Final answer:
Compact X-ray sources in binary systems with more than 3 solar masses are Black hole candidates, as they surpass the mass limits for white dwarfs and neutron stars.
Step-by-step explanation:
Compact X-ray sources in binary systems where the source has more than 3 solar masses (M⊙) are Black hole candidates. This is due to the fact that the mass threshold exceeds the maximum mass of a neutron star and far surpasses the Chandrasekhar limit for white dwarfs, which is about 1.4 M⊙. Observations of such systems often reveal (1) one star of the pair is not visible, (2) the flickering X-ray emission indicating an accretion disk, and (3) the massive unseen object, which rules out most other types of stellar remnants, pointing to the presence of a stellar-mass black hole. For example, the first discovered black hole binary system, Cygnus X-1, features a visible star and an invisible companion with a mass around 15 M⊙, indicating it is a black hole.