GCN Circular 1630

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

Two additional photometric nights of UBVRcIc all-sky photometry
for the 11x11 arcmin field centered at the afterglow coordinates
for GRB021004 (Shirasaki et al. GCN 1565
Not found
 
 
; Fox et al. GCN 1564
Not found
 
 
)
have been aquired with the USNOFS 1.0-m telescope.  Stars brighter
than V=14.0 are saturated and should be used with care.
The photometric data on the NOFS anonymous ftp site has
been updated:
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 comparison star given by Fox et al. has the following
updated coordinates, magnitudes and colors:

6.744892  18.948922 00:26:58.77  +18:56:56.1  J2000

    V     B-V    U-B    V-R   R-I     errors
  16.258  1.172  1.115  0.720 0.642   0.006 0.028 0.034 0.015 0.023

These colors suggest an approximate K5 spectral classification.

One possibility for the various bumps in the afterglow lightcurve
(suggested privately by B. Schaefer, D. Reichart and P. Price),
is that the Fox et al. comparison star is variable.
This has been investigated by performing differential
photometry with respect to an ensemble of other comparison
stars on the three photometric nights.  The BV results are
given below.

     HJD       V     B-V    verr   bverr
  52552.9804 16.256  1.172  0.003  0.006
  52556.7223 16.256  1.174  0.003  0.005
  52556.7751 16.258  1.176  0.002  0.004
  52556.8276 16.255  1.172  0.002  0.004
  52556.8802 16.259  1.166  0.002  0.004
  52556.9312 16.262  1.122  0.002  0.005
  52557.7823 16.258  1.168  0.002  0.004
  52557.8196 16.252  1.168  0.003  0.005
  52557.8569 16.248  1.181  0.003  0.005
  52557.8942 16.248  1.180  0.002  0.004
  52557.9315 16.255  1.173  0.002  0.004

The datapoint at HJD 2452556.9312 was contaminated by
a cosmic ray hit and so should be given lower weight.
All other measures indicate that the comparison star
is constant over the three nights of observation.  Unless
this star was doing something different between UTD 021005
and UTD 021009, there is no evidence that the afterglow
lightcurve changes are due to comparison star variation.
Using more than one comparison star (or at least including
a check star) when performing differential photometry, and also
selecting comp stars that are close to the program object in
magnitude and color space, is always recommended
to guard against possible variability of any comparison star.