis, rule out the galactic nucleus as the site of the GRB, since the u nusual morphology of the host may be the result of an ongoing galactic merg er, but our demonstration that this host galaxy has extremely blue optical- to-infrared colors more strongly supports an association between GRBs and s tar 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 ph ases: an early, rapid decline (f(nu) proportional to t(-1.6) for t < 0.1 da ys); a slower, intermediate decline power-law decay (f(nu) proportional to t(-1.1) for 0.1 < t < 2 days); and then a more rapid decay (at least as ste ep as f(nu) proportional to t(-18) for t > 2 days). The break to a steeper slope at late times may provide evidence that the optical emission from thi s GRB was highly beamed.