TITLE: GCN CIRCULAR NUMBER: 10635 SUBJECT: GRB 100418A: RTT150 observations during the optical afterglow maximum DATE: 10/04/20 18:39:42 GMT FROM: Irek Khamitov at TUG I. Bikmaev (KSU/AST), I. Khamitov (TUG), S. Melnikov, N. Sakhibullin (KSU/AST), R. Burenin, M. Pavlinsky, R. Sunyaev (IKI), Z. Eker (TUG), U. Kiziloglu (METU), E. Gogus (Sabanci Uni.) report: The optical afterglow of GRB 100418A (Marshall et al., GCN 10612, Filgas et al., GCN 10617) was observed with Russian-Turkish 1.5-m telescope (RTT150,Bakirlitepe, TUBITAK National Observatory, Turkey) in April 18/19 and April 19/20, 2010, in partially cloudy conditions. We obtained a serie of 300 sec images in Rc-band in April 18/19, and serie of 600 sec images in April 19/20. We found that during April 18 set OT shows strong brightness variations of Rc ~ 18-20 mag within hour time scale. In the Table below we give our Rc estimates relative to the nearby USNO-B1.0 star 1014-0271368 (R2Mag=17.32). April 18 UT (T-T0),h Rc err ================================= 21:43 0.5 20.29 +/-0.10 22:38 1.5 >20.50 3-sigma image limit 23:25 2.25 18.64 +/-0.04 23:51 2.6 18.30 +/-0.04 23:58 2.9 18.33 +/-0.02 ================================= April 20 UT (T-T0),h Rc err ================================= 00:40 27.5 18.88 +/-0.03 RTT-150 data shows that brightness of OT sharply increased from 20 to 18 mag at (T-T0) ~ 2 hours. The OT was observed at maximum brigtness atapproximately 2.6 hours after the burst and then its flux started to decline, which is consistent with other measurements made later on (Rumyantsev et al., GCN 10634, Filgas et al GCN 10617). The brightening of the OT is shown in the following RTT150 images: http://hea.iki.rssi.ru/grb/100418a/indexeng.html -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean.