TITLE: GCN CIRCULAR NUMBER: 41071 SUBJECT: EP250711a: NOT optical counterpart candidate DATE: 25/07/12 08:45:33 GMT FROM: Dong Xu at NAOC/CAS dxu@nao.cas.cn

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

We observed the field of EP250711a detected by EP/WXT (Wang et al., GCN 41070), using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained 5 x 200 s exposures in the SDSS z-band, starting at 03:02:59 UT on 2025-07-12, i.e., 9.87 hrs after the EP trigger.

An uncatalogued optical transient (OT) is detected within the EP/FXT error circle (Wang et al., GCN 41070) at coordinates

R.A.(J2000) = 19:28:37.94 Dec.(J2000) = +4:42:06.80

with an uncertainty of ~ 0.5 arcsec. The source has z ~ 20.2 mag, calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction. We think the source is likely the optical counterpart of EP250711a.

We also note that there is a source north-west to the OT, being offset only by ~ 0.9 arcsec in the PanSTARRS, which has z ~ 21.5 mag and may be physically connected to EP250711a.

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