TITLE: GCN CIRCULAR NUMBER: 13636 SUBJECT: GRB 120811C: RTT150 optical observations DATE: 12/08/13 17:29:30 GMT FROM: Rodion Burenin at IKI, Moscow A. Galeev (Kazan Federal University), I. Khamitov (TUG), I. Bikmaev, N. Sakhibullin (Kazan Federal University), R. Burenin, M. Pavlinsky, R. Sunyaev (IKI), Z. Eker (TUG), U. Kiziloglu (METU), E. Gogus (Sabanci Uni.) report: We continued the observations of the field of the Swift-BAT GRB 120811C (Barlow et al. GCN 13622, Galeev et al., GCN 13626) with Russian-Turkish 1.5-m telescope (RTT150, Bakirlitepe, TUBITAK National Observatory, Turkey). The second epoch observations were started at 12 Aug, 18:38 UT, i.e. approximately 27.05 hours after the burst. We made a serie of 300s images in R using TFOSC instrument. The afterglow was clearly detected in the combined image. Using USNO-B1 stars as reference, we estimated the afterglow magnitude as m_R=22.3+-0.2. Fitting this point together with the data from RTT150 first epoch observations (Galeev et al., GCN 13626), gives the power law decay with index alpha = 1.04 +- 0.05, consistent with late time decay in X-rays, as it was observed with XRT (Evans et al., GCN 13630). When all RTT150 data are compared with the data of MASTER telescopes (Denisenko et al., GCN 13623, Ivanov et al., 13635), the power law break at few hours time scale is found, approximately consistent with the position of second break observed in X-rays with XRT (Evans et al., GCN 13630). However, better cross-calibration between RTT and MASTER photometry should be done for more reliable light curve modelling. The light curve and direct images of the field can be found at: http://hea.iki.rssi.ru/grb/120811c/indexeng.html