TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1273 SUBJECT: GRB011121, possible supernova association DATE: 02/03/15 17:16:11 GMT FROM: Peter Garnavich at U of Notre Dame P. M. Garnavich, S. T. Holland (Notre Dame), S. Jha, R. P. Kirshner, D. Bersier, and K. Z. Stanek (CfA) We have reanalyzed R-band images of GRB 011121 obtained with the Walter Baade 6.5m Magellan telescope on 2001 Dec. 4.32 in light of the recently released HST images taken near the same time (Bloom, GCN 1260). The brightness of the afterglow in the Magellan data deviated from the steep powerlaw decline seen in the first three days after the burst and we attributed this to contamination from the host galaxy (e.g. Phillips et al. GCN 1164). However the Magellan magnitude of R=23.0+\-0.1, obtained by point-spread-function fitting is consistent with the WFPC2 magnitude of f702w=23.16+\-0.08 which has minimal host contamination. Both observations are nearly 2 magnitudes brighter than the early-time extrapolation. The full light curve is well fit by an initial powerlaw with index of 1.71+\-0.05 plus the light curve of SN 1998bw stretched and faded to a redshift of z=0.36 (Infante et al. GCN 1152). The Dec. 4 data is 10 days after explosion in the rest frame, while SN 1998bw peaked 17 days after GRB 980425, so later epochs may show a small rise in the f702w magnitude before final fading. The light curve can be viewed at http://www.nd.edu/~pgarnavi/grb011121/grb011121_sn.ps Assuming a flat cosmology with Omega_m=0.3 and a Hubble parameter of 65 km/s/Mpc, we derive an absolute magnitude for the possible supernova on Dec. 4 of Mv=-19.3+\-0.2 (corrected for a large Galactic extinction but not for host extinction), consistent with SN 1998bw near maximum. These observations could be strong evidence for a SN-GRB connection. This message may be cited.