TITLE: GCN CIRCULAR NUMBER: 29850 SUBJECT: Swift GRB 210421A: Global MASTER-Net observations report DATE: 21/04/21 00:42:22 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 GRB 210421A ( M. J. Moss et al., GCN 29849) errorbox 13 sec after notice time and 47 sec after trigger time at 2021-04-21 00:28:17 UT, with upper limit up to 17.1 mag. The observations began at zenith distance = 54 deg. The sun altitude is -30.5 deg. The galactic latitude b = 5 deg., longitude l = 210 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1597889 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 52 | MASTER-OAFA | C | 10 | 16.4 | 82 | MASTER-OAFA | C | 10 | 16.4 | 116 | MASTER-OAFA | C | 20 | 16.8 | 160 | MASTER-OAFA | C | 30 | 17.0 | 215 | MASTER-OAFA | C | 40 | 17.1 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited.