GCN 44208: GRB 260310A/AT2026fgk: Kilonova-Catcher long term optical monitoring

2026-04-03T13:30:44.895Z | rev 0 | event: GRB 260310A
M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (IJCLab), M. Coughlin (UMN),S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We have continued to observe the field of GRB 260310A detected by Fermi/GBM (Fermi GBM team, GCN 43951) and AstroSat CZTI (Salunke et al., GCN 43958) with the GRANDMA citizen science project Kilonova-catcher (KNC) over the past week mainly in Rc and sdssr filters (One observation every night). Our last night observations were performed with a CDK17 telescope operated by M. Freeberg. Our observations started at TGRB+23.9 days and were taken with Rc filters. 

In our stacked frames, subtracted from the Legacy Survey DR10 template image, we still detect the optical counterpart reported by many teams (O'Neill et al. 2026, TNS Discovery Report 294132; 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 et al., GCN 44065;  Brosio et al., GCN 44067; Pankov et al. GCN 44073; Perley et al. GCN 44096; Passaleva et al. GCN 44104; Kang et al;, GCN 44119; Effress et al., GCN 44122; de Ugarte Postigo et al., GCN 44124; Guelfand et al. GCN 44125; O'Connor et al., GCN 44137; Liu et al., GCN 44172)

We report our last 3 nights follow-up results in the table below: 
+------------------+-----------+-----------+----------------+--------------+
| Tmid-TGRB (days) | Exp (s)   | Filter    | Magnitude      | Instrument   |
+==================+===========+===========+================+==============+
| 21.82            | 44 x 180s | Rc (Vega) | 19.92 +/- 0.12 | CDK17        | 
| 22.82            | 50 x 180s | Rc (Vega) | 20.01 +/- 0.15 | CDK17        | 
| 23.95            | 50 x 180s | Rc (Vega) | 20.22 +/- 0.12 | CDK17        | 
+------------------+-----------+-----------+----------------+--------------+

The KNC members will keep monitoring the source as long as it stays detectable.

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the Johnson-Cousins filters were calibrated using the GAIA DR3 Synphot catalog.

We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). 

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).