TITLE: GCN CIRCULAR NUMBER: 42391 SUBJECT: EP251023a: NOT optical counterpart detection DATE: 25/10/23 10:01:31 GMT FROM: liuxing@nao.cas.cn

X. Liu, Z.P. Zhu, L.B. He, J. An, S.Q. Jiang, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), Niko Pyykkinen (NOT) report on behalf of a large collaboration:

We observed the field of the EP251023a (Wu et al., GCN 42388) using the 2.56m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observation started at 05:55:11 UTC on 2025-10-23, i.e., 3.4 hours after the EP trigger, and a series of frames were obtained in the Sloan r- and z- bands.

Within the EP/FXT localization error region (Wu et al., GCN 42388), the optical counterpart (Wortley et al., GCN 42387; Li et al., GCN 42389) is also clearly detected in both r- and z- bands with magnitudes as follows:

T-mid (hr) | Filter | Exposure | Mag 3.49 | r | 3x200s | 18.57 +- 0.01 3.72 | z | 5x150s | 18.36 +- 0.01

calibrated against nearby Pan-STARRS stars and not corrected for Galactic extinction.

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