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 × 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 × 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 × 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.