GCN 44073: GRB 260310A: continued optical observation at Mondy

2026-03-21T22:21:34.340Z | rev 0 | event: GRB 260310A
N. Pankov (IKI), A. Volnova (IKI), E. Klunko (ISTP), A. S. Moskvitin
(SAO RAS), O. Spiridonova (SAO RAS), A. Pozanenko (IKI) report on
behalf of GRB IKI FuN.

We continued observation the GRB260310A/AT2026fgk detected by
Fermi/GBM (Fermi GBM team, GCN 43951) and AstroSat (Salunke et al.,
GCN 43958) with AZT-33IK telescope of Mondy observatory. We obtained
series in B and R filters on 2026-03-21.

The OT discovered by GOTO (O'Neill et al. 2026, TNS Discovery Report
294132) and observed earlier (Konno et al., GCN 43974; Hinds et al.,
GCN 43977; Lipunov et al., GCN 43978;Martin-Carrillo et al., GCN
43979; 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; Moreno Méndez et
al., GCN 43980; Becerra et al., GCN 43991; Li et al., GCN 43993; Stein
et al., GCN 43996; Watson et al., GCN 44001; Lai et al., GCN 44002;
Izzo et al., GCN 44003; Brivio et al., GCN 44004; Moskvitin et al.,
GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al.,
GCN 44022; Belkin et al., GCN 44043; Li et al., GCN 44044; Gupta et
al., GCN 44051; Becerra et al., GCN 44059; Volnova et al., GCN 44060;
Busmann et al., GCN 44061; Zheng t al., GCN 44065) is clearly detected
in our images. Preliminary photometry is the following

Date UT_start t_mid - T0,  exp, mag, err,  filter, UL(3sigma)
                          days                s

2026-03-21 16:35:57 11.48526 30*120 B 20.19 0.04 23.3
2026-03-21 16:04:50 11.46366 15*120 R 19.21 0.04 23.4

The photometry is based on the nearby calibrations stars of PS1
catalogue converted to B, R using the Lupton (2005) transformations.
The magnitudes of the AT2026fgk are not corrected for Galactic
extinctions.

The light curve obtained from our observations and GCNs can be found
in https://heaiki.ru/lvc/GRB260310A/GRB260310A_LC.png
We can assume that the light curve becomes steeper 9 days after long
lasting plateau phase