TITLE: GCN CIRCULAR NUMBER: 21830 SUBJECT: GRB 170906A: MASTER-OAFA (Argentina) early automatic OT detection DATE: 17/09/06 09:01:51 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, E. Gorbovskoy, N.Tyurina, V.Kornilov, D.Kuvshinov, P.Balanutsa, A.Kuznetsov, V.V.Chazov, D. Vlasenko, I.Gorbunov, A.V.Krylov Lomonosov Moscow State University, Sternberg Astronomical Institute, Moscow State University R. Podesta, Carlos Lopez and F. Podesta Observatorio Astronomico Felix Aguilar (OAFA) Hugo Levato and Carlos Saffe Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE) R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias K. Ivanov, O. Gres, N.M. Budnev, S. Yazev, O. Chuvalaev, V. Poleshchuk Irkutsk State University A. Tlatov, V.Senik, A.V. Parhomenko, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory V. Yurkov, A. Gabovich, Yu. Sergienko Blagoveschensk Educational State University, Blagoveschensk D. Buckley, S. Potter, A. Kniazev, M. Kotze South African Astronomical Observatory MASTER-OAFA robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in Argentina was pointed to the Swift GRB170906A (Siegel rt al. GCN21821) 44 sec after trigger time at 2017-09-06 00:43:56 UT with 10 sec exposition. Robot found 1 optical transient within SWIFT error-box (ra=13:35:49.19 dec=-47:06:01.9 r=0.61(90%)) at RA, DEC = 13h 35m 49.3s , -47d 6m 02.7s We see OT brightenning from unfiltered mag 15.6 up to 14.4 at 2017-09-06 00:50:54 UT (midle time). The 5-sigma upper limit on the single images has been about 16.8mag This OT was also detected by UVOT (Siegel et al GCN 21821) and REM (Covino et al. GCN 21825). The observations made on zenit distance = 64 degrees, galaxy latitude b = 15 degree. The moon (100 % bright part) is 35 degrees above the horizon. The distance between moon and object is 113 The sun altitude is -31.7 degree. The object can be observed till 2017-09-06 04:04:24. The message may be cited.