Weighing in on black hole binaries with BPASS: LB-1 does not contain a 70M$_{\odot}$ black hole

TL;DR

This study uses stellar population models to show that the LB-1 system likely contains a black hole of 4-7 solar masses, not the previously claimed 70 solar masses, aligning with current stellar evolution theories.

Contribution

The paper demonstrates that the LB-1 system's properties are consistent with moderate-mass black holes, challenging earlier claims of an extremely massive black hole.

Findings

LB-1's black hole mass is likely 4-7 solar masses.

Gaia distance measurements are accurate for LB-1.

The system aligns with typical Galactic black hole binaries.

Abstract

The recent identification of a candidate very massive 70 M(Sun) black hole is at odds with our current understanding of stellar winds and pair-instability supernovae. We investigate alternate explanations for this system by searching the BPASS v2.2 stellar and population synthesis models for those that match the observed properties of the system. We find binary evolution models that match the LB-1 system, at the reported Gaia distance, with more moderate black hole masses of 4 to 7 M(Sun). We also examine the suggestion that the binary motion may have led to an incorrect distance determination by Gaia. We find that the Gaia distance is accurate and that the binary system is consistent with the observation at this distance. Consequently it is highly improbable that the black hole in this system has the extreme mass originally suggested. Instead, it is more likely to be representative of the typical black hole binary population expected in our Galaxy.