GCN Circular 1276

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
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 ). 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
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 .

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
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 ) and subject to the
estimated Galactic extinction of A_V=1.64 towards GRB 011121 (see GCN
1158
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 ). 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.