TITLE:   GCN CIRCULAR
NUMBER:  44043
SUBJECT: GRB 260310A / AT2026fgk: LCO optical observations
DATE:    26/03/18 06:59:09 GMT
FROM:    Sergey Belkin at Monash University <sergey.belkin@monash.edu>

S. Belkin and M. Shrestha (Monash U) report:

We observed the optical counterpart AT2026fgk of GRB 260310A, detected by Fermi/GBM (Fermi GBM Team, GCN 43951) and AstroSat/CZTI (Salunke et al., GCN 43958), with the Las Cumbres Observatory 1m telescope at McDonald Observatory equipped with the Sinistro instrument.

Observations began on 2026-03-14 at 06:12:09 UT, approximately 4.05 days after the trigger. We obtained 4 x 60 s exposures in the r band and 6 x 60 s exposures in the u band. In the stacked images we clearly detect the reported optical counterpart AT2026fgk (e.g. Lipunov et al., GCN 43954; Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Mendez et al., GCN 43980; de Ugarte Postigo et al., GCN 43984; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo, GCN 44003; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al., GCN 44022), discovered by GOTO (O’Neill et al., TNS Discovery Report 294132).

The preliminary photometry from the stacked images is:

Date (UT)            T-T0 (days)   Filter   Exp. time (s)     Magnitude    3-sigma UL
========================================================================================
2026-03-14 06:14:13    4.0535        r         60 x 4      18.58 +/- 0.02    21.79
2026-03-14 06:21:49    4.0588        u         60 x 6      19.47 +/- 0.04    21.12

The r-band photometry is calibrated against Pan-STARRS field stars. The u-band photometry is calibrated against Gaia DR3 synthetic SDSS u-band stars. The quoted values are not corrected for Galactic extinction. Further follow-up is encouraged.