TITLE:   GCN CIRCULAR
NUMBER:  42158
SUBJECT: GRB 251007A: NOT optical observations
DATE:    25/10/08 06:46:01 GMT
FROM:    Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>

D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), T. Pursimo (NOT), A. L. Bouquin (NOT and DTU Space) report on behalf of a larger collaboration:

We observed the optical afterglow of GRB 251007A (Ambrosi et al., GCN 42146; Wu et al., GCN 42154) with the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started on 2025 Oct 8.16 UT (8.2 hr after the trigger), as soon as the target was visible from La Palma, and consisted of images in the SDSS g, r, and i filters.

In a stack of nine 200 s images, we measure

i = 21.07 +- 0.07

at a mean epoch of 2025 Oct 8.17 UT (8.46 hr after the trigger). This magnitude is in the AB system, is calibrated against nearby stars from the Pan-STARRS catalog, and is not corrected for Galactic extinction.

The afterglow looks red, with a color r-i ~ 0.4, which corresponds to a spectral slope alpha ~ 1.9 (F_nu propto nu^-alpha). This is consistent with the bright i-band detection by SVOM/C-GFT (Wu et al., GCN 42154).