TITLE: GCN CIRCULAR NUMBER: 21091 SUBJECT: GRB 170510A: Global MASTER Net possible OT observations DATE: 17/05/12 14:36:50 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, E. Gorbovskoy, V.Kornilov, N.Tyurina, D.Kuvshinov, A.V.Krylov, I.Gorbunov, P.Balanutsa, A.Kuznetsov, V.V.Chazov, D. Vlasenko Lomonosov Moscow State University, Sternberg Astronomical Institut D.Buckley, S. Potter, A.Kniazev, M.Kotze South African Astronomical Observatory R.Podesta, C.Lopez, F. Podesta Observatorio Astronomico Felix Aguilar (OAFA) , National University of San Juan, Argentina H. Levato, C. Saffe Instituto de Ciencias Astronomicas,de la Tierra y del Espacio (ICATE), San Juan, Argentina K.Ivanov, S.Yazev, N.M.Budnev, O.Gres, O.Chuvalaev, V.A.Poleshchuk, O. Ershova Irkutsk State University V.Yurkov, Yu.Sergienko Blagoveschensk Educational State University, Blagoveschensk A. Tlatov, V.Senik, A.V. Parhomenko, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias MASTER-SAAO robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in SAAO was starting survey on the LAT GRB170510.22 error-box (Bissaldi et al., GCN #21080) 22480 sec after notice time and 41530 sec after trigger time at 2017-05-10 16:44:36 UT. The 5-sigma upper limit on our first (180s exposure) set is about 18.9 mag We marginaly see optical candidate (Cucchiara et al. GCN 21087) at XRT (Evans et al. GCN #21082, Melandri et al. GCN #21086) position with unfiltered mag ~ 19 m . Ra, Dec = 10 40 36.76 -39 12 43.8 (+/-1 arcsec) We did not see object 2.5 hours late with limit 19.5 . So X-ray Source 1 (Melandri et al. GCN #21086) is possible OT. The message may be cited.