AGN corona

The term AGN corona indicates an X-ray source near or surrounding an active galactic nucleus (AGN), the theorized cause of observed Compton reflections, which are presumed to be such reflections of the corona's X-rays by accretion-disk material and/or other of the AGN's surrounding clouds. Many AGNs are too distant to resolve and all this must be deduced from spectral signatures and light curves. The coronas are a subject of current theory, and models devised to explain observations have led to theories regarding their size (corona size), whether the source is steady or sporadic, and whether it surrounds the supermassive black hole (SMBH) or is merely nearby. One model, the lamppost model, presumes the (X-ray emitting) corona does not surround the SMBH but occupies a relatively small volume located on the SMBH's rotation axis. Reverberation mapping uses the time interval between a change in the amount of X-rays (from the corona) and an apparently-corresponding change in the Compton reflection; such a time interval yields a minimum distance between the X-ray source and the Compton reflection. The existence and consistency of such time intervals is a reason for presuming the corona is in a distinct, separate location. The corona is presumed to be electrons, which I believe are presumed to be held in place by features of a magnetic field, somewhat analogous to some aspects of the Sun's activity.