The Soft X-ray Spectrum of Scattering-Dominated AGN

Authors: Netzer, H.1, Turner, T.J.2,3, George, I.M.2,3
Affiltns: 1Tel Aviv University, 2 LHEA+ NASA/GSFC, 3 USRA,
Journal: ApJ, 504, 680
Publication Date: 1998 Sep 10

Abstract

This paper discusses the properties of scattering-dominated active galactic nuclei (AGNs). We define these to be AGNs for which the direct line of sight to the continuum source is obscured by Compton-thick material. The aim is to construct, for the first time, a model consistent with X-ray line luminosities, line ratios, and various luminosity indicators. The ASCA spectra of six such sources show several X-ray lines that can be reliably measured, mostly due to highly ionized magnesium, silicon, sulfur, and iron. These enable us to investigate the physical conditions of the scattering material. The sources show evidence of He-like and H-like iron lines that are likely to be produced in hot (T~1e6 K) photoionized gas. By measuring the equivalent widths of the lines, and by constructing a diagnostic line-ratio diagram, we demonstrate that the silicon and magnesium lines are produced by the same gas that emits the highly ionized iron lines. The properties of this gas are rather different from the properties of warm absorbers in type I AGNs. Neutral 6.4 keV iron lines are also detected, originating in a different component which can be either Compton-thin or Compton-thick. The best-measured iron lines suggest an enhancement of nFe/nH by a factor of ~2 compared to solar, in both the hot and cool Compton-thin components. We further show that in four of the sources, the Fe K_alpha 6.4 keV/H_beta (4861 Angstrom) line ratio is consistent with that predicted for typical narrow-line region clouds, provided the column density is larger than 10^{22.5} cm^{-2), alpha_{ox} is smaller than 1.3, and the reddening-corrected H_beta is known. For some sources, this is a viable alternative to the commonly assumed Compton-thick medium as the origin of the 6.4 keV iron line.

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