What structure of orbiting hot gas commonly powers a bright black-hole X-ray source?

An accretion disk is the structure of orbiting hot gas that commonly powers a bright black-hole X-ray source.

Material falling toward a black hole usually carries angular momentum, so it cannot plunge straight inward. Instead, it spreads into a flattened rotating disk. Friction and turbulence within the disk convert gravitational potential energy into heat, raising the gas to temperatures that can produce visible light, ultraviolet radiation, or X-rays.

The disk is outside the event horizon; the black hole itself does not shine. In many systems, magnetic fields also help launch narrow jets of particles perpendicular to the disk. The exact appearance depends on the black hole’s mass, spin, feeding rate, and the properties of the surrounding gas.

Accretion disks are not unique to black holes. They also occur around young stars, white dwarfs, and neutron stars. Their presence therefore indicates rapid accretion, not automatically a black hole.