TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1276 SUBJECT: GRB 011121: Third HST Epoch DATE: 02/03/19 07:14:41 GMT FROM: Shri Kulkarni at Caltech S. R. Kulkarni, J. S. Bloom, P. A. Price, D. E. Reichart, Caltech and B. Schmidt, Mount Stromlo Observatory report on behalf of a larger collaboration: Here we report on the third HST epoch (19 Dec 2001 UT) of WFPC2 images of GRB 011121 (see GCN 1274). The third epoch was requested for observations 1 week after the second epoch but HST scheduling restrictions resulted in the data being taken earlier. We have performed PSF-fitting photometry on the transient and using the prescription of Dolphin and Holtzman (D-H) we measure the following magnitudes: Epoch delta T Filter D-H Magnitude (days) --------------------------------------------- 3 27.24 F555W 25.08 +/- 0.07 3 27.30 F702W 23.65 +/- 0.04 3 28.10 F814W 23.16 +/- 0.06 3 28.16 F850LP 22.72 +/- 0.06 --------------------------------------------- Note: (1) These magnitudes are measured magnitues and have not been corrected for extinction. (2) The magnitude system here is the same as that in GCN 1274. We have converted these magnitudes to fluxes and the light curve can be found at http://www.astro.caltech.edu/~der/1121lc_0.75mag.eps. We assumed that the excess over the power law decay of the afterglow is due to an underlying SN. To this end, we assumed assumed an SN 1998bw template (corrected for extinction of A_V=0.19), redshifted to that of the host galaxy of GRB 011121 (z=0.36, GCN 1152) and subject to the estimated Galactic extinction of A_V=1.64 towards GRB 011121 (see GCN 1158). A simple fit ("chi-by-eye") requires the 1998bw contribution to be dimmed by 0.75 mag. Even so, the fit is only approximate with clear deviations in some bands (e.g. F555W). A full-fledged proper fitting of the data must address two points. First (a minor concern) is the heavy extinction towards this direction. Second (a major issue) is that there is great evidence that cosmologically located GRBs are not spherical but jetted sources with opening angles of only a few degrees (e.g. Frail et al. 2001, ApJ 562, 55). In contrast, the asymmetry in SN 1998bw is sufficiently mild that this issue has been debated for some time in the literature. The fact that we see GRB 011121 means that we are seeing the explosion along the symmetry axis (or close to it). [The opening angle for GRB 011121, as with other GRBs, can be estimated from broad-band data. Indeed, if the true gamma-ray energy release from GRB 011121 is similar to those estimated from other cosmologically located GRBs then we estimate an opening angle of over 8 degrees]. Continuing, if cosmologically GRBs have an underlying SN then these SN are likely to be severely asymmetric. These strong asymmetries, more than any any other quantity (e.g., the amount of synthesized Nickel), will strongly affect the light curve (see Hoflich et al. 1999, ApJ 524, L107). Rapid rise in the SN light is easily explained by rapid expansion along the polar axis and the decline can be expected to be rapid as well. Thus we should be prepared to see a large diversity in the light curves of the underlying SN in cosmologically located GRBs. Within these expectations, it appears that the case for an underlying SN in GRB 011121 is well established.