2004MNRAS.347L..18K

Chandra observations of the Cartwheel galaxy reveal a population of ultraluminous X-ray sources (ULXs) with lifetimes ≲ 10⁷ yr associated with a spreading wave of star formation which began some 3x10⁸ yr ago. A population of high-mass X-ray binaries provides a simple model: donor stars of initial masses M₂≳ 15 M_☉transfer mass on their thermal time-scales to black holes of masses M₁≳ 10 M_☉.

For alternative explanations of the Cartwheel ULX population in terms of intermediate-mass black holes (IMBHs) accreting from massive stars, the inferred production rate ≳10^–6 yr^–1 implies at least 300 IMBHs, and more probably 3x10⁴, within the star-forming ring. These estimates are increased by factors of η^–1 if the efficiency η with which IMBHs find companions of ≳15 M_☉within 10⁷ yr is <1. Current models of IMBH production would require a very large mass (≳10¹⁰ M_☉) of stars to have formed new clusters. Further, the accretion efficiency must be low (≲ 6x10^–3) for IMBH binaries, suggesting super-Eddington accretion, even though intermediate black hole masses are invoked with the purpose of avoiding it.

These arguments suggest either that, to make a ULX, an IMBH must accrete from some as yet unknown non-stellar mass reservoir with very specific properties, or that most if not all ULXs in star-forming galaxies are high-mass X-ray binaries.