TITLE: GCN CIRCULAR NUMBER: 45396 SUBJECT: EP260806a: SAO RAS multicolor optical observations DATE: 26/08/18 15:10:31 GMT FROM: Nicolai Pankov at IKI <colinsergesen@gmail.com> N. Pankov (IKI), A. Moskvitin (SAO RAS), A. Pozanenko (IKI), O. Spiridonova (SAO RAS), S. Kaisin (SAO RAS) report on behalf of IKI-GRB-FuN: We observed the field of the X-ray transient EP260806a detected by EP (Li et al., GCN Circ. 45299) with the Zeiss-1000 1-meter and the BTA 6-meter telescopes of Special Astrophysical Observatory (SAO RAS) on six epochs between 2026-08-07T00:56:47 and 2026-08-10T23:19:45 UT. We have applied image subtraction using PS1 g,r images as reference and clearly detected the optical counterpart (He et al., GCN 45300; de Ugarte Postigo et al., GCN 45302; Eyles-Ferris et al., GCN 45303; Ma et al., GCN 45304; de Ugarte Postigo et al., GCN 45306; Anumarlapudi et al., GCN 45312; Abidkhanov et al., GCN 45354; Hua et al., GCN 45385; Maksut et al., GCN 4538) at each epoch. The observational summary and preliminary photometry are presented below: Date UTstart t-T0 Exp. Filter Mag Err. Telescope (mid, d) (n*s) 2026-08-07 00:56:47 0.08539 7*120 Rc 19.519 0.030 Zeiss-1000 2026-08-07 23:18:25 1.01898 4*300 Rc 18.842 0.055 Zeiss-1000 2026-08-08 23:18:25 2.02679 5*300 Rc 18.783 0.043 Zeiss-1000 2026-08-10 02:53:28 3.16181 12*20 r 18.877 0.008 BTA-6m 2026-08-10 04:25:25 3.22439 2*20 g 17.650 0.140 BTA-6m 2026-08-10 00:57:26 3.08632 3*300 Rc 18.618 0.007 Zeiss-1000 2026-08-10 23:19:45 4.02827 6*300 Rc 18.935 0.084 Zeiss-1000 2026-08-11 23:26:37 5.02719 6*300 Rc 18.618 0.007 Zeiss-1000 The photometry is based on set of nearby stars from USNO-B1.0 (for Rc) and PS1 (for g,r) filters, respectively. No correction for the Galactic extinction was applied. The plot of the light curve derived from own data and GCN photometry is available online at: https://heaiki.ru/lvc/r/GRBs/EP260806a/EP260806a_lc.png. We observe a significant g-r color evolution from -0.24 +/- 0.13 at ~0.5 days (Anumarlapudi et al., GCN 45312) to -1.23 +/- 0.16 at ~3.2 days since trigger. The observed light curve behavior is consistent with an AGN outburst, as was suggested by de Ugarte Postigo et al. (GCN 45302). View this GCN Circular online at https://gcn.nasa.gov/circulars/45396. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...