GCN 43182: EP251220a: NOT optical observations

A. P. C. van Hoof (Radboud), G. Corcoran (UCD), J. An (NAOC), R. A. J. Eyles-Ferris (U. Leicester), D. B. Malesani (DAWN/NBI and Radboud), D. Xu (NAOC), J. Sanchez-Sierras (Radboud), P. G. Jonker (Radboud), A. Martin-Carrillo (UCD), A. Kadela (NOT and NBI), report on behalf of a larger collaboration:

We observed the optical counterpart (Sánchez Álvarez et al., GCN 43167; Kumar et al., GCN 43169; Rakotondrainibe et al., GCN 43170; Busmann et al., GCN 43181) of EP251220a (Liu et al., GCNs 43162, 43172) with the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. A sequence of 4x300 s exposures were taken in SDSS r-band starting on 2025-12-21 at 19:37:19.906 UT (~35.18 hr since the EP trigger).

The source is well detected in the stacked image with a preliminary photometry of

r = 21.79 +/- 0.05 (AB),

calibrated against nearby stars from the Pan-STARRS catalog and not corrected for Galactic extinction. Our photometry is consistent with the temporal index found by Rakotondrainibe et al. (GCN 43170).