TITLE: GCN CIRCULAR NUMBER: 37035 SUBJECT: EP240804a: NOT optical counterpart detection DATE: 24/08/05 05:07:21 GMT FROM: Zipei Zhu at NAOC <zpzhu@nao.cas.cn> J. An, S.Y. Fu, S.Q. Jiang, X. Liu, Z.P. Zhu, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), B.N. Hauptmann (NOT) report on behalf of a large collaboration: We observed the field of EP240804a detected by Einstein Probe (EP, Wang et al., GCN 37034), using the ALFOSC instrument mounted on the Nordic Optical Telescope (NOT), with 3x300 s photometry in the SDSS-r band. Observations started at 02:55:25 UT on 2024-08-05, i.e. 5.28 hr after the trigger. An uncatalogued decaying source is detected in the individual as well as the stacked frames within the EP/FXT error circle (Wang et al., GCN 37034) and localized at coordinates R.A. (J2000) = 22:30:38.84 Dec. (J2000) = -39:07:20.06 with an uncertainty of ~ 0.5 arcsec. The source has r ~ 20 mag, calibrated with SkyMapper and not corrected for Galactic extinction. There is no known object at the above position by checking MPC. We thus conclude that the source is the optical counterpart candidate of EP240804a. Further observations are encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/37035. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...