  We analyse the X-ray spectrum of the black hole X-ray binary MAXI J1820+070
using observations from XMM-Newton and NuSTAR during 'hard' states of its
2018-2019 outburst. We take a fully Bayesian approach, and this is one of the
first papers to present a fully Bayesian workflow for the analysis of an X-ray
binary X-ray spectrum. This allows us to leverage the relatively
well-understood distance and binary system properties (like inclination and
black hole mass), as well as information from the XMM-Newton RGS data to assess
the foreground X-ray absorption. We employ a spectral model for a `vanilla'
disc-corona system: the disc is flat and in the plane perpendicular to the axis
of the jet and the black hole spin, the disc extends inwards to the innermost
stable circular orbit around the black hole, and the (non-thermal) hard X-ray
photons are up-scattered soft X-ray photons originating from the disc thermal
emission. Together, these provide tight constraints on the spectral model and,
in combination with the strong prior information about the system, mean we can
then constrain other parameters that are poorly understood such as the disc
colour correction factor. By marginalising over all the parameters, we
calculate a posterior density for the black hole spin parameter, $a$. Our
modelling suggests a preference for low or negative spin values, although this
could plausibly be reproduced by higher spins and a modest degree of disc
truncation. This approach demonstrates the efficacy and some of the
complexities of Bayesian methods for X-ray spectral analysis.
