The Eclipsing Black Hole X-ray Binary M33 X-7: Understanding the Current Properties

TL;DR

This study investigates the formation of the M33 X-7 black hole binary, revealing that single star evolution models cannot fully explain the observed properties, implying prior interactions are necessary.

Contribution

It demonstrates the limitations of single star evolution models in explaining M33 X-7's properties and highlights the need to consider binary interactions.

Findings

Single star models cannot account for the observed companion star's luminosity and mass.

A best-fit model suggests a ~13 solar mass black hole and a ~54 solar mass companion.

Prior interactions between the black hole progenitor and companion are likely essential.

Abstract

We explore the formation and evolution of the black hole X-ray binary system M33 X-7. In particular, we examine whether accounting for systematic errors in the stellar parameters inherent to single star models, as well as the uncertainty in the distance to M33, can explain the discrepancy between the observed and expected luminosity of the ~70 solar masses companion star. Our analysis assumes no prior interactions between the companion star and the black hole progenitor. We use four different stellar evolution codes, modified to include a variety of current stellar wind prescriptions. For the models satisfying the observational constraints on the donor star's effective temperature and luminosity, we recalculate the black hole mass, the orbital separation, and the mean X-ray luminosity. Our best model, satisfying simultaneously all observational constraints except the observationally inferred companion mass, consists of a ~13 solar masses black hole and a ~54 solar masses companion star. We conclude that a star with the observed mass and luminosity can not be explained via single star evolution models, and that a prior interaction between the companion star and the black hole progenitor should be taken into account.