  The recent discovery of periodic pulsations from several members of the
ultraluminous X-ray source (ULX) family in nearby galaxies as well as in our
own galaxy unveiled the nature of the accreting compact object. Neutron stars
rather than black holes are currently believed to power a substantial number of
ULXs whether or not pulsations are observed. The detection of cyclotron
absorption lines in the X-ray spectrum of a ULX provides an alternative way to
identify the compact object as a neutron star. Among the non-pulsating ULXs,
the presence of a cyclotron resonance scattering feature (CRSF) in the spectrum
of M51 ULX-8 has been reported. In the present work, the magnetic field
strength on the surface of the neutron star in M51 ULX-8 is inferred from the
energy of the observed CRSF to estimate the beaming fraction in X-ray emission
and more importantly the observable range for the elusive neutron-star spin
period to be hopefully discovered by the forthcoming space missions in the near
future.
