TITLE: GCN CIRCULAR NUMBER: 20502 SUBJECT: GRB 170121B/Fermi trigger 506702667 MASTER-Amur OT detection DATE: 17/01/22 22:22:36 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, E. Gorbovskoy, E.Popova, N.Tyurina, V.Kornilov, D.Kuvshinov, N.Tyurina, A.V.Krylov, I.Gorbunov, P.Balanutsa, A.Kuznetsov, V.V.Chazov, D.Vlasenko Lomonosov Moscow State University, Sternberg Astronomical Institut of MSU V.Yurkov, Yu.Sergienko Blagoveschensk Educational State University, Blagoveschensk K.Ivanov, S.Yazev, N.M.Budnev, O.Gres, O.Chuvalaev, V.A.Poleshchuk, O. Ershova Irkutsk State University A. Tlatov, V.Senik, A.V. Parhomenko, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory 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 D.Buckley, S. Potter, A.Kniazev, M.Kotze South African Astronomical Observatory R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in Blagoveschensk was pointed to the Fermi trigger Number 506702667 72 sec after notice time and 171 sec after trigger time at 2017-01-21 14:47:14 UT. MASTER-Amur auto-detection system discovered OT source at (RA, Dec) = 04h 50m 17.73s -09d 16m 02.0s . The OT unfiltered magnitude is 14.9m (limit 16.0 m). The OT is seen in 5 images. OT detected before maximum and shown decay on last images 4 minutes after detection. We have reference image without OT on 2017-01-21.61560 UT with unfiltered magnitude limit 20.5 m. Spectral observations are required. The message may be cited. [GCN OPS NOTE(23jan17): Per author's request, the Subject line was corrected from "170121A" to "170121B".]