TITLE: GCN CIRCULAR NUMBER: 39990 SUBJECT: Fermi GRB 250331D: Global MASTER-Net observations report DATE: 25/04/01 00:03:05 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs lipunov@xray.sai.msu.ru

V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, , D.Vlasenko, I.Panchenko, A.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, 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. 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-Kislovodsk robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) started inspect of the Fermi GRB 250331D ( Fermi GBM team, GCN 39961) errorbox 62486 sec after notice time and 62523 sec after trigger time at 2025-03-31 23:11:20 UT, with upper limit up to 17.9 mag. The observations began at zenith distance = 71 deg. The sun altitude is -35.8 deg.

The galactic latitude b = -7 deg., longitude l = 128 deg.

Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2829033

We obtain a following upper limits.

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

62554 | 2025-03-31 23:11:20 | MASTER-Kislovodsk | (02h 54m 34.96s , +65d 33m 33.8s) | C | 60 | 17.7 | 63431 | 2025-03-31 23:25:58 | MASTER-Kislovodsk | (02h 54m 36.68s , +65d 33m 23.8s) | C | 60 | 17.9 | 64508 | 2025-03-31 23:43:54 | MASTER-Kislovodsk | (02h 53m 45.13s , +65d 44m 21.5s) | C | 60 | 17.7 | Filter C is a clear (unfiltred) band.

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

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