TITLE: GCN CIRCULAR NUMBER: 40080 SUBJECT: GRB 250404A / EP250404a: NOT optical observations DATE: 25/04/05 15:27:15 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group antonio.martin-carrillo@ucd.ie
A. Martin-Carrillo (UCD), B. Schneider (LAM), A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud Univ.), J. An (NAOC), D. Xu (NAOC), G. Corcoran (UCD), Dimple (U. Birmingham), M. A. Diaz Teodori (NOT and Turku Univ.), report on behalf of a larger collaboration:
We observed the field of EP250404a (Hu et al., GCN 40051), likely associated with GRB 250404A detected by Fermi/GBM (Fermi GBM team, GCN 40050; Mukherjee & Meegan, GCN 40067) using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained exposures in the SDSS-r (3x180 s) and SDSS-z (2x180 s) bands, starting at 22:09:17 UT on 2025-04-04 (7.82 hr after the EP trigger).
The optical counterpart reported by Jiang et al. (GCN. 40052), Perez-Garcia et al. (GCN 40053), Du et al. (GCN 40058), Zhu et al. (GCN 40061), Konno et al. (GCN 40063), Odeh et al. (GCN 40064), He et al. (GCN 40069), Pérez-Fournon et al. (GCN 40071), Jelinek et al. (GCN 40072), Rakotondrainibe et al. (GCN 40073), Taguchi et al. (GCN 40074), Ruocco et al. (GCN 40076) is well detected in the stacked images, with the following preliminary magnitudes:
r = 20.43 +/- 0.05 (7.90 hr after trigger); z = 19.79 +/- 0.06 (8.04 hr after trigger).
The photometric calibration was performed using nearby stars from the Pan-STARRS catalog and the magnitudes are not corrected for Galactic extinction.
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