[ITAL]Hubble Space Telescope[/ITAL] and Palomar Imaging of GRB 990123: Implications for the Nature of Gamma-Ray Bursts and Their Hosts

We report on HST and Palomar optical images of the field of GRB 990123,obtained on 8 and 9 February 1999. We find that the optical transient (OT)associated with GRB 990123 is located on an irregular galaxy, with magnitudeV=24.20 +/- 0.15. The strong metal absorption lines seen in the spectrum of theOT, along with the low probability of a chance superposition, lead us toconclude that this galaxy is the host of the GRB. The OT is projected withinthe ~1'' visible stellar field of the host, nearer the edge than the center. Wecannot, 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 ongoinggalactic merger, but our demonstration that this host galaxy has extremely blueoptical to infrared colors more strongly supports an association between GRBsand star formation. We find that the OT magnitude on 1999 Feb 9.05, V = 25.45+/- 0.15, is about 1.5 mag fainter than expected from extrapolation of thedecay rate found in earlier observations. A detailed analysis of the OT lightcurve suggests that its fading has gone through three distinct phases: an earlyrapid decline (f_{nu} \propto t^{-1.6} for t < 0.1 days), a slower intermediatedecline power-law decay (f_{nu} \propto t^{-1.1} for 0.1 < t < 2 days), andthen a more rapid decay (at least as steep as (f_{\nu} \propto t^{-1.8} for t >2 days). The break to steeper slope at late times may provide evidence that theoptical emission from this GRB was highly beamed.Comment: Accepted for publication in Astrophysical Journal (Letters). Fourteen pages. Three encapsulated figure