A0620-00

A0620-00 (variable star designation V616 Monocerotis) is a binary system in Monoceros, about 3,300 light-years away. A K-type main-sequence star orbits an unseen companion every 7.75 hours; the companion is too massive to be a neutron star and must be a stellar-mass black hole, one of the nearest known.

The system flared in X-rays in 1917 and again in 1975, when the Ariel 5 satellite saw it as the brightest X-ray source in the sky. It is classified as an X-ray nova, and its black hole was identified in 1986.