TITLE: GCN CIRCULAR NUMBER: 41996 SUBJECT: GRB 250919A / EP250919a: Global MASTER Net before, during and after trigger optical observations DATE: 25/09/26 06:58:48 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs lipunov@xray.sai.msu.ru

R.Podesta, C.Francile, F. Podesta, E. Gonzalez (OAFA, San Juan Uni., Argentina);

N.Tiurina, E.Gorbovskoy, V.Lipunov, A.Kuznetsov, A.Sankovich, G.Antipov, P.Balanutsa, Yu. Tselik, A. Sosnovskiy, M.Shilova, A.Chasovnikov, V.Topolev, K.Zhirkov, Ya.Kechin, V.Senik, D.Vlasenko(Lomonosov MSU, SAI, Moscow),

D.Buckley, (SAAO, South Africa)

O.Gress, N.Budnev (ISU, Irkutsk),

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)

The MASTER-OAFA robotic wide-field camera, located in Argentina (OAFA observatory of San Juan National University [1], during a regular survey, obtained an image of the region of the sky containing the point with coordinates MASTER OT J195412.89-485016.4 (GCN 41893) and source (Li et al., GCN 41877) before, during and after the Fermi trigger moment (GBM GCN 41874, GCN 41876, GCN 41890; LAT GCN 41884, Ttrigger=00:28:52) of GRB 250919A .

Very bright optical counterpart MASTER OT J195412.89-485016.4 promptly detected at the agnitude ~ 9 m.

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).

The full 4 nights light curve is currently being processed.

[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#625

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