TITLE: GCN CIRCULAR NUMBER: 18511 SUBJECT: GRB 151027B: MASTER-NET early OT detection DATE: 15/10/28 13:24:39 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs 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 E. Gorbovskoy, V. Lipunov, N.Tyurina, V.Kornilov, P.Balanutsa, A.Kuznetsov, D.Kuvshinov, D. Vlasenko, E.Popova Lomonosov Moscow State University, Sternberg Astronomical Institute K.Ivanov, S.Yazev, N.M.Budnev, O.Gres, O.Chuvalaev, V.A.Poleshchuk Irkutsk State University A. Tlatov, A.V. Parhomenko, D. Dormidontov, V.Sennik Kislovodsk Solar Station of the Pulkovo Observatory V.Yurkov, Yu.Sergienko, D.Varda, E.Sinyakov Blagoveschensk Educational State University, Blagoveschensk V.Krushinski, I.Zalozhnih Ural Federal University, Kourovka Hugo Levato and Carlos Saffe Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE) Claudio Mallamaci, Carlos Lopez and Federico Podest Observatorio Astronomico Felix Aguilar (OAFA) Two MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in SAAO and IAC was pointed to the GRB151027B (Ukwatta et. al GCN 18499) 25 sec after notice time and 156 sec after trigger time at 2015-10-27 22:43:19 UT. On our first (30s exposure) set we haven`t found optical transient (Malesani et. al. GCN 18501, Xu et. al. GCN 18505, Wiseman et. al GCN 18507) within SWIFT error-box. The 2-sigma upper limit has been about 17.7 mag. The observations were significantly complicate the closest distance to the Moon (~ 35 d). The Moon background light is picked up the sky background on our cameras more than 10 times. Therefore, we could not get deep enough image. Nevertheless, we detect OT on a several coadd images in consequent time in SAAO. The first time we see OT 475 seconds after the trigger with magnitude ~ 18.9. OT had a slight brightening 1736 seconds after the trigger to 18.5 mag. The preliminary photometry available at table below. Table 1. GRB151027B MASTER-SAAO Early photometry. T-Ts T-Tm Exptime Mag.(P/+P\) Coadd 159 174 2 x 30 >17.7 1 201 221 2 x 40 >18.0 1 252 277 2 x 50 >18.3 1 313 343 2 x 60 >18.3 1 384 424 2 x 80 >18.4 1 159 311 2 x 260 >19.0 5 475 842 2 x 690 18.9 5 1235 1736 2 x 900 18.5 5 2264 3280 2 x 1800 18.9 10 3499 4515 2 x 1800 19.1 10 T - is trigger time, Ts - exposure start time, Tm - exposure middle time. We see OT in separate polarizations on s/n~5 confidence lewel. Table 1 shows the averaged data from two mutually perpendicular polarization. The message may be cited.