An active galactic nucleus (AGN) is a central region of a galaxy with very high luminosity. Excess emission has been observed in radio, infrared, visible light, X-ray, and gamma rays bands. A galaxy hosting one is called an active galaxy. X-rays are useful for identifying AGNs because virtually all produce them and they penetrate the surrounding galaxy, which have no X-ray sources strong enough to lead to confusion.
The radiation is assumed to be due to accretion of mass by a supermassive black hole. Often associated with a nucleus is a relativistic jet. The radiation as well as matter spun from an accretion disk is called the AGN outflow or AGN feedback (see star formation feedback) for which several possible effects are theorized:
The outflow creates a region of plasma whose heat causes it to have a lower density than the surrounding gas of the same pressure. The term AGN bubble is used for the region. The plasma can give off X-rays.
The extremely high luminosity of some AGNs (in theory, higher than could be sustained) has led to theories of pulses.
Some observed periodicity seems unlikely to be produced by a black hole, suggesting involvement of a pulsar.