TITLE: GCN CIRCULAR NUMBER: 17721 SUBJECT: GRB 150413A: RTT150 optical observations DATE: 15/04/18 09:36:15 GMT FROM: Rodion Burenin at IKI, Moscow I. Bikmaev, E. Irtuganov, N. Sakhibullin (KFU/AST), R. Burenin, M. Pavlinsky, R. Sunyaev (IKI), I. Khamitov, H. Kirbiyik (TUG), U. Kiziloglu (METU), E. Gogus (Sabanci Uni.) report: The field of the optical afterglow of GRB 150413A (Markwardt et al., GCN 17688, Ivanov et al., GCN 17689) was observed with Russian-Turkish 1.5-m telescope (RTT150, Bakirlitepe, TUBITAK National Observatory, Turkey) using the TFOSC instrument. We obtained series of Rc exposures in April 13/14 (50 frames x 300 sec, UT = 20:27 - 02:30 ) and in April 14/15 (4 frames x 600 sec, UT = 22:43 - 00:28, in partially cloudy conditions). The optical transient is clearly detected in each images. Using USNO-B1 1618-0098185 star at RA,DEC = 12:41:09.18 +71:50:32.4 (J2000) with R2 = 15.59 mag as a reference we estimated the R-magnitudes of the afterglow: T-T0, Mag. +/- Mag.err (days) 0.2932 17.39 +/- 0.03 0.5200 18.70 +/- 0.05 1.4318 21.4 +/- 0.2 RTT-150 light curve in April 13/14, 2015, can be found at: http://hea.iki.rssi.ru/grb/150413a/indexeng.html The break in the light curve at (T-T0) = 0.33 days is evident (break is seen also in V-band data of Bassano Bresciano Observatory, Quadri et al, GCN 17704). Before the break at (T-T0) = 0.33 days, the power law decay index is =~0.9, and it is close to 3.0 after that. Note, that decay index is close to 2.0 after (T-T0) = 0.7 days. Therefore, there are multiple breaks in the OT light curve.