TITLE: GCN CIRCULAR NUMBER: 23381 SUBJECT: IceCube-181023A: MASTER Net optical alert and follow up observations DATE: 18/10/25 00:13:07 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V.Lipunov, V.Kornilov, N.Tiurina, E. Gorbovskoy, A.Kuznetsov, D.Zimnukhov, D.Kuvshinov, P.Balanutsa, V.Vladimirov, D.Vlasenko Lomonosov Moscow State University, Physics Department, SAI N.M.Budnev, O. Gres, Yu.Ishmuhametova Irkutsk State University (API ISU) D. Buckley, South African Astronomical Observatory (SAAO) R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias (IAC) A. Tlatov, V.Senik, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory R. Podesta, F. Podesta, C. Lopez, C.Francile Observatorio Astronomico Felix Aguilar (OAFA) H.Levato, Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE) A. Gabovich, V. Yurkov, Yu. Sergienko Blagoveschensk Educational State University (BSPU) MASTER Global Robotic Net started automatically IC181023A alert (from socket coordinates) observations 22 sec after notice time (MASTER-Tavrida) and continued 1 day later as IceCube-181023A (Blaufuss et al.GCN 23375,Pizzuto et al. GCN 23380) inspection at MASTER-Kislovodsk. MASTER-Tavrida robotic telescope (MASTER Net: Lipunov et.al. 2010, Advances in Astronomy, vol. 2010, 30L http://www.hindawi.com/journals/aa/2010/349171/), located in Lomonosov MSU, SAI Crimean astronomical station, was pointed automatically to the IC181023A error-box (taken from the socket) 22 sec after notice time and 66 sec after trigger time at 2018-10-23 16:38:38 UT. We observed this socket error-box with unfiltered mlim=18.3 and not found new OT, connected with EHE neutrino astrophysical source. The galactic latitude b = 7 deg., longitude l = 19 deg. The observations made on zenit distance = 68 deg. The sun altitude was -20.7 deg. The moon (99 % bright part) was 21 deg. above the horizon. The distance between moon and object was 109 deg. MASTER-Kislovodsk started IceCube-181023A (Blaufuss et al.GCN 23375 (RA:270.18 (-1.70,+2.00 deg); Dec: -8.57 (-1.30,+1.25 deg)) inspection with mlim=17.0 between clouds at 2018-10-24 15:31:39 UT The observations made on zenit distance = 63 deg. The moon (100 % bright part) was 11 deg. above the horizon. The distance between moon and object was 121 deg. The sun altitude was -15.2 deg. Observations and more accurate reduction will be continued. This message may be cited