TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1135 SUBJECT: GRB 010921: HST Detection of the Afterglow and Host Galaxy DATE: 01/11/08 04:07:12 GMT FROM: Josh Bloom at CIT GRB 010921: HST Detection of the Afterglow and Host Galaxy J. S. Bloom, S. R. Kulkarni, F. A. Harrison, T. J. Galama, P. A. Price, S. G. Djorgovski, D. W. Fox, E. Berger, and D. A. Frail report on behalf the larger Caltech-NRAO-CARA GRB Collaboration: "We observed the field of GRB 010921 (Ricker et al., GCN #1096; Hurley et al., GCN #1097; Price et al., GCN #1107) on 2001 October 26-27 UT using the WFPC F450W, F555W, F606W, F814W, F850LP filters on the Hubble Space Telescope for a total of 1200 s integration in each bandpass. The observations were conducted as part of the HST-GO Proposal #8867 (S. Kulkarni, P.I.). An astrometric comparison of the HST images with the Price et al. discovery image from Palomar reveals that the transient position is consistent (to better than 1 sigma) with a red compact source O.41 arcsec to the East of the center of a galaxy. The Palomar localization is inconsistent (at the 3-sigma level) with the center of the galaxy (note: no correction has yet been made to the Palomar transient position in the Palomar image for the contribution of the galaxy to the centroid position; correcting for this effect would tend to push the Palomar transient position further from the galaxy center). Given that the red color of the compact source is consistent with the Price et al. spectral slope and that the localization is consistent, we identify this source as the afterglow of GRB 010921 and the galaxy (z=0.45; Djorgovski et al. #1108) as the host." An image of the host and the optical transient may be found at: http://www.astro.caltech.edu/~jsb/GRB/grb010921-host-reg.ps The 1 and 3-sigma uncertainty contours of the OT position measured from the Palomar discovery images is shown. Links to false-color images of the WFPC field and close-up images of the OT-host system may be found at: http://www.astro.caltech.edu/~jsb/grb010921 Analysis of the broadband afterglow spectra is underway and more HST observations are scheduled. This message may be cited.