TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  2240
SUBJECT: GRB 021004: HST observations
DATE:    03/05/21 11:37:48 GMT
FROM:    Andrew Levan at U.of Leicester  <anl@star.le.ac.uk>

Andrew Levan (U. Leicester/STScI), Andrew Fruchter (STScI), Johan Fynbo
(U. Aarhus), Paul Vreeswijk (ESO) and Javier Gorosabel (IAA-CSIC/STScI)
report for the GOSH collaboration.

The field of GRB 021004 has been observed using the ACS/WFC on HST
in four epochs: the 7th, 11th and 22nd Oct and 26th Nov 2002.

A difference image of the last two epochs, which were dithered, reveals a
declining point source whose position is consistent within the astrometric
errors (~0."01) with the peak of the light in the later image.  The source
is marginally resolved in that image (with a FWHM of 0".11, compared to a
point source FWHM of ~0."09) indicating a host galaxy directly underlying
the OT. This differs from the astrometry of Mirabal et al
(astro-ph/0303616) who find the OT position well offset from the host. As
the source lies on the peak of the light, and the object is only
marginally resolved, it is possible that the OT continues to contribute
significantly to the observed brightness even at the later epoch. Hence
the magnitude found for the source in November, F606W(AB)=24.3 +/- 0.1,
is an upper limit on the true brightness of the host galaxy.

We have scheduled late time observations of GRB 021004 which will enable
an accurate determination of both the host galaxy magnitude and the late
time decay of the afterglow.

Images of the field can be found at

http://www.stsci.edu/~fruchter/GRB/021004