TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1117 SUBJECT: GRB 010921: Possible supernova component DATE: 01/10/28 06:09:46 GMT FROM: Paul Price at RSAA, ANU at CIT P.A. Price, S.R. Kulkarni, D.W. Fox, E. Berger, J.S. Bloom, S.G. Djorgovski, D.A. Frail, T.J. Galama, F.A. Harrison, A. Mahabal, D.E. Reichart, R. Sari and S.A. Yost of the Caltech-NRAO-CARA GRB collaboration report: We re-observed the optical transient associated with GRB 010921 (GCN ##1107,1108) with the Palomar 200-inch telescope in Sloan g'r'i'z' on 2001 Oct 19.1 UT. Magnitudes of the counterpart were measured relative to stars in the field calibrated with the Palomar 60-inch telescope and the USNOFS 1.0-metre telescope (GCN #1100). We find that, at this epoch, the counterpart has a spectral flux distribution that peaks between the r' and i' bands, and falls away on the red and blue sides as ~ nu^3.5 and ~ nu^-2.1 respectively. This spectral flux distribution finds a natural explanation in terms of a supernova underlying the GRB afterglow (Bloom et al. 1999; Reichart 1999; Galama et al. 2000). If this interpretation is correct, the source should continue to fade beyond R ~ 22 mag over a timescale of a few weeks. We strongly encourage further monitoring of this source, especially in the blue and near infrared. This message may be cited. References: Bloom, J.S. et al., 1999, Nature, 401, 453. Reichart, D.E., 1999, ApJ, 521, L111. Galama, T.J. et al., 2000, ApJ, 536, 185.