A wide star-black-hole binary system from radial-velocity measurements

TL;DR

This study reports the discovery of a wide binary system in our galaxy consisting of a B-type star and a massive black hole, identified through radial velocity measurements, challenging existing stellar evolution models.

Contribution

It provides the first radial velocity evidence for a wide star-black-hole binary with a very massive black hole in a high-metallicity environment.

Findings

Black hole mass estimated at ~68 solar masses

Orbital period of the binary system is 78.9 days

System is a wide binary with implications for stellar evolution theories

Abstract

All stellar mass black holes have hitherto been identified by X-rays emitted by gas that is accreting onto the black hole from a companion star. These systems are all binaries with black holes below 30 M$_{\odot}$$^{1-4}$. Theory predicts, however, that X-ray emitting systems form a minority of the total population of star-black hole binaries$^{5,6}$. When the black hole is not accreting gas, it can be found through radial velocity measurements of the motion of the companion star. Here we report radial velocity measurements of a Galactic star, LB-1, which is a B-type star, taken over two years. We find that the motion of the B-star and an accompanying H$\alpha$ emission line require the presence of a dark companion with a mass of $68^{+11}_{-13}$ M$_{\odot}$, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. The gravitational wave experiments have detected similarly massive black holes$^{7,8}$, but forming such massive ones in a high-metallicity environment would be extremely challenging to current stellar evolution theories$^{9-11}$.