TITLE: GCN CIRCULAR NUMBER: 30760 SUBJECT: Swift GRB210903.58: Global MASTER-Net observations report DATE: 21/09/04 13:51:32 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 GRB210903.58 (trigger No 1071762,09h 01m 45.05s , -40d 19m 14.5s, R=0.0016) errorbox 42372 sec after notice time and 42385 sec after trigger time at 2021-09-04 01:34:44 UT, with upper limit up to 17.5 mag. The observations began at zenith distance = 82 deg. The sun altitude is -42.1 deg. MASTER-OAFA robotic telescope located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB210903.58 errorbox 64806 sec after notice time and 64820 sec after trigger time at 2021-09-04 07:48:39 UT, with upper limit up to 17.9 mag. The observations began at zenith distance = 79 deg. The sun altitude is -38.7 deg. The galactic latitude b = 4 deg., longitude l = 263 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1708691 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 42476 | MASTER-SAAO | C | 180 | 16.8 | 43295 | MASTER-SAAO | C | 180 | 17.5 | 44439 | MASTER-SAAO | C | 180 | 17.4 | 45952 | MASTER-SAAO | C | 180 | 16.4 | 47353 | MASTER-SAAO | C | 180 | 15.8 | 48056 | MASTER-SAAO | C | 180 | 16.2 | 50216 | MASTER-SAAO | C | 180 | 15.0 | 64910 | MASTER-OAFA | C | 180 | 17.1 | 65324 | MASTER-OAFA | C | 180 | 17.3 | 65747 | MASTER-OAFA | C | 180 | 17.4 | 66756 | MASTER-OAFA | C | 180 | 17.6 | 67365 | MASTER-OAFA | C | 180 | 17.8 | 67779 | MASTER-OAFA | C | 180 | 17.9 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited.