TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1274 SUBJECT: GRB 011121: HST Observations reveal an intermediate-time multicolor DATE: 02/03/18 05:24:55 GMT FROM: Josh Bloom at CIT GRB 011121: HST Observations reveal an intermediate-time multicolor bump J. S. Bloom, P. A. Price, S. R. Kulkarni, D. E. Reichart (Caltech), D. A. Frail (NRAO), E. Berger on behalf of the Caltech-NRAO-CARA GRB Collaboration, report: As a part of our AO-10 HST GRB program, we observed the afterglow (Wyrzykowski et al.; GCN #1150) of GRB 011121 (Piro et al.; GCN #1147). We obtained WFPC2 observations through multiple filters, designed to detect or constrain underlying supernovae following low-redshift GRBs. The observations presented here were obtained on 2001 Dec 4-5 and 2001 Dec 14-16. Recently, Garnavich et al. (GCN #1273) noted that the optical afterglow of GRB 011121 on day 13 showed an excess in the F702W filter over an extrapolation of fluxes measured at earlier epochs in the R-band filter. They suggested this excess is indicative of a contribution from an underlying supernova (SN) as has been discussed for some GRBs (e.g. GRB 980326, GRB 970228). Here we report further analysis of the HST data. In the following, we give the PSF-fit photometry for the first two epochs of our program in an 0.5 arcsec radius using the current WFPC zero-points from Dolphin (see http://www.noao.edu/staff/dolphin/wfpc2_calib/ ). The magnitudes have been corrected for the (sometimes considerable) charge-transfer inefficiency using the Dolphin methodology but not corrected for Galactic extinction; the uncertainties do not reflect the errors in the absolute zero-points. These brightnesses also include an uncertain contribution from the host galaxy at the position of the transient. As per the discussion in Holtzmann et al. 1995, the "infinite aperture" magnitudes will be ~0.1 mag brighter, Epoch delta T Filter ST Magnitude (days) --------------------------------------------- 1 13.09 F450W 24.64 +/- 0.07 1 13.16 F555W 23.88 +/- 0.05 1 13.23 F702W 23.16 +/- 0.05 1 14.02 F814W 22.79 +/- 0.03 1 14.15 F850LP 22.51 +/- 0.06 2 23.03 F555W 24.43 +/- 0.04 2 23.09 F702W 23.33 +/- 0.03 2 24.83 F814W 22.98 +/- 0.03 2 24.96 F850LP 22.59 +/- 0.09 --------------------------------------------- We confirm that the excess seen on day 13 in F702W is also present in other filters (F450W, F555W, F814W, and F850LP). Second, the excess is seen even at epoch 2 (day 23-25). Thanks to our extensive multi-band data, this is the first unambiguous detection of an intermediate-time bump (>~ 10 days) in a GRB afterglow simultaneously in more than 3 filters. As is common in the SN interpretation, we took the multi-band light curves of SN 1998bw and transformed the same to the redshift of GRB 011121 (z = 0.36, GCN #1152). In the SN 1998bw interpretation, one would expect the brightness to have increased by 0.03 mag in the F555W filter, 0.23 mag in the F702W filter, 0.30 mag in the F814W filter, and 0.19 mag in the F850LP filter between these two epochs. Instead, we find the brightness to decrease by 0.55 +/- 0.06 mag in the 555W filter, 0.17 +/- 0.06 mag in the 702W filter, 0.19 +/- 0.04 mag in the 814W filter, and 0.08 +/- 0.11 mag in the 850LP filter. Thus this curious bump is inconsistent with an underlying SN similar to SN 1998bw. We caution that all these excesses are measured with respect to extrapolations of early time ground based data and such extrapolations have not included possible jet breaks." This message can be cited.