Constraints on the binary black hole hypothesis for system LB-1

TL;DR

This paper investigates the nature of a 70-solar-mass dark object in LB-1, using H-alpha emission line analysis to rule out the binary black hole hypothesis and support a single black hole origin.

Contribution

It provides observational constraints that challenge the binary black hole explanation for LB-1, favoring a single black hole formed by stellar collapse.

Findings

Current binary black hole hypothesis is ruled out due to merger time constraints.

Previous binary black hole scenario is unlikely because of low orbital eccentricity.

The dark mass is most consistent with a single black hole formed by stellar collapse.

Abstract

At about 70 solar masses, the recently-discovered dark object orbited by a B-type star in the system LB-1 is difficult to understand as the end point of standard stellar evolution, except as a binary black hole (BBH). LB-1 shows a strong, broad H-alpha emission line that is best attributed to a gaseous disk surrounding the dark mass. We use the observed H-alpha line shape, particularly its wing extension, to constrain the inner radius of the disk and thereby the separation of a putative BBH. The hypothesis of a current BBH is effectively ruled out on the grounds that its merger time must be a small fraction of the current age of the B star. The hypothesis of a previous BBH that merged to create the current dark mass is also effectively ruled out by the low orbital eccentricity, due to the combination of mass loss and kick resulted from gravitational wave emission in any past merger. We conclude that the current dark mass is a single black hole produced by the highly mass-conserving, monolithic collapse of a massive star.