Magnetic Flare Origin of X-Rays in Active Galactic Nuclei: Photoionization Evidence

Abstract

We present full-disk X-ray reflection spectra for two currently popular accretion flow geometries for active galactic nuclei (AGNs): (1) the lamppost model, which is frequently used to discuss the iron-line reverberation in AGNs, and (2) the model in which X-rays are produced in magnetic flares above a cold accretion disk. The lamppost spectra contain several spectroscopic features that are characteristic of highly ionized material that is not seen in the X-ray spectra of most AGNs. The magnetic flare model, on the other hand, produces reflected spectra that are roughly a superposition of a power law and a neutral-like reflection and an iron Kα line; thus, these spectra are more in line with typical AGN X-ray spectra. Furthermore, because of the difference in the ionization structure of the illuminated material in the two models, the line equivalent width increases with the X-ray luminosity LX for the lamppost and decreases with LX for the flare model. In light of these theoretical insights, the recent iron-line reverberation studies of AGNs, the X-ray Baldwin effect, and the general lack of X-ray reflection features in distant quasars all suggest that, for high accretion rates, the cold accretion disk is covered by a thick, completely ionized Thomson skin. Because the latter is only possible when the X-rays are concentrated in small emitting regions, we believe that this presents strong evidence for the magnetic flare origin of X-rays in AGNs.