TITLE: GCN CIRCULAR NUMBER: 21391 SUBJECT: GRB 170728B: MASTER-IAC first minute short GRB early OT detection DATE: 17/07/30 13:25:56 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, E. Gorbovskoy, V.Kornilov, N.Tyurina, D.Kuvshinov, A.V.Krylov, I.Gorbunov, P.Balanutsa, A.Kuznetsov, V.V.Chazov, D. Vlasenko Lomonosov Moscow State University, Sternberg Astronomical Institut of MSU R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias R.Podesta, C.Lopez, F. Podesta Observatorio Astronomico Felix Aguilar (OAFA), National University of San Juan, Argentina H. Levato, C. Saffe Instituto de Ciencias Astronomicas,de la Tierra y del Espacio (ICATE), San Juan, Argentina O.Gres, K.Ivanov, S.Yazev, N.M.Budnev, O.Gres, O.Chuvalaev, V.A.Poleshchuk, Irkutsk State University V.Yurkov, Yu.Sergienko Blagoveschensk Educational State University, Blagoveschensk A. Tlatov, V.Senik, A.V. Parhomenko, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory D.Buckley, S. Potter, A.Kniazev, M.Kotze South African Astronomical Observatory MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in IAC was pointed to the GRB170728B (Cenko et al., GCN Circ. #21371) 28 sec after notice time and 39 sec after trigger time at 2017-07-28 23:04:01 UT. On our first (10s exposure) set we found optical transient within SWIFT XRT error-box ( D'Elia et al., GCN #21376; ) in two polarizations! The 5-sigma upper limit has been about 16.9 mag We 100% found faiding OT on first 10 minutes images with m ~ 16.5 up to 18 mag. RA, DEC = 15h 51m 55s.5 , +70d 7m 19s.3 Error = +- 1.4 arcsec The faiding OT may be coincide with D'Avanzo et al. position (GCN #21372) and Heintz et al., (GCN #21374) and Izzo et al., (GCN #21381) and The observations made on zenit distance = 47 degrees, galaxy latitude b = 40 degree. The moon (34 % bright part) is 8 degrees above the horizon. The distance between moon and object is 78 The sun altitude is -33.7 degree. The error box can be observed till sunrise at 2017-07-29 06:25:17. The message may be cited.