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 magnitude V = 24.20 \pm 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 GRB .
The OT is projected within the \sim 1 ^ { \prime \prime } visible stellar field of the host , nearer the edge than the center .
We can not , 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 on 1999 Feb 9.05 , V = 25.45 \pm 0.15 , is about 1.5 mag fainter than expected from 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 _ { \nu } \propto t ^ { -1.6 } for t < 0.1 days ) , a slower intermediate decline power-law decay ( f _ { \nu } \propto t ^ { -1.1 } for 0.1 < t < 2 days ) , and then 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 the optical emission from this GRB was highly beamed .