Question

What makes us think that the star system Cygnus X-1 contains a black hole?

A. It emits X rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a neutron star.

B. No light is emitted from this star system, so it must contain a black hole.

C. The fact that we see strong X-ray emission tells us that the system must contain a black hole.

D. Cygnus X-1 is a powerful X-ray burster, so it must contain a black hole.