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

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}$.