The connection between merging double compact objects and the ultraluminous X-ray sources

We explore the different formation channels of merging double compact objects (DCOs: BH–BH/BH–NS/NS–NS) that went through an ultraluminous X-ray phase (ULX: X-ray sources with apparent luminosity exceeding 10³⁹ erg s⁻¹⁠). There are many evolutionary scenarios which can naturally explain the formation of merging DCO systems: isolated binary evolution, dynamical evolution inside dense clusters and chemically homogeneous evolution of field binaries. It is not clear which scenario is responsible for the majority of LIGO/Virgo sources. Finding connections between ULXs and DCOs can potentially point to the origin of merging DCOs as more and more ULXs are discovered. We use the STARTRACK population synthesis code to show how many ULXs will form merging DCOs in the framework of isolated binary evolution. Our merger rate calculation shows that in the local Universe typically 50 per cent of merging BH–BH progenitor binaries have evolved through a ULX phase. This indicates that ULXs can be used to study the origin of LIGO/Virgo sources. We have also estimated that the fraction of observed ULXs that will form merging DCOs in future varies between 5 per cent and 40 per cent depending on common envelope model and metallicity.