TITLE: GCN CIRCULAR NUMBER: 30118 SUBJECT: Swift GRB210603.41: Global MASTER-Net observations report DATE: 21/06/03 09:58:39 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, E. Gorbovskoy, V.Kornilov, N.Tyurina, 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), 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), O.A. Gres, N.M. Budnev, O.Ershova (Irkutsk State University, API), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), V. Yurkov, A. Gabovich, Yu. Sergienko (Blagoveschensk Educational State University) MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB210603.41 (trigger No 1053220,15h 55m 16.56s , -54d 02m 45.6s, R=0.05) errorbox 40 sec after trigger time at 2021-06-03 09:46:26 UT, with upper limit up to 16.7 mag. The observations began at zenith distance = 66 deg. The sun altitude is -20.8 deg. The galactic latitude b = -1 deg., longitude l = 328 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1627464 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 45 | MASTER-OAFA | C | 10 | 15.9 | 75 | MASTER-OAFA | C | 10 | 15.8 | 109 | MASTER-OAFA | C | 20 | 16.2 | 154 | MASTER-OAFA | C | 30 | 16.6 | 208 | MASTER-OAFA | C | 40 | 16.7 | 273 | MASTER-OAFA | C | 50 | 16.7 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited.