Allak, S.2025-01-272025-01-2720220035-87111365-2966https://doi.org/10.1093/mnras/stac2887https://hdl.handle.net/20.500.12428/27776Studying Ultraluminous X-ray sources (ULXs) in the optical wavelengths provides important clues about the accretion mechanisms and the evolutionary processes of X-ray binary systems. In this study, three (C1, C2, and C3) possible optical counterparts were identified for well-known neutron star (NS) candidate M51 ULX-8 through advanced astrometry based on the Chandra and Hubble Space Telescope (HST) observations, as well as the GAIA optical source catalogue. Optical periodic modulation of 125.5 d with an amplitude of 0.14 mag was determined for C3, which has evidence to represent the optical nature of ULX-8 using one-year (2016-2017) 34 HST Advanced Camera for Surveys (ACS)/Wide Field Camera (WFC) observations. Moreover, surprisingly, the observed optical fluxes of C3 exhibit a bi-modal distribution. This could mean that there is a possible correlation between the optical and the X-ray flux variabilities of the ULX-8. The possible scenarios which are frequently mentioned in the literature proposed for the nature of optical emission and optical super-orbital period. The most probable scenario is that the optical emission could have originated from the accretion disc of the ULX-8.eninfo:eu-repo/semantics/openAccessastrometrygalaxies: individual: M51 ULX-8X-rays: binaries: Optical counterparts of M51 ULX-8Article51733495350410.1093/mnras/stac2887Q1WOS:0008727119000042-s2.0-85145255550Q1