New estimate of the black hole mass in the binary system MWC 656

Abstract

This work consists of the study of the MWC 656 binary system. It was the first binary system composed of a Be star and a black hole ever found. Black holes orbiting Be star are of great interest in the study of supernova explosions and formation of a binary system composed of a neutron star and a black hole because their merging in nearby galaxies could be detected via gravitational waves. We analysed the radial velocities from the photo-spheric He I absorption lines generated in the Be star and from the He II emission line generated in the accretion disc of the compact object. Two single-line fits and one double line fit are presented. From the single-line fits we observed that the epoch and the argument of periastron correspond to those of a binary system. However, we found di_erent eccentricities: e=0.41±0.08 from the Be star orbit and e=0.09±0.03 from the black hole orbit. In addition, from the double-line fit, fixing e to 0.2, we estimated the mass range of the compact object to be M2 =3.0±7.4M which still implies the presence of a black hole. Furthermore, we calculated that a mass loss of ΔM=(3.6±1.9) M_ in a supernova explosion is enough to explain an eccentricity of 0.2±0.1 and the measured space velocity of the binary system.