Population X: Are the super-Eddington X-ray sources beamed jets in microblazars or intermediate mass black holes?

Abstract

Recent X-ray observations reveal an increasing number of X-ray sources in nearby galaxies exceeding luminosities of Lx ≳ 2 × 1039 ergs-1. Assuming isotropic emission, the Eddington limit suggests a population of intermediate-mass black holes of M• ≫ 10 M⊙. However, Markoff et al. (2001a) proposed that jets may be contributing to the X-ray emission from X-ray binaries (XRBs), implying that some X-ray sources may be relativistically beamed. This could reduce the required black hole masses to standard values. To test this hypothesis, we investigate a simple X-ray population synthesis model for X-ray point sources in galaxies with relativistic beaming and compare it with an isotropic emission model. The model is used to explain a combined data set of X-ray point sources in nearby galaxies. We show that the current distributions are consistent with black hole masses M⊙ ≲ 10 and bulk Lorentz factors for jets in microquasars of γj ∼ 5. Alternatively, intermediate mass black holes up to 1000 M⊙ are required which are distributed in a powerlaw with roughly dN/dM ∼ M-2.