Exploring the formation of binary systems of Ultraluminous X-ray Sources
Abstract
Ultraluminous X-ray sources (ULXs) are observationally defined as non-nuclear extragalactic X-ray point sources with luminosity exceeding the Eddington limit for a ∼10 solar mass compact object. They are the key to exploring extreme super-Eddington accretion regimes and the unknown distribution of black hole (BH) masses above ∼20 solar masses. To address these issues and shed light on the nature of ULXs, we started a systematic investigation of the dynamics and emission properties of BH binaries in young star clusters for different metallicities, computing optical colour-magnitude diagrams for all systems that enter an active Roche lobe accretion phase. The calculation accounts for the emission of both the donor star and the irradiated accretion disc, under different assumptions on the physical regime of the accretion disc. Comparison with the available X-ray data and HST photometry of ULX counterparts shows that binary systems hosting a rather massive stellar BH originating from the direct collapse of a massive low-metallicity star and formed through dynamical interaction in the cluster can produce a ULX with emission properties consistent with the observed ones. Important constraints on the range of allowed BH masses and on the properties of the accretion flow are also derived.
- Publication:
-
The X-ray Universe 2014
- Pub Date:
- July 2014
- Bibcode:
- 2014xru..confE.211Z