TITLE: GCN CIRCULAR NUMBER: 21179 SUBJECT: GRB 170531B: MASTER Net optical counterpart refind position and magnitude DATE: 17/06/01 11:21:45 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs N.Tyurina, V. Lipunov, V.Kornilov, E.Gorbovskoy, I.Gorbunov, D.Kuvshinov, A.V.Krylov, 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 (Lipunov et al., GCN #21173). Id Date time Exp Proc.type Limit* Tube OT Err UT sec 5sigma 1240587 2017-05-31 22:09:44 90 Alert (SWIFT) 18.3 EAST 18.3 0.5 1240589 2017-05-31 22:11:23 110 Alert (SWIFT) 18.4 EAST 18.4 0.5 1240591 2017-05-31 22:13:24 130 Alert (SWIFT) 18.5 EAST 18.4 0.5 1240593 2017-05-31 22:15:44 160 Alert (SWIFT) 18.5 EAST 18.5 0.5 1240596 2017-05-31 22:18:35 180 Alert (SWIFT) 18.5 EAST 18.5 0.5 1240652 2017-05-31 22:09:44 490 SumAlert(SWIFT) 19.1 EAST 18.3 0.2 1240597 2017-05-31 22:21:59 180 Alert (SWIFT) 18.3 EAST no - 1240599 2017-05-31 22:25:24 180 Alert (SWIFT) 18.5 EAST no - ................................................................. 1240629 2017-05-31 23:16:43 180 Alert (SWIFT) 18.5 EAST no - * - unfiltered magnitude with respect to USNO B1.0 stars reference stars m=0.2B + 0.8R We refind OT preliminary astrometric position (Lipunov et al., GCN #21173) which was done visually. Our refind position are: RA,DEC = 19 07 32.19 -16 25 06.9 3 sigma error = (+/-)2 arcsec (the source is marginaly visible). Our pixel is 2 arcsec. The offset from X-ray Swift position (Osborne at all) is 1.9 arcsec . The position of the Postigo et al. ( GCN 21177) is inside our error box (offset ~ 1.9 arcsec). The possible OT position Pozanenko et al.( GCN #21174) offset ~ 8.2 arc sec. The OT detection image is available. 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.