{
  "bibcode": "2002GCN..1630....1H",
  "body": "A. Henden (USRA/USNO) reports on the behalf of the USNO GRB team:\n\nTwo additional photometric nights of UBVRcIc all-sky photometry\nfor the 11x11 arcmin field centered at the afterglow coordinates\nfor GRB021004 (Shirasaki et al. GCN 1565; Fox et al. GCN 1564)\nhave been aquired with the USNOFS 1.0-m telescope.  Stars brighter\nthan V=14.0 are saturated and should be used with care.\nThe photometric data on the NOFS anonymous ftp site has\nbeen updated:\nftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/grb021004.dat\nThe astrometry in this file is based on linear plate solutions\nwith respect to UCAC2.  The external errors are less than 100mas.\n\nThe comparison star given by Fox et al. has the following\nupdated coordinates, magnitudes and colors:\n\n6.744892  18.948922 00:26:58.77  +18:56:56.1  J2000\n\n    V     B-V    U-B    V-R   R-I     errors\n  16.258  1.172  1.115  0.720 0.642   0.006 0.028 0.034 0.015 0.023\n\nThese colors suggest an approximate K5 spectral classification.\n\nOne possibility for the various bumps in the afterglow lightcurve\n(suggested privately by B. Schaefer, D. Reichart and P. Price),\nis that the Fox et al. comparison star is variable.\nThis has been investigated by performing differential\nphotometry with respect to an ensemble of other comparison\nstars on the three photometric nights.  The BV results are\ngiven below.\n\n     HJD       V     B-V    verr   bverr\n  52552.9804 16.256  1.172  0.003  0.006\n  52556.7223 16.256  1.174  0.003  0.005\n  52556.7751 16.258  1.176  0.002  0.004\n  52556.8276 16.255  1.172  0.002  0.004\n  52556.8802 16.259  1.166  0.002  0.004\n  52556.9312 16.262  1.122  0.002  0.005\n  52557.7823 16.258  1.168  0.002  0.004\n  52557.8196 16.252  1.168  0.003  0.005\n  52557.8569 16.248  1.181  0.003  0.005\n  52557.8942 16.248  1.180  0.002  0.004\n  52557.9315 16.255  1.173  0.002  0.004\n\nThe datapoint at HJD 2452556.9312 was contaminated by\na cosmic ray hit and so should be given lower weight.\nAll other measures indicate that the comparison star\nis constant over the three nights of observation.  Unless\nthis star was doing something different between UTD 021005\nand UTD 021009, there is no evidence that the afterglow\nlightcurve changes are due to comparison star variation.\nUsing more than one comparison star (or at least including\na check star) when performing differential photometry, and also\nselecting comp stars that are close to the program object in\nmagnitude and color space, is always recommended\nto guard against possible variability of any comparison star.",
  "circularId": 1630,
  "createdOn": 1034439994000,
  "email": "aah@nofs.navy.mil",
  "subject": "GRB021004, UBVRcIc field photometry update",
  "submitter": "Arne A. Henden at USNO/USRA  <aah@nofs.navy.mil>",
  "eventId": "GRB 021004"
}