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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 Msun transfer mass on their thermal timescales to black holes of masses M_1 > 10 Msun. For alternative explanations of the Cartwheel ULX population in terms of intermediate-mass black holes (IMBH) 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 eta^-1 if the efficiency eta with which IMBHs find companions of > 15 Msun within 10^7 yr is <1. Current models of IMBH production would require a very large mass ($ga 10^{10}msun$) 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.
We review observations of ultraluminous X-ray sources (ULXs). X-ray spectroscopic and timing studies of ULXs suggest a new accretion state distinct from those seen in Galactic stellar-mass black hole binaries. The detection of coherent pulsations ind
Although ultra-luminous X-ray sources (ULX) are important for astrophysics due to their extreme apparent super-Eddington luminosities, their nature is still poorly known. Theoretical and observational studies suggest that ULXs could be a diversified
Mass outflows from Eddington-limited accreting compact objects appear to be a very widespread phenomenon. They may provide the soft excess observed in quasars and ULXs, and imply that such objects have a major effect on their surroundings. In particu
The recent discovery of a neutron star accretor in the ultra-luminous X-ray source M82 X-2 challenges our understanding of high-mass X-ray binary formation and evolution. By combining binary population synthesis and detailed mass-transfer models, how
We study spectral variability of 11 ultraluminous X-ray sources (ULX) using archived XMM-Newton and Chandra observations. We use three models to describe the observed spectra: a power-law, a multi-colour disc (MCD) and a combination of these two mode