TITLE: GCN CIRCULAR NUMBER: 19612 SUBJECT: GRB 160625B: Global MASTER-Net prompt optical emission detection DATE: 16/06/28 14:05:38 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs E. Gorbovskoy, V. Lipunov, N.Tyurina, V.Kornilov, P.Balanutsa, A.Kuznetsov, D.Kuvshinov, Lomonosov Moscow State University, Sternberg Astronomical Institute R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias D.Buckley, S. Potter, A.Kniazev, M.Kotze South African Astronomical Observatory O.Gres, K.Ivanov, S.Yazev, N.M.Budnev, O.Chuvalaev, V.A.Poleshchuk Irkutsk State University A. Tlatov, A.V. Parhomenko, D. Dormidontov, V.Senik Kislovodsk Solar Station of the Pulkovo Observatory V.Yurkov, Yu.Sergienko, A.Gabovich, E.Sinyakov Blagoveschensk Educational State University, Blagoveschensk Ricardo Podesta, Carlos Lopez and Federico Podest Observatorio Astronomico Felix Aguilar (OAFA) Hugo Levato and Carlos Saffe Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE) MASTER-IAC robotic telescope (MASTER-Net: http://observ.pereplet.ru) was pointed to the GRB 160625B (Burns et al., GCN19581) 31 sec after GBM notice time and 57 sec after GBM trigger (131 sec before LAT Trigger, Dirirsa et al., GCN 19580) at 2016-06-25 22:41:13UT . Starting from this time we continiously observed Swift X-ray error box (Melandri et al., GCN 19585) by MASTER-IAC wide field camera with 5 sec unfiltered exposition. We have several thousands frames. The OT appears at 22 43 30 (+- 2.5 sec) UT and rised to maximum ~8 mag after ~5 sec at RATIR position (Troja et al., GCN 19588). The prompt optical emission strongly correlated with Konus Wind gamma ray light curve ( Svinkin et al., GCN 19604 ). MASTER-IAC robotic telescope started OT error box observation 43 sec after LAT notice time and 95 sec after trigger time at 2016-06-25 22:44:59 in two polarisations. The OT was about ~ 9.7 mag at this moment ( 0.2B + 0.8R with respect to USNOB1.0 stars) The delay with publication connected with installation new MASTER telescope at Argentina (http://master.sai.msu.ru/masternet/). The message may be cited.