TITLE:   GCN CIRCULAR
NUMBER:  39770
SUBJECT: GRB 250317B: NOT optical observations suggest an optical plateau phase
DATE:    25/03/18 13:35:17 GMT
FROM:    Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>

Z. P. Zhu (NAOC), D. B. Malesani (DAWN/NBI and Radboud Univ.), B. Schneider (LAM), A. Martin-Carrillo (UCD), A. Henderson de la Fuente (NOT), report on behalf of a larger collaboration:

We observed the field of the GRB 250317B (Zhao et al., GCN 39753) using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained 2 exposures of 300 s each in the SDSS-r band starting at 22:43:24 UT on 2025-03-17 (20.56 hr after the trigger). 

The optical counterpart reported by Watson et al. (GCN 39754), Palmerio et al. (GCN 39755), Pérez-Fournon et al. (GCN 39757), Ghosh et al. (GCN 39759), Ferro et al. (GCN 39764) and Schneider et al. (GCN 39767) is well detected in the stacked image, with a preliminary magnitude of r = 21.7 +/- 0.1 mag (AB). The photometric calibration was performed using nearby stars from the Pan-STARRS catalog and the magnitude is not corrected for Galactic extinction.

We note that the measured magnitude is consistent with those reported by Schneider et al. (GCN 39767), obtained ~1 hr before our observation, and Thoene et al. (GCN 39769), obtained about 0.6 hr after our observation, suggesting a possible optical plateau or rebrightening episode.

Further observations are planned and encouraged to monitor the evolution of the optical afterglow.