  Identifying the compact object in ultraluminous X-ray sources (ULXs) has
to-date required detection of pulsations or a cyclotron resonance scattering
feature (CRSF), indicating a magnetised neutron star. However, pulsations are
observed to be transient and it is plausible that accretion onto the neutron
star may have suppressed the surface magnetic field such that pulsations and
CRSFs will be entirely absent. We may therefore lack direct means to identify
neutron star systems whilst we presently lack an effective means by which to
identify black hole ULXs. Here we present a possible method for separating the
ULX population by assuming the X-ray, mHz quasi-periodic oscillations (QPOs)
and day timescale periods/QPOs are associated with Lense-Thirring precession of
the inflow and outflowing wind respectively. The precession timescales combined
with the temperature of the soft X-ray component produce planes where the
accretor mass enters as a free parameter. Depending on the properties of the
wind, use of these planes may be robust to a range in the angular momentum
(spin) and, for high accretion rates, essentially independent of the neutron
star's surface dipole field strength. Our model also predicts the mHz QPO
frequency and magnitude of the phase-lag imprinted due to propagation through
the optically thick wind; in the case of NGC 5408 X-1 we subsequently infer a
black hole mass and high spin. Finally, we note that observing secular QPO
evolution over sufficient baselines may indicate a neutron star, as the
precession responds to spin-up which is not readily observable for black hole
primaries.
