TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1594 SUBJECT: GRB021004: Update Bisei photometric data DATE: 02/10/07 09:06:45 GMT FROM: Yuji Urata at RIKEN K. Matsumoto (Okayama Univ.), T. Kawabata (BAO), K. Ayani (BAO), Y.Urata (Titech / RIKEN), H. Yamaoka (Kyushu Univ.), N. Kawai (Titech / RIKEN) report: "We have corrected our photometric data at Oct. 4 14:13 UT of the GRB021004 afterglow (reported in GCN1567), which was obtained by using the R-band photometric reference USNO star RA=00:26:58.77, DEC=+18:56:56.1 as R=15.52 in the field photometry (Henden, GCN 1583), and present the additional preliminary results as follows. epoch start time UT exposure R-mag. 1 Oct. 4 13:57 60s 16.38 2 Oct. 4 14:13 60s 16.36 3 Oct. 4 14:55 60s 16.63 4 Oct. 4 16:31 60s x 3 17.03 Their typical errors (1 sigma) are 0.12 mag. We find that our point 1 is fainter than the magnitude extrapolated back from the observations at later epochs (decay index 1.32, Weidinger et al. GCN 1573). This suggests that the break occurred during our observations between 0.08 d and 0.2 d after the burst. Analyses of the full data set of our time-resolved photometry in VRI-bands are underway." This message may be cited.