TITLE: GCN CIRCULAR NUMBER: 30173 SUBJECT: Swift GRB210610.83: Global MASTER-Net OT detection DATE: 21/06/10 20:39:05 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, E. Gorbovskoy, V.Kornilov, D.Kuvshinov, N.Tyurina, P.Balanutsa, A.Kuznetsov, D. Vlasenko, F. Balakin Lomonosov Moscow State University, Sternberg Astronomical Institute, Moscow State University A. Tlatov, V.Senik, A.V. Parhomenko, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory K. Ivanov, O. Gres, N.M. Budnev, S. Yazev, O. Chuvalaev, V. Poleshchuk Irkutsk State University V. Yurkov, A. Gabovich, Yu. Sergienko Blagoveschensk Educational State University, Blagoveschensk R. Podesta, Carlos Lopez and F. Podesta Observatorio Astronomico Felix Aguilar (OAFA) R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias D. Buckley, S. Potter, A. Kniazev, M. Kotze South African Astronomical Observatory MASTER-Kislovodsk robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the GRB210610.83 145 sec after trigger time at 2021-06-10 19:53:52 UT. On our 3-th (130s exposure) set , obtained 656 sec after tigger time at 2021-06-10 20:02:24 UT, we found 1 optical transient within Swift error-box (ra=243.938 dec=14.3906 r=0.05) brighter than 17.7. T-Tmid Date Time Expt. Ra Dec Mag ---------|---------------------|-------|-----------------|-----------------|------- 721 2021-06-10 20:02:24 130 (16h 15m 40.49s , +14d 23m 56.4s) 15.2 The 5-sigma upper limit has been about 17.7mag The message may be cited.