TITLE: GCN CIRCULAR NUMBER: 41520 SUBJECT: GRB 250823A: NOT optical observations DATE: 25/08/24 08:54:25 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk> D. B. Malesani (DAWN/NBI and Radboud), L. Izzo (INAF/OAC and DARK/NBI), A. Martin-Carrillo (UCD), B. P. Gompertz (Birmingham), A de Ugarte Postigo (LAM), A. Henderson de la Fuente (NOT) report on behalf of a larger collaboration: We observed the optical counterpart (Gompertz & Dimple, GCN 41512; Fernandez-Garcia et al., GCN 41513; Strausbaugh & Cucchiara, GCN 41514) of the Swift and Fermi GRB 250823A (Siegel et al., GCN 41511; Smith et al., GCN 41517) using the Nordic Optical Telescope (NOT) located in La Palma (Canary Islands, Spain). Observations were carried out in the r and z filters. Calibrated against nearby Pan-STARRS pointlike objects, we report the following magnitudes: | Filter | Mean epoch (UT) | Time since trigger (hr) | Exp time (s) | Magnitude (AB) | | ------ | --------------- | ----------------------- | ------------ | -------------- | | r | 2025 Aug 24.022 | 4.36 | 3x300 | 21.89 +- 0.05 | | z | 2025 Aug 24.036 | 4.69 | 5x250 | 21.15 +- 0.12 | These values are not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41520. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...