TITLE: GCN CIRCULAR NUMBER: 41893 SUBJECT: GRB 250919A / EP250919a: MASTER bright afterglow detection 24h after GRB time DATE: 25/09/20 00:38:17 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru> V.M.Lipunov (Lomonosov MSU, Moscow), R.Podesta, C.Francile, F. Podesta, E. Gonzalez (OAFA), A.Kuznetsov, A.Sankovich, G.Antipov, P.Balanutsa, E.Gorbovskoy, N.Tiurina, M.Shilova, A.Chasovnikov, V.Topolev, K.Zhirkov, Ya.Kechin, V.Senik, D.Vlasenko(Lomonosov MSU), D.Buckley, (SAAO, South Africa) O.Gress, N.Budnev (ISU, Irkutsk), A.Sosnovskij (Crao RAS), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), V.M.Pillet, R.Rebolo Lopez (The Instituto de Astrofisica de Canarias, Spain), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez,J.Martinez,A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysic Observatory, Mexico) MASTER Global robotic Net (Lipunov et al. 2010) started Fermi GRB 250919A (GBM GCN 41874, GCN 41876, GCN 41890; LAT GCN 41884, Ttrigger=00:28:52) observation in alert mode (Lipunov et al. GCN 41875) at MASTER-OAFA and continued as EP GRB 250919 (GCN 41879) since its notice time (Lipunov et al. 41880) Bright optical counterpart MASTER OT J195412.89-485016.4 was detected with peak structure light curve. 24h after GRB detection (GCN 41874) this optical counterpart has unfiltered m_OT~18.3 MASTER observations will be continued This transient was first publishied as EP250919a optical counterpart by Las Cumbres observatory (GCN 41877, t_detection=03:02:52.800 UTC) and as Fermi GRB 250919A/EP250919a optical counterpart by Gemini-South (LEvan et al. GCN 41883) with reshift detection z = 1.145 X-ray counterpart was also detected by Swift (Osborne et al. GCN 41885) with OT afterglow detection by UVOT (Oates et al. GCN 41886), . NuSTAR (GCN 41888) and Svom (41882). View this GCN Circular online at https://gcn.nasa.gov/circulars/41893. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...