TITLE: GCN CIRCULAR NUMBER: 42850 SUBJECT: GRB 251126A: Kilonova-Catcher optical afterglow detection DATE: 25/11/26 21:24:46 GMT FROM: Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com> E. Broens (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 251126A detected by Swift (Caputo et al., GCN 42843) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a private telescope T-BRO operated by E. Broens. Our observations started at TGRB+35 min and were taken unfiltered. In our stacked frames, subtracted from the PanSTARRS DR2 template image, we marginally detect the optical counterpart reported by GIT (Swain, GCN 42844). Despite the pretty low SNR (while the source is visible by eye in our image), we performed a forced photometry at the position we found to be at RA, Dec (J2000): 99.9440, +54.9864. We report our follow-up results in the table below: +---------------+-----------+---------+----------------+-------------+ | Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument | +===============+===========+=========+================+=============+ | 0.83 | 10 x 180s | r (AB) | 19.43 +/- 0.42 | T-BRO | +---------------+-----------+---------+----------------+-------------+ Compared to the magnitude reported by GIT (Swain, GCN 42844) the source has significantly faded even when considering our marginal detection which induced large error bars on the source photometry. Therefore, we confirm that this source is likely the optical afterglow of GRB 251126A. This is in agreement with the detections also reported by the Nanshan/HMT team (Fu et al., GCN 42848) and the Asiago team (Reguitti et al., GCN 42849). All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the Clear filters were calibrated using the PanSTARRS DR1 catalog using the PS1-r filter. 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/42850. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...