Ultraluminous X-ray sources ( ULXs ) with L _ { x } > 10 ^ { 39 } ergs s ^ { -1 } have been discovered in great numbers in external galaxies with ROSAT , Chandra , and XMM .
The central question regarding this important class of sources is whether they represent an extension in the luminosity function of binary X-ray sources containing neutron stars and stellar-mass black holes ( BHs ) , or a new class of objects , e.g. , systems containing intermediate-mass black holes ( 100 – 1000 M _ { \odot } ) .
We have carried out a theoretical study to test whether a large fraction of the ULXs , especially those in galaxies with recent star formation activity , can be explained with binary systems containing stellar-mass black holes .
To this end , we have applied a unique set of binary evolution models for black-hole X-ray binaries , coupled to a binary population synthesis code , to model the ULXs observed in external galaxies .
We find that for donor stars with initial masses \ga 10 ~ { } M _ { \odot } the mass transfer driven by the normal nuclear evolution of the donor star is sufficient to potentially power most ULXs .
This is the case during core hydrogen burning and , to an even more pronounced degree , while the donor star ascends the giant branch , though the latter phases lasts only \sim 5 % of the main sequence phase .
We show that with only a modest violation of the Eddington limit , e.g. , a factor of \sim 10 , both the numbers and properties of the majority of the ULXs can be reproduced .
One of our conclusions is that if stellar-mass black-hole binaries account for a significant fraction of ULXs in star-forming galaxies , then the rate of formation of such systems is \sim 3 \times 10 ^ { -7 } yr ^ { -1 } normalized to a core-collapse supernova rate of 0.01 yr ^ { -1 } .