Ultraluminous X-ray sources and star formation

Chandra observations of the Cartwheel galaxy reveal a population of ultraluminous X-ray sources (ULXs) with lifetimes < 10 7 yr associated with a spreading wave of star formation which began some 3 x 10 8 yr ago. A population of high-mass X-ray binaries provides a simple model: donor stars of initial masses M 2 ≥ 15 M ○. transfer mass on their thermal time-scales to black holes of masses M 1 ≥ 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 3 x 10 4 , 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 10 7 yr is <1. Current models of IMBH production would require a very large mass (≥10 10 M ○. ) of stars to have formed new clusters. Further, the accretion efficiency must be low (≥ 6 x 10 -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.