Predicting ULX demographics from geometrical beaming

The ultraluminous X-ray source (ULX) population is known to contain neutron stars, but the relative number of these compared to black hole primaries is unknown. Assuming classical super-critical 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 neutron stars to black holes. Conversely - and perhaps more importantly - given even a crude estimate for the spatial densities, an estimate of the fraction of the population containing neutron stars will begin to constrain the mean mass of black holes in ULXs.