TITLE: GCN CIRCULAR NUMBER: 45677 SUBJECT: Swift GRB 260921A: Global MASTER-Net observations report DATE: 26/09/21 09:59:36 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs M. Segura, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA) V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya..Kechin, V.Senik, A.Chasovnikov, K.Labsina (Lomonosov MSU) D.Buckley (SAAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity) A.Sosnovskij (Lomonosov MSU) A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory) L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) R.Rebolo (The Instituto de Astrofisica de Canarias) O.Gress, N.Budnev(ISU) MASTER-OAFA robotic telescope [1] located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 260921A ( K. L. Page et al., GCN 45676) errorbox 58 sec after notice time and 80 sec after trigger time at 2026-09-21 09:30:32 UT, with upper limit up to 15.5 mag. Observations started at twilight. The observations began at zenith distance = 76 deg. The sun altitude is -12.8 deg. The galactic latitude b = 39 deg., longitude l = 233 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3508447 We obtain a following upper limits. Tmid-T0 | Site |Filt.| Expt. | Limit| Comment _________|_____________________|_____|_______|______|________ 91 | MASTER-OAFA | C | 20 | 15.2 | 117 | MASTER-OAFA | C | 20 | 15.3 | 148 | MASTER-OAFA | C | 30 | 15.3 | 183 | MASTER-OAFA | C | 30 | 15.2 | 224 | MASTER-OAFA | C | 40 | 15.5 | 275 | MASTER-OAFA | C | 50 | 15.1 | 337 | MASTER-OAFA | C | 60 | 15.3 | 407 | MASTER-OAFA | C | 70 | 14.9 | 494 | MASTER-OAFA | C | 90 | 14.6 | 600 | MASTER-OAFA | C | 110 | 15.3 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. [1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http : // www.pereplet.ru/lipunov/625.html