TITLE: GCN GRB OBSERVATION REPORT NUMBER: 255 SUBJECT: GRB 990123: Reduced HST Images DATE: 99/02/09 19:22:51 GMT FROM: Andrew S. Fruchter at STScI Andrew Fruchter, Kailash Sahu, Harry Ferugson, Mario Livio and Mark Metzger on behalf of the larger HST GRB collaboration report: We have reduced the HST STIS CCD clear (50CCD) images of GRB 990123 announced earlier today by the Director of STScI, Steven Beckwith, in GCN 254. We find the optical transient to be superposed on an extended irregular galaxy. A quick photometric reduction shows that the OT has a magnitude of V=25.4 +/- 0.1 and the galaxy has V=24.3 +/- 0.15. The total integrated magnitude of the galaxy and OT is therefore V~24, which agrees with Palomar 5-m observations taken approximately 12 hours before the HST observations, and which will be reported in a succeeding GCN. No correction to the STIS magnitude has yet been made to take into account the colors of the objects, which could change the estimated magnitudes from the broad STIS clear filter by ~0.1 mag. The faint magnitude of the OT suggests that it may be declining more rapidly than the earlier power-law would predict, as suggested by the MDM group in GCN 248. The probable host galaxy is itself composed of several irregular pieces, extended over a region more than 1" across. The major fraction of the light in the host is to the north of the OT, thus agreeing with the probable extension reported by Djorgovski et al. in GCN 243. A gif image as well as the reduced FITS files will be posted by 16:00 EST today (9 February 1999) on http://www.stsci.edu/~fruchter/GRB/990123 We welcome the use of the reduced FITS image by others; however, we ask that those who do use this image cite this GCN, or a formal publication that may succeed it (the bibliographic reference of which would be available on the above web page).