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 45389) 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).