TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3853 SUBJECT: GRB050820A: RTT150 optical observations DATE: 05/08/22 12:40:21 GMT FROM: Irek Khamitov at TUG I. Bikmaev, A. Galeev, N. Sakhibullin (KSU/AST), R. Burenin, M. Pavlinsky, R. Sunyaev (IKI), I. Khamitov, Z. Aslan (TUG), U. Kiziloglu (METU), E. Gogus (Sabanci Uni.), report: We have observed the OT of GRB 050820A (GCN 3829) with the Russian-Turkish 1.5-m telescope (Bakirlitepe, TUBITAK National Observatory, Turkey) in two consequative nights . We made a series of exposures in Rc between UT 18:35 - 22:15, August 20, 2005, and in BRcIc between UT 22:46-01:15, August 21/22, 2005. Moonlight produced variable background in all images for both nights. The afterglow is clearly detected in all bands. Using Henden's stars (GCN 3845) present in the frames, we estimated the magnitudes as follow: t-t0 Band mag merr exptime (hours, midtime) (sec,total) 18.05 Rc 19.48 +/- 0.01 900 41.42 Rc 20.38 +/- 0.05 1800 40.93 B 21.00 +/- 0.06 1800 41.55 Ic 19.83 +/- 0.07 1800 On the basis of our Rc data, we estimeted an alpha index of power-law decay for the observed period as ~ -1.0. The JPG-images of BRcIc and color combined image are available at: http://www.tug.tubitak.gov.tr/~irekk/grb/grb050820a/grb050820A_b.JPG http://www.tug.tubitak.gov.tr/~irekk/grb/grb050820a/grb050820A_r.JPG http://www.tug.tubitak.gov.tr/~irekk/grb/grb050820a/grb050820A_i.JPG http://www.tug.tubitak.gov.tr/~irekk/grb/grb050820a/grb050820_bri.JPG This message may be cited.