TITLE:   GCN CIRCULAR
NUMBER:  44006
SUBJECT: GRB 260310A / AT2026fgk: SAO RAS optical observations
DATE:    26/03/14 15:46:52 GMT
FROM:    Alexander Moskvitin at SAO RAS <mosk@sao.ru>

A. Moskvitin (SAO RAS), O. Spiridonova (SAO RAS),
A. Pozanenko (IKI), N. Pankov (IKI), A. Volnova (IKI),
report on behalf of GRB follow-up team and IKI-GRB-FuN.

We observed the field of the AT2026fgk, potential counterpart 
of the GRB 260310A (Fermi GBM team, GCN 43951; Hamburg and Meegan, 
GCN 43975; Salunke et al., GCN 43958) with the Zeiss-1000 
1m telescope of the SAO RAS equipped with the CCD photometer.
We obtained series of 300 sec images in B, V, Rc, Ic filters 
on March 12/13 and 13/14. The observations carried out 
under a good weather conditions and typical seeing of 1".5--2".5.

The candidate discovered by GOTO (O'Neill et al. 2026, TNS Discovery 
Report 294132) and observed by numerous observatories (Konno et al., 
GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978;
Martin-Carrillo et al., GCN 43979; Moren Méndez et al., GCN 43980;
de Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; 
Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Li et al., 
GCN 43993; Jayaraman et al., GCN 43994; Stein et al., GCN 43996;
Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001; 
Lai et al., GCN 44002; Izzo, GCN 44003; Brivio et al., GCN 44004; 
Rhodes et al., GCN 44005) is clearly detected in our images 
with the following brightness.

Date       UT_start  t_mid - T0, filter  exp,   mag   +/- err  
                     days                s     
2026.03.12T23:27:00  2.80565     B       5*300  18.77 +/- 0.01
2026.03.12T23:09:50  2.79577     Rc      6*300  18.00 +/- 0.01
2026.03.13T23:56:42  3.82450     B       5*300  18.92 +/- 0.01
2026.03.13T23:45:30  3.81664     Rc      4*300  18.14 +/- 0.01

This preliminary photometry is based on the nearby stars from 
PanSTARRS catalogue. Magnitudes of standards converted to B, Rc 
with the Lupton 2005 equations. The magnitudes of the AT2026fgk 
are not corrected for Galactic and host extinction 
and may be contaminated by the host galaxy.

We can not trace any colour evolution between the two epochs 
(2.8 and 3.8 days after GRB trigger).