TITLE: GCN CIRCULAR NUMBER: 28502 SUBJECT: Swift GRB 200925B: Global MASTER-Net OT detection DATE: 20/09/26 00:05:19 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, N.Tyurina, E. Gorbovskoy, V.Kornilov, P.Balanutsa, A.Kuznetsov, F.Balakin, V.Vladimirov, D. Vlasenko, I.Gorbunov, D.Zimnukhov, V.Senik, T.Pogrosheva, D.Kuvshinov, D. Cheryasov (Lomonosov Moscow State University, SAI, Physics Department), O.A. Gres, N.M. Budnev, O.Ershova (Irkutsk State University, API), R. Podesta, C.Lopez, F. Podesta, C.Francile (Observatorio Astronomico Felix Aguilar OAFA), R. Rebolo, M. Serra (The Instituto de Astrofisica de Canarias), D. Buckley (South African Astronomical Observatory), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), V. Yurkov, A. Gabovich, Yu. Sergienko (Blagoveschensk Educational State University) MASTER-Tavrida robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, SAI Crimea astronomical station) was pointed to the Swift GRB200925A (trigger No 997453,16h 27m 24.24s , +78d 23m 38.4s, R=0.05) errorbox 14 sec after notice time and 58 sec after trigger time at 2020-09-25 21:51:35 UT, with upper limit up to 18.4 mag. The observations began at zenith distance = 53 deg. The sun altitude is -47.0 deg. On our 4-th (40s exposure) set , obtained 229 sec after tigger time at 2020-09-25 21:54:16 UT, we found an optical transient within Swift error-box (ra=246.851 dec=+78.394 r=0.05). T-Tmid Date Time Expt. Ra Dec Mag ---------|---------------------|-------|-----------------|-----------------|----- 229 2020-09-25 21:54:16 40 (246.7866 d, +78.3896 d) 17.9 The OT position in within enhanced XRT position (GCN 28500) and is coincident with OSN afterglow candidate position (A. de Ugarte Postigo et al, GCN 28501) The 5-sigma upper limit has been about 18.4 mag The message may be cited.