TITLE: GCN CIRCULAR NUMBER: 41967 SUBJECT: GRB 250920C: Kilonova-Catcher optical afterglow detection DATE: 25/09/24 14:37:22 GMT FROM: Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com> M. Freeberg (KNC), C. Andrade(UMN), D. Turpin (CEA-Saclay/Irfu), 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 250920C detected by Fermi/GBM (Fermi GBM team, GCN 41903), Swift/BAT (Gupta et al., GCN 41904), SVOM/GRM (Wang et al., GCN 41929) and Konus-WIND (Panteleeva et al., GCN 41957) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the iTelescope T30 telescope operated by M. Freeberg. Our observations started at TGRB+3.3hr. In our stacked frames, subtracted from the PanSTARRS DR2 template image, we detect the optical/IR counterpart reported by GOTO (Wortley et al., GCN 41907) and further confirmed by many other teams (Strausbaugh et al.,GCN 41911; Ghosh et al., GCN 41913; Kuin et al., GCN 41920; Mohan et al., GCN 41925; Sanchez-Ramirez et al., GCN 41928; Guiffreda et al., GCN 41943; Mo et al., GCN 41947; de Ugarte Postigo et al., GCN 41951; de Wet et al., GCN 41952; Izzo et al., GCN 41955; Zheng et al., GCN 41956; Zhu et al., GCN 41965) We report some of our first image follow-up results in the table below +---------------+----------+-----------+----------------+--------------+ | Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument | +===============+==========+===========+================+==============+ | 3.49 | 3 x 300s | Rc (Vega) | 18.93 +/- 0.07 | iT30 | +---------------+----------+-----------+----------------+--------------+ 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/). View this GCN Circular online at https://gcn.nasa.gov/circulars/41967. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...