TITLE: GCN CIRCULAR NUMBER: 41157 SUBJECT: GRB 250725A: MASTER-OAFA optical detection of very fast afterglow DATE: 25/07/25 08:51:52 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs lipunov@xray.sai.msu.ru

P. Rudominskaya (Lomonosov, MSU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (OAFA), V.Lipunov (Lomonosov MSU), E.Gorbovskoy, A.Kuznetsov, G.Antipov, P.Balanutsa, A.Chasovnikov, N.Tiurina, V.Topolev, K.Zhirkov, Ya.Kechin, V.Senik, D.Vlasenko (Lomonosov MSU), O.Gress, N.Budnev (ISU), A.Sosnovskij (Crao RAS), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory

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) was pointed to the GRB250725.08 26 sec after notice time and 45 sec after trigger time at 2025-07-25 01:52:22 UT. On our first (10s exposure) set we found 1 optical transient within Swift error-box (ra=35.3833 dec=-82.7836 r=0.05) brighter than 17.7.

T-Tmid Date Time Expt. Ra Dec Mag ---------|---------------------|-------|-----------------|-----------------|------- 50 2025-07-25 01:52:22 10 (02h 21m 55.67s , -82d 47m 28.1s) 15.8 The 5-sigma upper limit has been about 17.7mag

The automatical light curve is the very fast fall F ~ t^-2 This very similar to short GRB behavior. The large optical telescope observation is very welcome! The message may be cited.

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