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GCN Circular 1459

Subject
GRB020331: Late-time Optical Light Curve
Date
2002-07-24T16:15:14Z (22 years ago)
From
Alicia Soderberg at Caltech <ams@astro.caltech.edu>
A. M. Soderberg, D. W. Fox, and S. R. Kulkarni (Caltech) report
on behalf of the larger Caltech-NRAO-CARA GRB collaboration.

"We have observed the optical afterglow associated with GRB 020331
(GCN 1334Loading...
 
 ) with HST+STIS (Clear) at four epochs spanning the period
25-50 days following the burst (one orbit per epoch).

Registration of the STIS data against ground-based images allows us to
identify the afterglow amidst a complex of at least three galaxies,
including the presumed host galaxy which lies under the afterglow
itself.  Aperture photometry on the optical transient + host yields
the following R-equivalent magnitudes for the first three epochs:

    HST Epoch   T (days since burst)     R (mag)
       1        23.9                     24.54 +/- 0.07
       2        36.6                     25.01 +/- 0.10
       3        42.7                     24.76 +/- 0.09

We have derived our R-equivalent magnitudes by assuming
(R-Clear)=-0.67 as given by the STIS Exposure Time Calculator for
a source with power-law spectrum (index=-1).
These values imply a power-law flux decay index over this interval with
index alpha=0.75, compatible with previous ground-based estimates
(Ricker et al., GCN 1382Loading...
 
 ).

Preliminary analysis of the fourth epoch, T=50 days, indicates a
rebrightening of the optical afterglow by 0.2 +/- 0.1 mag as compared
with our Epoch 3.  We have requested additional HST observations to
help us clarify whether this rebrightening is statistically significant.

A finding chart of the field can be found at:

	http://www.astro.caltech.edu/~ams/grb020331.html

This message may be cited."