TITLE: GCN CIRCULAR NUMBER: 36761 SUBJECT: EP240625a: NOT optical counterpart candidate DATE: 24/06/25 16:25:42 GMT FROM: Dong Xu at NAOC/CAS dxu@nao.cas.cn

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

We observed the field of EP240625a detected by EP/WXT (Pan et al., GCN 36757) using the the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. We obtained 16x30 s Sloan z-filter images at a median time of 1.52 hr after the EP trigger.

An uncatalogued optical transient (OT), within the 10 arcsec EP/FXT error circle (Pan et al., GCN 36760), is marginally detected in the stacked z-band image at coordinates:

R.A. (J2000)= 20:42:55.47 Dec. (J2000)= -15:58:24.00

with an uncertainty of ~ 0.2 arcsec. It has z = 20.7 +/- 0.2 mag , calibrated against nearby Pan-STARRS stars and not corrected for Galactic extinction. Given the low S/N for the OT, we cannot completely rule out it being due to background fluctuation.

Further observations are scheduled.

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