TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1630 SUBJECT: GRB021004, UBVRcIc field photometry update DATE: 02/10/12 16:26:34 GMT FROM: Arne A. Henden at USNO/USRA 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; Fox et al. GCN 1564) 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.