EP260628c/EP J223759.5+531421: NOT optical upper limit

ATel #17885; S. Q. Jiang (NAOC), Z. P. Zhu (NACO), J. An (NAOC), X. Liu (NAOC), L. B. He (NAOC), D. Xu (NAOC), S. Y. Fu (HUST), J. P.U. Fynbo (DAWN/NBI), V. Vuolteenaho (NOT)
on 12 Jul 2026; 08:07 UT
Credential Certification: Dong Xu (dxu@nao.cas.cn)

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We observed the magnetar candidate EP260628c/EP J223759.5+531421 discovered by the Einstein Probe mission (Yang et al., ATel #17859; Rea et al., ATel #17870; Lai et al., ATel #17875; Gotz et al., ATel #17876; Coti Zelati et al., ATel #17877)using the 2.56m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observation started at 2026-06-30T04:10:37.119 UT, i.e., 1.50 days post-burst, and a series of Sloan z-band frames were obtained. No uncatalogued optical transient is detected in the stacked image within the EP/FXT error circle, down to a 3-sigma limiting magnitude of z > 22.2, calibrated with Pan-STARRS DR2 stars in the field.