TITLE: GCN CIRCULAR NUMBER: 30747 SUBJECT: Swift GRB 210901A: Global MASTER-Net observations report DATE: 21/09/01 17:23:41 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru> 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-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB 210901A ( S. Laha et al., GCN 30746) errorbox 6860 sec after notice time and 6950 sec after trigger time at 2021-09-01 17:00:26 UT, with upper limit up to 18.6 mag. Observations started at twilight. The observations began at zenith distance = 49 deg. The sun altitude is -8.9 deg. The galactic latitude b = -30 deg., longitude l = 318 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1706835 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 7040 | MASTER-SAAO | C | 180 | 17.4 | 7220 | MASTER-SAAO | C | 540 | 17.8 | Coadd 7240 | MASTER-SAAO | C | 180 | 17.9 | 7439 | MASTER-SAAO | C | 180 | 18.3 | 7639 | MASTER-SAAO | C | 180 | 18.6 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited.