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.