TITLE: GCN CIRCULAR NUMBER: 44462 SUBJECT: GRB 260310A / AT2026fgk: photometric monitoring and tentative maximum of a possible SN DATE: 26/05/04 19:46:05 GMT FROM: Alina Volnova at IKI RAS <alinusss@gmail.com> N. Pankov (IKI), A. Pozanenko (IKI), A. Volnova (IKI), E. Schekotikhin (IKI), E. Klunko (ISTP), M. Eselevich (ISTP), A. Moskvitin (SAO RAS), O. Spiridonova (SAO RAS), V. Rumyantsev (CrAO), A. Novichonok (KIAM), A. Kochergin (UAFO IAA), I. Sokolov (INASAN), M. Krugov (FAI), R. Inasaridze (AbAO), A. Tarasenkov (INASAN) report on behalf of IKI GRB-FuN: We observed the optical counterpart AT2026fgk (a.k.a. GOTO26buh, O'Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65; Konno et al., GCN 43974) associated with GRB 260310A (The Fermi GBM team, GCN 43951; Salunke et la., GCN 43958; Hamburg & Meegan, GCN 43975; Jayaraman et al., GCN 43994). Observations began on March 13, 2026, and continue to this day. Observations were performed with the following telescopes: AZT-33IK (Mondy), Zeiss-1000 (SAO RAS), Zeiss-1000 (Koshka), ZTSh (CrAO), ORI-50 (UAFO), Zeiss-2000 (Terskol), AZT-20 (Assy), AS-32 (AbAO) with BR-filters, and BVRI-filters on a few epochs. We clearly detect the counterpart (Lipunov et al., GCN 43954, 43978; Martin-Carrillo et al., GCN 43979, 44000; Moreno Méndez et al., GCN 43979; Hsu et al., GCN 43986; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991, 44059; Jin-Ji Li et al., GCN 43993; Stein et al., GCN 43996; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo, GCN 44003; Brivio et al., GCN 44004; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021, 44067; Pawar et al., GCN 44022; Belkin & Shrestha, GCN 44043; H. L. Li et al., GCN 44044; Gupta et al., GCN 44051; Passaleva et al., GCN 44058, 44104; Volnova et al., GCN 44060; Busmann et al., GCN 44061; Zheng & Filippenko, GCN 44065; Pankov et al., GCN 44073; Serrau et al., GCN 44112; Kang et al., GCN 44119; Effress et al., GCN 44122; Guelfand et al., GCN 44125; O'Connor et al., GCN 44137; Liu et al., GCN 44172; Freeberg et al., GCN 44208). We confirm the break reported by Perly at al. (GCN 44096) on the 8th day after the trigger. After subtracting the image using the template obtained on April, 21 in the R filter with AZT-33IK, we constructed the afterglow + supernova light curve (LC). We note, that the point source is still clearly detected on the image on May, 02. We note that the afterglow power law decay index of -2.1 is steeper than reported by Perly at al. (GCN 44096) and may be affected by the still glowing optical counterpart. Subtracting the power-law afterglow with the power law decay index of -2.1 from the light curve gives us a possible supernova light curve, shown in https://heaiki.ru/lvc/GRB260310A/GRB260310A_subtracred_host_and_PL__afterglo... (for the LC construction we use only observations obtained with AZT-033IK, Mondy.) Thus we can tentatively confirm the maximum of bump which was suggested as possible SN associated with GRB 260310A in R-filter of about 21 days after the GRB trigger reported earlier (Freeberg et al., GCN 44248), however more faint and with apparent magnitude of about R ~ 21.8. Thus absolute magnitude at the redshift of z=0.153 (Hinds et al., GCN 43977; de Ugarte Postigo et al., GCN 43984) would be not fainter than M_R = -17.5. Initially SN rising was suggested by SED evolution (de Ugarte Postigo et al., GCN 44124). We continue to monitor the optical counterpart of the GRB 260310A. View this GCN Circular online at https://gcn.nasa.gov/circulars/44462. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...