TITLE: GCN CIRCULAR NUMBER: 22797 SUBJECT: GRB 180618A: MASTER OT observation (a short GRB with extended emission) DATE: 18/06/19 12:00:58 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs N.Tyurina, V. Lipunov, E. Gorbovskoy, V.Kornilov, A.Kuznetsov, I. Gorbunov, D. Vlasenko, D.Zimnukhov, D.Kuvshinov, P.Balanutsa, V.Vladimirov Lomonosov Moscow State University, Sternberg Astronomical Institute, Moscow State University R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias 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, C. Saffe Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE) D. Buckley South African Astronomical Observatory O. Gres, N.M.Budnev , Yu.Ishmuhametova Irkutsk State University A. Gabovich, V. Yurkov, Yu. Sergienko Blagoveschensk Educational State University, Blagoveschensk MASTER-Kislovodsk robotic telescope (MASTER-Net: http://observ.pereplet.ru Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) , located in Kislovodsk Solar Station of Pulkovo observatory, was pointed to the Swift GRB180618A (LaPorte et al. GCN #22790; Sakamoto et al., GCN #22796) 25 sec after notice time and 41 sec after trigger time at 2018-06-18 00:43:53 UT. On our first (10s exposure) polarization set we haven`t found optical transient within SWIFT error-box (ra=169.933 dec=73.8478 r=0.05). The 5-sigma upper limit has been about 15.2 mag The observations made on zenit distance = 61 degrees, galaxy latitude b = 42 degree. The moon (24 % bright part) below the horizon (The altitude of the Moon is -31 degree ). Observations started at twilight and bad transparency. The sun altitude is -6.8 degree. MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in IAC was pointed to the GRB180618A 1734 sec after trigger time at 2018-06-18 01:12:07 UT. On our first (180s exposure) set we found ~19 mag optical transient within SWIFT UVOT error-box (LaPorte et al. GCN #22790; Guidorzi et al., GCN #22792;). The 5-sigma upper limit has been about 19.0 mag on single (180 s) and 20.5 on coadd (1800 s) exposures. ==================================================================== The observations made on zenit distance = 64 degrees, galaxy latitude b = 42 degree. The moon (24 % bright part) below the horizon (The altitude of the Moon is -15 degree ). The sun altitude is -38.3 degree. The object can be observed till sunrise at 2018-06-18 06:08:21 The message may be cited.