  Measurements of the spin of stellar mass black holes (BHs) are now possible
both through LIGO observations of binary BH mergers and for BHs in X-ray binary
systems. The spins of BHs as inferred from LIGO observations suggest that BH
spins are on the lower end of what is expected for a ``flat'' distribution of
spins, while those from BHs in X-ray binaries tend to be large. Superradiance,
a process that can effectively reduce the spin of BHs before they merge, could
explain the lower observed spins in binary BH mergers for a non
self-interacting light boson. In this paper, we use Bayesian analysis to infer
the posterior probability distribution for the mass of a light boson that could
fit LIGO data. We also analyze spins of BHs from X-ray binaries, and find that
the X-ray binary data can be explained by superradiance due to a light boson
with large self-interactions. We infer the mass range for such a boson that is
consistent with the X-ray binary data.
