Abstract
The reported discovery of a cyclotron resonance scattering feature (CRSF) in the spectrum of M51 ULX-8 may provide an important clue as to the nature of the magnetic field in those ultraluminous X-ray sources hosting neutron stars. In this paper we present the covariance (linearly correlated variability) spectrum of M51 ULX-8 on long (> 2000s) timescales. This allows us to unambiguously decompose the spectrum which requires multiple components in order to explain the broad-band emission. Having a clearer picture of the spectral decomposition leads to various tests for the dipole field strength of the neutron star which can be extended to other ULXs when certain criteria are met. In the case of M51 ULX-8, we rule out a very strong (~10^15 G) dipole solution with either a sub- or super-critical disc. Instead, our tests indicate an upper limit on the dipole field of ~10^12 G, and a classical super-critical inflow, similar to that inferred in other ULXs found to harbour neutron stars, although we do not rule out the presence of an additional, strong (~10^15 G) multipole field falling off steeply with distance from the neutron star.
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