The connection between merging double compact objects and the Ultraluminous X-ray Sources

TL;DR

This paper investigates how ultraluminous X-ray sources (ULXs) are connected to the formation of merging double compact objects (DCOs), suggesting ULXs are significant indicators of the origins of gravitational wave sources.

Contribution

It demonstrates, using population synthesis, that about 50% of merging black hole binaries in the local universe pass through a ULX phase, linking ULXs to gravitational wave progenitors.

Findings

Approximately 50% of local merging BH-BH binaries evolve through ULX phases.

The fraction of ULXs that will become merging DCOs ranges from 5% to 40%.

ULXs can serve as observational probes for the origins of LIGO/Virgo sources.

Abstract

We explore the different formation channels of merging double compact objects (DCOs: BH-BH/BH-NS/NS-NS) that went through a ultraluminous X-ray phase (ULX: X-ray sources with apparent luminosity exceeding $10^{39}$ 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% 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% to 40% depending on common envelope model and metallicity.