How can gas falling toward a neutron star or black hole generate X-rays?

Gas falling toward a neutron star or black hole generates X-rays primarily through the formation of an accretion disk. As matter from a companion star is pulled toward the compact object, it possesses angular momentum that prevents it from falling in a straight line, forcing it to spiral inward instead.12

Within this disk, viscosity and turbulence act as friction, causing the gas to lose energy and angular momentum as it drifts inward. This process converts the gravitational potential energy of the infalling material into intense heat. Because the material is accelerated to extreme speeds—often approaching the speed of light—the disk reaches temperatures ranging from thousands to millions of kelvins. At these high temperatures, the disk emits significant amounts of radiation in the X-ray spectrum.13

Additionally, parts of the disk may evaporate to form a low-density, high-temperature corona that emits hard X-rays, while in some systems, gas may be channeled directly to the poles of a neutron star to produce pulsed X-ray emissions.13