TITLE: GCN CIRCULAR NUMBER: 8173 SUBJECT: GRB 080903: Astrotel-Caucasus optical observations DATE: 08/09/03 05:40:54 GMT FROM: Denis Denissenko at IKI, Moscow S. Korotkiy (Ka-Dar Observatory), T. Kryachko, B. Satovskiy (Astrotel Observatory, KSU/AST) and D. Denisenko (IKI) report: We have observed the error box of Swift gamma-ray burst GRB 080903 (Grupe et al., GCN 8169) with 0.3-m f/7.8 Astrotel-Caucasus telescope (+ Apogee Alta U9000 CCD camera) at Kazan State University Mountain Station (Karachay-Cherkessia, Russia). The first 120-sec unfiltered exposure was taken at 01:18:21 UT, 358 seconds after the burst trigger. Inside the XRT error circle we have found a new source not present on DSS plates at the following coordinates (J2000.0): 05 47 10.05 +51 15 50.9 (astrometry relative to USNO-A2.0 catalogue, accuracy +/-0.2"). Using the nearby USNO-A2.0 1350-06155883 (05 47 16.36 +51 15 56.4) with R=18.1 as a comparison star, we have measured CR (unfiltered with Red zero point) magnitude 19.1 for the afterglow candidate. The limiting magnitude of the image is 20.5. Four additional images were obtained with the approaching twilight and brightening sky background. The object is still present on the second 120-sec exposure obtained at 01:22:09 UT, but not detected on the next images. The crop of Astrotel-Caucasus image is available at: http://www.astroalert.su/files/grb080903_crop_x4_size.jpg Color-combined (BRIR) DSS image of the same area for comparison: http://hea.iki.rssi.ru/~denis/GRB080903-BRIR-crop.jpg