TITLE: GCN CIRCULAR NUMBER: 41534 SUBJECT: LIGO/Virgo/KAGRA S250818k: MASTER predicovery limits of the AT2025ulz/ZTF25abjmnps KN candidate DATE: 25/08/25 08:21:58 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, N.Tiurina, D.Vlasenko, P.Balanutsa, I.Panchenko, A.Sankovich, A.Chasovnikov K.Zhirkov, G.Antipov, I.Gorbunov, A.Sosnovskij, Yu.Tselik, Ya.Kechin, V.Senik, V.Topolev (Lomonosov MSU), O.Gress, N.Budnev, O.Ershova (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), D.Buckley (SAAO), 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) Global MASTER robotic net [1] observed the field with AT2025ulz/ZTF25abjmnps, reported by Stein et al. (GCN 41414) and possibly associated with the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440) before and after trigger time. At the location of AT2025ulz the twin MASTER600-Tunka telescope (Cmos QHY 6060, FOW = 18 square degress) did not detect a source with next 5-sigma limits: Date UT Tstart-Ttrig Exp Filter tube lim (middle) (day) (s) 250815 14:55:18 -3.12 60 Clear East 19.8 250822 16:52:05 3.95 8x60 Clear West 20.9 250823 15:37:06 4.90 38x60 Clear West 21.8 The first observation was a lucky coincidence. Since there was no automatic LVC alert, subsequent observations had to be carried out in the Internet telescope mode, taking into account the specific quality of the weather. [1] V.M. Lipunov, V.G. Kornilov, E.Gorbovskoy, N. Tiurina & A.Kuznetsov, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http://www.pereplet.ru/lipunov/625.html#625 View this GCN Circular online at https://gcn.nasa.gov/circulars/41534. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...