  Studying 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 days with an amplitude of 0.14
magnitude was determined for C3 which has evidence to represent the optical
nature of ULX-8 using one-year (2016-2017) 34 HST ACS (Advanced Camera for
Surveys)/WFC (Wide Field Camera) 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 disk of the ULX-8.
