TITLE: GCN CIRCULAR NUMBER: 43211 SUBJECT: EP251222a: NTT/ULTRACAM optical observations DATE: 25/12/23 08:11:19 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk> D. B. Malesani (DAWN/NBI and Radboud), M. Stepney (CATA), D. Sahman (Sheffield), L. Cotter (UCD), G. Corcoran (UCD), P. G. Jonker (Radboud) report on behalf of a larger collaboration: We observed the field of the fast X-ray transient EP251222a (Huang et al., GCN 43200) using the ULTRACAM instrument on the ESO NTT telescope in La Silla (Chile). Observations consisted of 120 exposures by 15 s each, taken at a mean epoch 2025 December 23, 03:07:14 UT (11.81 hr after the EP trigger). The optical counterpart identified by Li et al. (GCN 43205) is well detected in our coadded image, for which we measure a magnitude r = 21.56 +- 0.08 (AB), calibrated against nearby stars from the Pan-STARRS catalog and not corrected for Galactic extinction. This confirms the rebrightening reported by Li et al. (GCN 43205). The consistency with the EP/FXT X-ray position (Huang et al., GCN 43200) makes this source the likely counterpart of EP251222a. Further follow-up is encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/43211. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...