The first black hole orbiting a Be star

Abstract

Stellar-mass black holes have been discovered in binary systems. The study of radial velocities of the companion stars allows us to restrict the masses of the compact objects. The Be/Compact Object binary system MWC 656 (also known as HD 215227) shows Fe II 4583Å and He II 4686Å emission lines (both double-peaked) on its spectrum. These lines indicate the existence of disks surrounding the binary components. Through the analysis of the radial velocities of He II (formed in the surrounding gas of the compact object) and Fe II (formed in the circumstellar disk around the Be star) emission lines using the SBOP program, the existence of a compact object with a mass in the range of 3.7 to 6.9 solar masses (within errors) is proved. As the lower value of the mass of the compact object is above the Tolman-Oppenheimer-Volkoff limit (3 solar masses), the Be star companion is classified as a black hole. Around 80 Be X-ray binaries have been found up to now. All but MWC 656 are supposed to be Be/Neutron star systems. For this reason, MWC 656 has been considered the first binary system consisting of a Be star and a black hole. Finally, the difficulty of detecting these Be/Black hole binaries through X-ray surveys is explained thanks to the study of black holes states