Which X-ray nova in Monoceros contains a black hole candidate and the star V616 Monocerotis? The answer is A0620-00.
A0620-00 is a binary system in Monoceros also known by the variable-star designation V616 Monocerotis. Its visible component is a K-type main-sequence star, while its unseen companion has a mass too large for a neutron star, making it a stellar-mass black-hole candidate.
The two objects orbit one another in only about 7.75 hours. Matter pulled from the companion forms an accretion disc around the black hole; friction and heating in that disc produce strong X-rays. The system’s changing visible brightness also reflects the companion’s distorted, ellipsoidal shape as it revolves.
A0620-00 produced major X-ray outbursts in 1917 and 1975. During the 1975 event, detected by the Ariel 5 satellite, it became the brightest X-ray source in the sky. Its black-hole nature was established through later dynamical studies, rather than by seeing the black hole directly.