TITLE: GCN CIRCULAR NUMBER: 7803 SUBJECT: GRB080603B: RTT150 optical observations, break in light curves DATE: 08/06/04 09:46:50 GMT FROM: Irek Khamitov at TUG R. Zhuchkov, I.Bikmaev, N. Sakhibullin (KSU/AST), I. Khamitov, Z. Eker (TUG), U. Kiziloglu (METU), E. Gogus (Sabanci Uni.) R. Burenin, M. Pavlinsky, R. Sunyaev (IKI), report: The optical counterpart (GCN7792, 7793, 7794, 7795, 7796, 7797, 7799) of GRB 080603B (GCN 7794, SWIFT trigger=313087) was observed with Russian-Turkish 1.5-m telescope (RTT150, Bakirlitepe, TUBITAK National Observatory, Turkey), starting at 03 June, 20:55 UT, i.e. ~1.3 hours after the burst, using Andor CCD. A series of 70 frames with 60sec exposure in the R and V bands were taken for a time span ~ 4.5 hours. The afterglow is clearly detected in all images. Using RTT150 preliminary calibrated magnitudes for USNO-A2 1575-03195804 star R.A. = 11 46 09.53, Dec. = +68 01 51.3, V=14.2, R=13.8 mag, the brightness of the OT was estimated on each frame. V,R light curves are shown at: http://www.tug.tubitak.gov.tr/~irekk/grb/grb080603B/GRB080603B_lc.jpg Higher dispersion at (T-T0) > 4 hours must be due to high airmass ( Z ~ 65-70 degrees). The VR-light curves are well fitted with two power law decay functions with a break about 3 hours after the burst. A power law decay indices were estimated in OT VR-light curves as follow: before the break alpha_R=-0.59+/-0.03 alpha_V=-0.60+/-0.03 after the break alpha_R=-1.14+/-0.09 alpha_V=-1.03+/-0.07 The alpha indices before the break are in good agreement with Klotz et al. (GCN7799) calculated for the time span between 1 and 4 hours after the burst. -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean.