TITLE: GCN CIRCULAR NUMBER: 21173 SUBJECT: GRB 170531B: MASTER Net OT detection DATE: 17/05/31 23:19:01 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, V.Kornilov, N.Tyurina, E.Gorbovskoy, 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 A. Tlatov, V.Senik, A.V. Parhomenko, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory N.M.Budnev, O.Gres, O. Ershova Irkutsk State University A.Gabovich, V.Yurkov, Yu.Sergienko Blagoveschensk Educational State University, Blagoveschensk 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 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 MASTER-Kislovodsk robotic telescope (2x4 square degrees, MASTER-Net: http://observ.pereplet.ru) was pointed to the GRB170531B (Evans et al., GCN #21171) 12 sec after notice time and 452 sec after trigger time on 2017-05-31 22:09:44 UT. There is marginally seen optical transient at the position 19h 07m 32.03s -16d 25' 10.4" with m_OT~19.0+-0.5 The 5-sigma upper limit is about 18.5mag The message may be cited. ==================================================================== The observations made on zenit distance = 64 degrees, galaxy latitude b = -11 degree. The moon (44 % bright part) below the horizon (The Moon altitude is -6 degree). The sun altitude is -22.7 degree. The object can be observed till sunrise at 2017-06-01 01:30:50 . The message may be cited.