TITLE: GCN CIRCULAR NUMBER: 23576 SUBJECT: GRB 181228A: Global MASTER Net OT detection DATE: 18/12/28 09:46:03 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, N.Tiurina, E. Gorbovskoy, V.Kornilov, P.Balanutsa, A.Kuznetsov, D. Vlasenko, D.Kobtsev (Lomonosov Moscow State University, SAI), R. Podesta, C.Lopez, F. Podesta, C.Francile (Observatorio Astronomico Felix Aguilar), H. Levato (Instituto de Ciencias Astronomicas, de la Tierra y del Espacio of San Juan National University, Argentina) R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres (The Instituto de Astrofisica de Canarias), D. Buckley (South African Astronomical Observatory), A. Tlatov, V.Senik, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), O. Gress, N.M. Budnev, S. Yazev, O. Chuvalaev (Irkutsk State University, API) V. Yurkov, A. Gabovich, Yu. Sergienko (Blagoveschensk Educational State University) MASTER-OAFA robotic telescope (Global MASTER Net http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the GRB181228A (Moss et al., GCN #23570) 26 sec after notice time and 47 sec after trigger time at 2018-12-28 02:54:05 UT. On our first (10s exposure) set we found 1 optical transient within SWIFT XRT error-box (Beardmore et al., GCN #23574) brighter than 17.5. T-Tmid Date Time Expt. Ra Dec Mag s UTC s ---------|-------------------|------|-----------------|-----------------|------- 52 2018-12-28 02:54:05 10 ( 03h 19m 14.80s , +13d 12' 02".80 ) 17.0 The 5-sigma upper limit has been about 17.5 mag . Coordinate error ~2". The galactic latitude b = -35 deg., longitude l = 170 deg. The observations made on zenit distance = 49 deg.The moon (64 % bright part) below the horizon (The altitude of the Moon is -14 deg. ). The sun altitude is -29.6 deg. The object can be observed till 2018-12-28 06:59:09 The message may be cited.