TITLE: GCN CIRCULAR NUMBER: 39405 SUBJECT: IceCube-250207A: MASTER flaring blazar QSOJ0854+2006 DATE: 25/02/21 11:20:59 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs lipunov@xray.sai.msu.ru

with Zhirkov effect

K. Labzina, K.Zhirkov, V.Lipunov, E.Gorbovskoy, G.Antipov, D.Vlasenko, A.Kuznetsov, N.Tyurina, P.Balanutsa, A.Chasovnikov, D.Kuvshinov, V.Topolev,Ya.Kechin,Yu.Tselik, K.Minkina (Lomonosov Moscow State University, SAI, Physics Department), 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)

MASTER600-Tunka robotic telescope  located in Russia (Applied Physics Institute, Irkutsk State University; Lipunov, Kornilov, Gorbovskoy, Tiurina & Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591, pp.http://www.pereplet.ru/lipunov/625.html#625) started IceCube-250207A (IceCube GCN#39203)of the Ice Cube Alert 250207.09 error box  ~11h after trigger time at 2025-02-07 13:53:05 UT, with upper limit up to  19.2 mag. (see cover map: https://master.sai.msu.ru/site/master2/observ.php?id=2770114)

We analyzed MASTER archive images since 2010 of the source, that can be related with this event. Global MASTER Net detected QSOJ0854+2006 inside 1 sigma. QSOJ0854+2006 (https://simbad.u-strasbg.fr/simbad/sim-coo) fading to 16.6m during last nights observations.

Fortunately, data from the ATLAS network (Tonry et al., 2018, PASP, 13, 164505) are available, which show that ~24 hours before the neutrino alert (05.13 feb 2025.) the blazar QSOJ0854+2006 was at its local standard brightness, it was 1 magnitude brighter: 15.1 . After a few days, the blazar returned to its original bright state. Thus, we report the observation of an Zhirkov effect discovered earlier (The Astrophysical Journal Letters, 896, L19, 2020) .

The MASTER archive light curve from 2010 year is available at http://www.pereplet.ru/lipunov/images/ATbbo_galaxy_arhive_midi.jpg

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