TITLE: GCN CIRCULAR NUMBER: 29606 SUBJECT: Swift GRB 210306A: Global MASTER-Net observations report DATE: 21/03/06 20:02:39 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-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 GRB 210306A ( V. D'Elia et al., GCN 29597) errorbox 57322 sec after notice time and 57335 sec after trigger time at 2021-03-06 19:49:32 UT, with upper limit up to 20.0 mag. The observations began at zenith distance = 17 deg. The sun altitude is -42.8 deg. The galactic latitude b = 37 deg., longitude l = 157 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1562037 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 30222 | MASTER- | C | 60 | 18.5 | 31162 | MASTER- | C | 60 | 18.4 | 32113 | MASTER- | C | 60 | 18.9 | 57426 | MASTER-Tavrida | C | 180 | 20.0 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited.