{
  "bibcode": "2002GCN..1276....1K",
  "body": "S. R. Kulkarni, J. S. Bloom, P. A. Price, D. E. Reichart, Caltech\nand B. Schmidt, Mount Stromlo Observatory  report\non behalf of a larger collaboration:\n\nHere we report on the third HST epoch (19 Dec 2001 UT) of WFPC2 images of\nGRB 011121 (see GCN 1274). The third epoch was requested for observations\n1 week after the second epoch but HST scheduling restrictions resulted\nin the data being taken earlier.\n\nWe have performed PSF-fitting photometry on the transient and using\nthe prescription of Dolphin and Holtzman (D-H) we measure the following\nmagnitudes:\n\nEpoch  delta T      Filter   D-H Magnitude\n       (days)\n---------------------------------------------\n3       27.24       F555W    25.08 +/- 0.07\n3       27.30       F702W    23.65 +/- 0.04\n3       28.10       F814W    23.16 +/- 0.06\n3       28.16       F850LP   22.72 +/- 0.06\n---------------------------------------------\nNote:  (1) These magnitudes are measured magnitues and have not been\ncorrected for extinction. (2) The magnitude system here is the same\nas that in GCN 1274.\n\nWe have converted these magnitudes to fluxes and the light curve can be\nfound at http://www.astro.caltech.edu/~der/1121lc_0.75mag.eps.  We\nassumed that the excess over the power law decay of the afterglow is\ndue to an underlying SN. To this end, we assumed assumed  an SN 1998bw\ntemplate (corrected for extinction of A_V=0.19), redshifted to that of\nthe host galaxy of GRB 011121 (z=0.36, GCN 1152) and subject to the\nestimated Galactic extinction of A_V=1.64 towards GRB 011121 (see GCN\n1158). A simple fit (\"chi-by-eye\") requires the 1998bw contribution to\nbe dimmed by 0.75 mag. Even so, the fit is only approximate  with clear\ndeviations in some bands (e.g. F555W).\n\nA full-fledged proper fitting of the data must address two points.\nFirst (a minor concern) is the heavy extinction towards this direction.\n\nSecond (a major issue) is that there is great evidence that cosmologically\nlocated GRBs are not spherical but jetted sources with opening angles of\nonly a few degrees (e.g. Frail et al. 2001, ApJ 562, 55).  In contrast,\nthe asymmetry in SN 1998bw is sufficiently mild that this issue has been\ndebated for some time in the literature.\n\nThe fact that we see GRB 011121 means that we are seeing the explosion\nalong the symmetry axis (or close to it).  [The opening angle for GRB\n011121, as with other GRBs, can be estimated from broad-band data.\nIndeed, if the true gamma-ray energy release from GRB 011121 is similar\nto those estimated from other cosmologically located GRBs then\nwe estimate an opening angle of over 8 degrees].\n \nContinuing, if cosmologically GRBs have an underlying SN then these SN\nare likely to be severely asymmetric. These strong asymmetries, more than\nany any other quantity (e.g., the amount of synthesized Nickel), will\nstrongly affect the light curve (see Hoflich et al. 1999, ApJ 524, L107).\nRapid rise in the SN light is easily explained by rapid expansion along\nthe polar axis and the decline can be expected to be rapid as well. Thus\nwe should be prepared to see a large diversity in the light curves of the\nunderlying SN in cosmologically located GRBs. Within these expectations,\nit appears that the case for an underlying SN in GRB 011121 is \nwell established.",
  "circularId": 1276,
  "createdOn": 1016522081000,
  "email": "srk@astro.caltech.edu",
  "subject": "GRB 011121: Third HST Epoch",
  "submitter": "Shri Kulkarni at Caltech  <srk@astro.caltech.edu>",
  "eventId": "GRB 011121"
}