TITLE: GCN CIRCULAR NUMBER: 44112 SUBJECT: GRB 260310A/AT2026fgk: Kilonova-Catcher late optical afterglow detection at about 15 days post trigger time DATE: 26/03/25 14:03:49 GMT FROM: Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com> M. Serrau, M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (OCA/IJCLab), M. Coughlin (UMN),S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration: We observed the field of GRB 260310 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). Our observations were performed with the ODK16 telescope at Observatoire Banon La Tuilerie, France operated by M. Serrau and the TEC160FL telescope operated by M. Freeberg. Our observations started at TGRB+14.8 days and were taken with sdss-g and sdss-r filters. In our stacked frames, subtracted from the Legacy Survey DR10 template image, we detect the optical counterpart reported 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 et al., GCN 44065; Brosio et al., GCN 44067; Pankov et al. GCN 44073; Perley et al. GCN 44096; Passaleva et al. GCN 44104) We report part of our follow-up results in the table below +------------------+-----------+--------+----------------+-------------+ | Tmid-TGRB (days) | Exp (s) | Filter | Magnitude | Instrument | +==================+===========+========+================+=============+ | 14.83 | 16 x 300s | r (AB) | 19.96 +/- 0.09 | ODK16 | | 15.17 | 17 x 180s | r (AB) | 19.82 +/- 0.12 | TEC160FL | +------------------+-----------+--------+----------------+-------------+ The KNC members will keep monitoring the source in the coming days. All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 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/). View this GCN Circular online at https://gcn.nasa.gov/circulars/44112. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...