TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1562 SUBJECT: GRB 020903: Optical observations at RIKEN DATE: 02/10/01 11:18:09 GMT FROM: Ken ichi Torii at RIKEN K. Torii (RIKEN) and H. Yamaoka (Kyushu Univ.) report: The entire WXM error circle of XRF 020903 (Ricker, et al. GCN 1530) was observed at RIKEN with the 0.25-m reflector equipped with unfiltered cooled CCD AP6E. The observation started at 2002 September 3.6479 UT (5.23 hours after the burst trigger) and 20-s exposures were repeated. The first 168 frames, obtained between 3.6479 and 3.6965 UT, have been stacked and inspected for the variable source (hereafter VS) found by Soderberg, et al. (GCN 1554). In our image (4".2/pixel) a spatially unresolvable source is detected at (R.A., Dec.) = (22:48:42.37, -20:46:13.0) (J2000). The position is derived by using 12 comparison stars from UCAC1 and the fitting error is 0".3 in both coordinates. This position is closer to the galaxy G2 (as tabulated in USNO-A2.0) rather than to the VS. We also find that the flux for the spatially unresolved G2+VS was 2.0+/-0.2 mag fainter than the star 1 in the finding chart of Soderberg et al. (GCN 1554), which is consistent with the magnitude difference (G2 - star 1 = +2.17) in GSC-2.2.1 F magnitude. These results, both astrometrically and photometrically, show that the VS was much fainter than the galaxy G2 during our observation. ==========