Predicting ultraluminous X-ray source demographics from geometrical beaming

The ultraluminous X-ray source (ULX) population is known to contain neutron stars (NS), but the relative number of these compared to black hole (BH) primaries is unknown. Assuming classical supercritical accretion and resultant geometrical beaming, we show that the observed population ratio can be predicted from the mean masses of each family of compact objects and the relative spatial density of NSs to BHs. Conversely – and perhaps more importantly – given even a crude estimate for the spatial densities, an estimate of the fraction of the population containing NSs will begin to constrain the mean mass of BHs in ultraluminous X-ray sources.