TITLE: GCN CIRCULAR NUMBER: 22055 SUBJECT: GRB 171027A: MASTER-OAFA (Argentina) early prompt optical observations DATE: 17/10/27 10:28:03 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, E. Gorbovskoy, N.Tyurina, V.Kornilov, D.Kuvshinov, P.Balanutsa, A.Kuznetsov, V.V.Chazov, D. Vlasenko, I.Gorbunov, A.V.Krylov Lomonosov Moscow State University, Sternberg Astronomical Institute, Moscow State University R. Podesta, Carlos Lopez and F. Podesta Observatorio Astronomico Felix Aguilar (OAFA) Hugo Levato and Carlos Saffe Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE) R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias K. Ivanov, O. Gres, N.M. Budnev, S. Yazev, O. Chuvalaev, V. Poleshchuk Irkutsk State University A. Tlatov, V.Senik, A.V. Parhomenko, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory V. Yurkov, A. Gabovich, Yu. Sergienko Blagoveschensk Educational State University, Blagoveschensk D. Buckley, S. Potter, A. Kniazev, M. Kotze South African Astronomical Observatory MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in OAFA was pointed to the GRB171027A ( D'Avanzo et al., GCN Circ 22053 ) 26 sec after notice time and 71 sec after trigger time at 2017-10-27 08:24:17 UT. On our first (10s exposure) set we haven`t found optical transient within SWIFT error-box (ra=61.6708 dec=-2.60556 r=0.05). The 5-sigma upper limit has been about 17.5 mag . This and next 2 images are prompt optical limit because GRB duration is ~ 120 sec (D'Avanzo et al., GCN Circ 22053 ) The OT limit are: ID UT Exp Type Limit 103456 08:24:17 10 Alert (SWIFT) 17.5 103463 08:24:17 70 SumAlert (SWIFT) 19.0 103473 08:24:17 550 SumAlert (SWIFT) 20.0 103457 08:25:09 20 Alert (SWIFT) 18.1 103459 08:26:10 40 Alert (SWIFT) 18.6 103467 08:27:34 210 SumAlert (SWIFT) 19.5 103460 08:27:34 50 Alert (SWIFT) 18.6 103461 08:29:07 70 Alert (SWIFT) 18.9 103462 08:31:00 90 Alert (SWIFT) 19.0 103472 08:33:15 450 SumAlert (SWIFT) 19.9 103464 08:33:15 120 Alert (SWIFT) 19.2 103466 08:36:00 150 Alert (SWIFT) 19.2 103468 08:39:15 180 Alert (SWIFT) 19.3 103476 08:43:00 540 SumAlert (SWIFT) 19.5 103470 08:43:00 180 Alert (SWIFT) 19.0 103471 08:46:43 180 Alert (SWIFT) 18.8 103474 08:50:25 180 Alert (SWIFT) 18.4 103477 08:54:08 180 Alert (SWIFT) 18.1 103478 08:57:53 180 Alert (SWIFT) 18.1 The observations made on zenit distance = 41 degrees, galaxy latitude b = -37 degree. The moon (45 % bright part) below the horizon (The altitude of the Moon is -33 degree ). Observations started at twilight. The sun altitude is -16.5 degree. The object can be observed till sunrise at 2017-10-27 09:44:50 The message may be cited.