TITLE: GCN CIRCULAR NUMBER: 43555 SUBJECT: EP260128a: NOT optical counterpart candidate DATE: 26/01/29 11:54:22 GMT FROM: L. B. He at NAOC <helb@bao.ac.cn> L.B. He, J. An, X. Liu, S.Q. Jiang, Z.P. Zhu, D. Xu (NAOC), S.Y. Fu (HUST), J.P.U. Fynbo (DAWN/NBI), N. Pyykkinen (NOT) report on behalf of a large collaboration: We observed the field of EP260128a (Zhou et al., GCN 43549) using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started at 00:13:10 UT on 2026-01-29, i.e., 33.18 minutes after the EP trigger, and we obtained 9 x 90 s frames in the Sloan r-band and 9 x 100 s frames in the Sloan z- band. The stacked r- and z- band images have 3-sigma depth of r ~ 24.1 and z ~ 23.5, calibrated with nearby Pan-STARRS1 DR2 stars and not corrected for Galactic extinction. A new optical source is detected in the stacked z-band image within the EP/FXT error circle (Zhou et al., GCN 43554) at coordinates RA = 08:33:18.08 (128.32533) DEC = -10:16:32.3 (-10.27565) to ~ 4.2-sigma and we measured z = 23.07 +/- 0.26 (AB) at 1.004 hrs post-trigger. This source is not present in the z-band Legacy Survey DR11 early image v2, which is a bit deeper than the z-band NOT image. This source is also not present in the stacked r-band NOT image, as well as the r-band Legacy Survey image that is deeper than the NOT image. Considering the r-band non-detection and the z-band (weak) detection for the NOT candidate, EP260128a might be a high-redshift (i.e., z > ~5) event. We also note that the counterpart candidate by Liverpool Telescope (Eyles-Ferris et al., GCN 43553) is not present in neither r-band nor z-band stacked NOT image. Further optical and near-infrared follow-ups are encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/43555. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...