TITLE: GCN CIRCULAR NUMBER: 40344 SUBJECT: Fermi GRB 250504A: Global MASTER-Net observations report DATE: 25/05/05 00:01:03 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs lipunov@xray.sai.msu.ru

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, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), 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-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) started inspect of the Fermi GRB 250504A ( Fermi GBM team, GCN 40342) errorbox 1 sec after notice time and 43 sec after trigger time at 2025-05-04 23:26:38 UT, with upper limit up to 17.3 mag. The observations began at zenith distance = 29 deg. The sun altitude is -70.3 deg.

The galactic latitude b = -6 deg., longitude l = 350 deg.

Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2860892

We obtain a following upper limits.

Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________

49 | 2025-05-04 23:26:38 | MASTER-SAAO | (17h 56m 41.66s , -40d 35m 44.3s) | C | 10 | 16.7 | 77 | 2025-05-04 23:27:07 | MASTER-SAAO | (17h 56m 41.67s , -40d 35m 44.8s) | C | 10 | 16.7 | 111 | 2025-05-04 23:27:36 | MASTER-SAAO | (17h 56m 41.71s , -40d 35m 45.1s) | C | 20 | 17.0 | 151 | 2025-05-04 23:28:15 | MASTER-SAAO | (17h 56m 41.72s , -40d 35m 45.3s) | C | 20 | 17.0 | 249 | 2025-05-04 23:29:43 | MASTER-SAAO | (17h 56m 41.65s , -40d 35m 44.1s) | C | 40 | 17.2 | 313 | 2025-05-04 23:30:42 | MASTER-SAAO | (17h 56m 41.63s , -40d 35m 43.6s) | C | 50 | 17.2 | 387 | 2025-05-04 23:31:51 | MASTER-SAAO | (17h 56m 41.55s , -40d 35m 43.1s) | C | 60 | 17.3 | 466 | 2025-05-04 23:33:10 | MASTER-SAAO | (17h 56m 41.50s , -40d 35m 42.3s) | C | 60 | 17.3 | 545 | 2025-05-04 23:34:29 | MASTER-SAAO | (17h 56m 41.40s , -40d 35m 41.4s) | C | 60 | 17.3 | 624 | 2025-05-04 23:35:48 | MASTER-SAAO | (17h 56m 41.29s , -40d 35m 40.5s) | C | 60 | 17.3 | 703 | 2025-05-04 23:37:08 | MASTER-SAAO | (17h 56m 41.19s , -40d 35m 39.7s) | C | 60 | 17.3 | Filter C is a clear (unfiltred) band.

The observation and reduction will continue. The message may be cited.

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