TITLE: GCN GRB OBSERVATION REPORT NUMBER: 712 SUBJECT: GRB 990123: Late-time HST/STIS 50CCD observations of the host DATE: 00/06/17 02:51:46 GMT FROM: Andrew S. Fruchter at STScI A. Fruchter (STScI), S. Thorsett (UCSB), R. Wijers (SUNY) report for the larger HST GRB collaboration: Public HST observations in the red (LP) STIS filter should shortly be available of the host galaxy of GRB 990123. We therefore report here on the results of late-time imaging of the host by our group using the wide-band open (50CCD) filter. The images described here can be found at http://www.stsci.edu/~fruchter/GRB , and will be submitted to a journal shortly. The field of GRB 990123 was reobserved by HST with the STIS CCD in open filter (50CCD) mode during two orbits on 7 February 2000, or 380 days after the GRB. The optical transient was no longer visible. However we have been able to use this image to further refine our earlier photometry on the OT (Fruchter et al. 1999a,b). We now find a V magnitude for the OT of 25.4 +/- 0.1 on 8 February 1999 and V = 27.7 +/- 0.15 on 23 March 1999, implying that the late time late-time light curve falls as t^{-1.7 +/- 0.1}. This represents a break of about 0.65 from the power-law decay a couple of days after outburst, and thus is roughly consistent with the beaming model of Meszaros and Rees (1999). However, this also agrees within the errors with the power-law of the early time light curve reported by ROTSE, and this may suggest a connection between the early and late-time emission. The GRB is found to reside near the edge of the visible stellar field of the host, and is not superposed on a strong region of star-formation. References: Akerlof, C. et al, 1999, Nature, 398 400. Fruchter, A. et al., 1999, Ap. J. (Letters), 519, L13. Fruchter, A. et al., 1999, GCN 354. Meszaros, P. and Rees, M., 1998, MNRAS, 306L, 39.