TITLE:   GCN CIRCULAR
NUMBER:  1571
SUBJECT: XRF020903, BVRI field photometry
DATE:    02/10/04 22:32:34 GMT
FROM:    Arne A. Henden at USNO/USRA  <aah@nofs.navy.mil>

A. Henden (USRA/USNO) reports on behalf of the USNO GRB team:

We have acquired BVRcIc all-sky photometry for
a 20x20 arcmin field centered at the variable object coordinates
for XRF020903 (Ricker et al. GCN 1530; Soderberg et al. GCN 1554)
with the USNOFS 1.0-m telescope on one photometric night.  Stars
brighter than V=14.0 are saturated and should be used with care.
We have placed the photometric data on our anonymous ftp site:
ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/xrf020903.dat
The astrometry in this file is based on linear plate solutions
with respect to UCAC2.  The external errors are less than 100mas.

While the night was photometric, the XRF field is towards
our southern horizon and so there is a higher risk of
zeropoint errors.  We estimate the present zeropoint errors to
be about 0.03mag.  We will be extending this
calibration with additional nights to ensure against
a systematic zeropoint error.  You should check the dates on
the .dat file prior to final publication to get the latest photometry.

In particular, the two stars marked on Soderberg et al. finding
chart have the following position and magnitudes:

           RA (J2000) DEC           V     B-V    V-R   R-I
Star 1  22:48:36.05  -20:44:34.8  16.381  0.789  0.455 0.399
Star 2  22:48:53.48  -20:43:23.1  14.743  0.466  0.304 0.313

These values match closely those given in Covino et al. (GCN 1563).

Calibration of the GRB021004 field will occur around October 9 UT.