How do accretion disks produce X-ray emission around black holes?

• A. Gas in the disk heats to millions of kelvin as it loses gravitational potential energy, emitting thermal X-rays; inner disk regions produce hard X-rays via Comptonization.
• B. X-rays come from dark matter annihilation in the halo.
• C. X-rays are only produced by stellar companions in binary systems.
• D. X-rays are emitted uniformly across the disk with no dependence on radius.

Answer: (A)

Accretion disks shine in X-rays because gas spiraling toward a black hole loses gravitational potential energy and heats up to extreme temperatures. As matter moves inward, the released energy heats the disk to about millions of kelvin, so the disk emits thermal radiation in the X-ray range. The hottest, innermost regions reach the highest temperatures, producing the soft-to-hard X-ray photons characteristic of a thermal spectrum with higher-energy tails.

Beyond this thermal glow, the very inner disk and a surrounding hot region called the corona contain energetic electrons. Soft photons from the disk scatter off these hot electrons, gaining energy in the process. This Comptonization boosts photons to higher energies, creating the hard X-ray component of the observed spectrum. So the X-ray emission has two natural parts: a soft, thermal X-ray component from the hot disk and a hard X-ray component from Compton upscattering in the inner regions.

The emission is not uniform across the disk; it increases in energy toward the inner regions where gravity does the most work and where the hot electrons for Comptonization reside. Other sources like dark matter annihilation or emission from a companion star don’t reproduce this radius-dependent, disk-based X-ray spectrum.