TITLE:   GCN CIRCULAR
NUMBER:  41980
SUBJECT: GRB 250924A: NOT optical observations
DATE:    25/09/25 07:54:34 GMT
FROM:    Laura Cotter at University College Dublin <laura.cotter@ucdconnect.ie>

L. Cotter (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), A. de Ugarte Postigo (LAM), B. Schneider (LAM), G. Corcoran (UCD), K. Valeckas (NOT and NBI), report on behalf of a larger collaboration:

We observed the optical afterglow of GRB 250924A (Siegel et al., GCN 41959) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager. A total of 3x300 s of imaging in the r band was acquired.

At the mid epoch of 2025 Sep 25.117 UT (18.49 hr after the Swift/BAT trigger), we measure a magnitude of r = 23.55 +/- 0.15 (AB), calibrated against nearby objects from the Pan-STARRS catalog, and not corrected for Galactic extinction.

Comparing our result to previous measurements (e.g. Pankov et al., GCN 41969), the afterglow is significantly fainter than the extrapolation using the decay rate observed at early times (Schneider et al., GCN 41963; Zhang & Filippenko, GCN 41970), indicating a steepening of the light curve.