TITLE: GCN GRB OBSERVATION REPORT NUMBER: 715 SUBJECT: GRB990123, HST/STIS observations of the host galaxy DATE: 00/06/18 17:15:20 GMT FROM: Stephen Holland at IFA, U of Aarhus Stephen Holland, Johan Fynbo, Bjarne Thomsen (University of Aarhus), Michael Andersen (University of Oulu), Gunnlaugur Bjornsson (University of Iceland), Jens Hjorth (University of Copenhagen), Andreas Jaunsen (University of Oslo), Priya Natarajan (Universities of Cambridge, & Yale), and Nial Tanvir (University of Hertfordshire) We have obtained 8224 seconds of STIS images with the F28X50LP (long pass) aperture of the host galaxy of GRB 990123. This data was taken as part of the Survey of the Host Galaxies of Gamma-Ray Bursts (Holland et al. GCN 698) approximately 509 days after the burst. Combined images are now available at "http://www.ifa.au.dk/~hst/grb_hosts/data/index.html". Using the light curve fits of Holland et al. (2000, submitted to A&A) we predict that the optical afterglow will have R = 31.6 on 15 June 2000, and thus will not be visible in the STIS images. Therefore, we used aperture photometry to determine the AB magnitudes, in the long pass filter, of the three knots found by Holland & Hjorth (1999, A&A, 344, L67). We find the following colours for the three extended knots where CL is the AB magnitude in the STIS clear aperture, LP is the AB magnitude in the STIS long pass aperture, and beta is the corresponding spectral index, f = k*nu^beta. Knot CL LP CL-LP beta 1 28.3 28.2 +0.1 +/- 0.4 -0.4 2 28.1 27.5 +0.6 +/- 0.4 -2.6 3 28.0 27.5 +0.5 +/- 0.4 -2.2 The GRB occurred on the southeast edge of Knot 1. This knot is approximately one sigma bluer than the other two knots, and two sigma bluer than the overall colour of the galaxy (V-R = 0.43 +/- 0.18; Castro-Tirado et al., 1999, Science 283, 2069). This suggests that Knot 1 might be undergoing stronger star formation than the rest of the galaxy.