Why are black holes often detected via X-ray binaries?

• A. They emit strong gamma rays.
• B. They are bright in radio due to jets.
• C. The companion star's light curves reveal the orbit.
• D. The accretion disk reaches X-ray-emitting temperatures, producing X-ray emission.

Answer: (D)

Gas from a companion star that is captured by a black hole forms an accretion disk. As this gas spirals inward, gravitational energy is converted into heat, heating the inner parts of the disk to temperatures of millions of kelvin. At those temperatures the disk emits primarily X-rays, producing a strong X-ray signal that we can detect from Earth. This X-ray emission is the hallmark we rely on to identify black holes in binary systems, because the black hole itself doesn’t shine; it’s the hot, in-falling material around it that glows in X-rays.

Other options aren’t as reliable indicators. Radio emission from jets can occur in some black hole systems, but not all, and it’s not the universal signature. The companion star’s light curve can reveal that there is a binary and help measure orbital motion, but it doesn’t by itself confirm a black hole. Gamma-ray emission exists in some extreme cases, but most black hole X-ray binaries are characterized by their X-ray output from the hot accretion disk, not gamma rays.