[vsnet-grb-info 440] GRB021004, BVRcIc field photometry

[GCN Circulars <gcncirc@lheawww.gsfc.nasa.gov>]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1583
SUBJECT: GRB021004, BVRcIc field photometry
DATE:    02/10/05 18:20:08 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
an 11x11 arcmin field centered at the afterglow coordinates
for GRB021004 (Shirasaki et al. GCN 1565; Fox et al. GCN 1564)
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/grb021004.dat
The astrometry in this file is based on linear plate solutions
with respect to UCAC2.  The external errors are less than 100mas.

The coordinates of the afterglow, based on UCAC2, are:
  00:26:54.68  +18:55:41.6  J2000
The error in RA is about 0.05arcsec; the error in Dec
is about 0.2arcsec.  These coordinates will be improved,
but are in good agreement with Fox et al.
The object is nor particularly red or blue,
so a typical field star should work for a comparison.

While the night was photometric, the GRB field was observed
at high airmass due to other programs on the 1.0m telescope.
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, and to include
U-band measures.  You should check the dates on
the .dat file prior to final publication to get the latest photometry.

In particular, the comparison star used by Fox et al.
has the following position and magnitudes:

    RA (J2000) DEC           V     B-V    V-R   R-I
00:26:58.77  +18:56:56.1  16.261  1.204  0.737  0.664

These values yield Rc=15.52, somewhat fainter than the
assumed magnitude in Fox.

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

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