Detection of 125.5-day Optical Periodic Modulation of the Neutron Star
  M51 ULX-8
  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 {\it 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 it was
also found that the X-ray fluxes and also the observed optical fluxes of C3
exhibit a bi-modal distribution. This could mean that there is a possible
correlation between the optical fluxes and the X-ray fluxes 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.
