Simulating the X-ray luminosity of Be X-ray binaries: the case for black holes versus neutron stars

TL;DR

This paper uses simulations to compare X-ray luminosities of Be star binaries with black holes versus neutron stars, suggesting black hole systems are significantly fainter due to smaller circumstellar discs.

Contribution

It demonstrates how disc size and density depend on compact object mass and eccentricity, explaining the luminosity differences between black hole and neutron star binaries.

Findings

Be/black hole binaries are up to 10 times fainter than Be/neutron star binaries with the same eccentricity.

Disc size and density are influenced by the mass of the compact object and orbital eccentricity.

X-ray luminosity is heavily dependent on the size and density of the Be star's circumstellar disc.

Abstract

There are over 100 Be stars that are known to have neutron star companions but only one such system with a black hole. Previous theoretical work suggests this is not due to their formation but due to differences in X-ray luminosity. It has also been proposed that the truncation of the Be star's circumstellar disc is dependent on the mass of the compact object. Hence, Be star discs in black hole binaries are smaller. Since accretion onto the compact object from the Be star's disc is what powers the X-ray luminosity, a smaller disc in black hole systems leads to a lower luminosity. In this paper, simulations are performed with a range of eccentricities and compact object mass. The disc's size and density are shown to be dependent on both quantities. Mass capture and, in turn, X-ray luminosity are heavily dependent on the size and density of the disc. Be/black hole binaries are expected to be up to ~10 times fainter than Be/neutron star binaries when both systems have the same eccentricity and can be 100 times fainter when comparing systems with different eccentricity.