TITLE: GCN CIRCULAR NUMBER: 30756 SUBJECT: Fermi GRB 210903B: Global MASTER-Net observations report DATE: 21/09/03 17:00:29 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-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) started inspect of the Fermi GRB 210903B ( Fermi GBM team, GCN 30754) errorbox 6219 sec after trigger time at 2021-09-03 02:14:25 UT, with upper limit up to 18.5 mag. The observations began at zenith distance = 67 deg. The sun altitude is -49.9 deg. The galactic latitude b = 17 deg., longitude l = 42 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=1708736 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 6309 | 2021-09-03 02:14:25 | MASTER-OAFA | (18h 01m 44.08s , +25d 58m 19.7s) | C | 180 | 18.5 | 6910 | 2021-09-03 02:24:26 | MASTER-OAFA | (18h 01m 44.86s , +25d 57m 20.9s) | C | 180 | 18.5 | 7310 | 2021-09-03 02:31:06 | MASTER-OAFA | (18h 01m 47.01s , +25d 59m 20.6s) | C | 180 | 18.5 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited.