TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1259 SUBJECT: GRB 010921: HST Observations DATE: 02/03/06 01:54:55 GMT FROM: Paul Price at RSAA, ANU at CIT P.A. Price, B.P. Schmidt (RSAA, ANU) and S.R. Kulkarni (Caltech) report on behalf of the larger REACT GRB collaboration: As a part of our AO-9 HST GRB program, we observed the afterglow (Price et al GCN #1107) of GRB 010921 (Ricker et al. GCN #1096). We obtained WFPC2 observations through multiple filters, designed to detect or constrain underlying supernovae for low-redshift GRBs. The observations were obtained on 2001 Oct 26, Nov 6, Nov 25 and 2002 Jan 4. We have drizzled, registered and subtracted the images to obtain host-subtracted fluxes at each of the first three epochs, assuming the flux of the GRB in the final epoch image is negligible. The light-curve is available from http://www.mso.anu.edu.au/~pap/grb010921/ We identify a break in the light-curve at approximately 35 days which may be due to collimation of the ejecta. If so, then we calculate a jet-corrected gamma-ray energy release of 6.7 x 10^50 erg, consistent with the clustering of gamma-ray energy releases found by Frail et al. (2001, ApJ, 562, L55). SN 1998bw at z = 0.451 should peak in the F702W and F814W bands, at approximately 4 uJy. No evidence for such a component in the light-curve is seen in the F555W, F702W, F814W and F850LP bands. These measurements rule out the existence of a SN with a luminosity greater than approximately 20% that of SN 1998bw underlying the GRB. We note, however, that we cannot rule out the existence of a more typical SN Ib/c, such as SN 1994I. We strongly recommend that observers planning a search for a SN underneath a GRB afterglow do not assume that the SN will be as luminous as SN 1998bw. A more detailed analysis of the data is in progress. This message may be cited. [GCN ED NOTE (06Mar02): A type was corrected: "6.7 x 10^51" to "6.7 x 10^50".]