TITLE: GCN CIRCULAR NUMBER: 18427 SUBJECT: GRB 151021A: MASTER-NET bright OT detection DATE: 15/10/21 06:46:01 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, E. Gorbovskoy, N.Tyurina, V.Kornilov, P.Balanutsa, A.Kuznetsov, D.Kuvshinov, D. Vlasenko Lomonosov Moscow State University, Sternberg Astronomical Institute D.Buckley, S. Potter, A.Kniazev, M.Kotze South African Astronomical Observatory K.Ivanov, S.Yazev, N.M.Budnev, O.Gres, O.Chuvalaev, V.A.Poleshchuk Irkutsk State University A. Tlatov, A.V. Parhomenko, D. Dormidontov, V.Sennik Kislovodsk Solar Station of the Pulkovo Observatory V.Yurkov, Yu.Sergienko, D.Varda, E.Sinyakov Blagoveschensk Educational State University, Blagoveschensk V.Krushinski, I.Zalozhnih, A. Popov Ural Federal University, Kourovka Hugo Levato and Carlos Saffe Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE) Claudio Mallamaci, Carlos Lopez and Federico Podest Observatorio Astronomico Felix Aguilar (OAFA) MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in SAAO was pointed to the GRB151021A 25 sec after notice time and 50 sec after trigger time at 2015-10-21 01:30:06 UT in two polarizations. On our first (10s exposure) set we found 1 optical transient within SWIFT error-box (Melandri et. al GCN 18425) brighter then 16. date time Tmid-T0 Coord2000 Mag 2015-10-21 01:30:06 55.0 (22h 30m 34.42s , -33d 11m 50.3s) 14.86 The 5-sigma upper limit has been about 16.0 mag The message may be cited. [GCN OPS NOTE(21oct15) Per author's request, the GBB name in the Subject-line was changed from 0921A to 1021A.]