TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1453
SUBJECT: GRB 020410:  Discovery of Probable OT by HST
DATE:    02/07/16 20:59:47 GMT
FROM:    Andrew S. Fruchter at STScI  <fruchter@stsci.edu>

A.S. Fruchter, A.J. Levan, I. Burud (STScI), P.E. Nugent (LBNL) report
for the larger GOSH Collaboration:

We observed the field of GRB 020410 (GCN 1349) with HST using the STIS
CCD in open (50CCD) mode on the 8 May and 14 June 2002.   The 50" field
of STIS was centered on the refined NFI BeppoSax postion of Nicastro
et al. (GCN 1374), which has a 90% confidence error-circle radius of
20".

We find a variable source located at RA=22h 06m 31.87s, DEC=-83d 49m 28.3"
where the astrometry has been derived from the headers of the two HST
images (which agree to approximately 0."5).  This position is offset
from the center of the NFI error circle by approximately 6".

Using a zeropoint of V=26.3 for the wide-band STIS 50CCD image, we find
the source had a magnitude of V=25.35 on 8 May and 26.90 on 14 June,
where the uncertainty in the photometry is dominated by the color
correction (which one might expect to be of order 0.1 mags).  The source is
largely point-like, though it lies near a 25th magnitude galaxy, and
shows possible evidence of a faint underlying object.

The decline of ~1.55 magnitudes between the two observation dates is
equivalent to an effective power-law decay of -1.65.  While this value
is completely consistent with the late-time decay of the optical
afterglow of a GRB, we cannot rule out the possibility that a
supernova, unrelated to the GRB, occurred in our search region.  Given
the rates of discovery by HST of supernovae in deep images of random
fields (c.f. Gilliland, Nugent and Phillips 1999) we conservatively
estimate a ~5% chance that this could be such a chance superposition.
Further observations, which we expect will give us both a late-time
magnitude and color, should allow us to distinguish between these
possibilities.

Were the source to have faded consistently as a power law of exponent
-1.65 from early after the burst, then its magnitude would have been
~17 at 6 hours after burst when observations by Castro-Tirado et al.
(GCN 1355), which reached the DSS-2 limit, failed to detect a
counterpart.  BeppoSAX observations (GCN 1358, 1366) taken 20 and 54
hours after burst indicated that the x-ray afterglow associated with
the GRB fell by only a factor of two, consistent with an effective
power law decline of -0.9.  Therefore a break may be the most likely
explanation for the non-detection of this transient in early optical
observations.

If, as expected, this is indeed the OT of GRB 020410, this would be the
first time that HST (or any orbiting observatory) has discovered the OT
of a GRB.

The HST images of the field of GRB 020410 can be found at
http://www.stsci.edu/~fruchter/GRB/020410.