The Difference in Narrow Fe K\alpha Line Emission Between Seyfert 1 and Seyfert 2 Galaxies

TL;DR

This study compares the narrow Fe Kalpha line emission between Seyfert 1 and Seyfert 2 galaxies, revealing anisotropy and the need for correction factors to use the line as an intrinsic AGN luminosity indicator.

Contribution

It provides the first observational evidence of anisotropic narrow Fe Kalpha emission and discusses implications for AGN unification models and torus structure.

Findings

Seyfert 2 galaxies have significantly weaker Fe Kalpha lines than Seyfert 1s.

The difference supports anisotropic emission of the Fe Kalpha line.

Results align with models of a clumpy obscuring torus.

Abstract

We compile a sample of 89 Seyfert galaxies with both [OIV] 25.89 um line luminosities observed by Spitzer IRS and X-ray spectra observed by XMM EPIC. Using [OIV] emission as proxy of AGN intrinsic luminosity, we find that although type 2 AGNs have higher line equivalent width, the narrow Fe Kalpha line in Compton-Thin and Compton-Thick Seyfert 2 galaxies are 2.9 +0.8 -0.6 and 5.6 +1.9 -1.4 times weaker in terms of luminosity than Seyfert 1 galaxies respectively. This indicates different correction factors need to be applied for various types of AGNs before the narrow Fe Kalpha line luminosity could serve as intrinsic AGN luminosity indicator. We also find Seyfert 1 galaxies in our sample have on average marginally larger line width and higher line centroid energy, suggesting contamination from highly ionized Fe line or broader line emission from much smaller radius, but this effect is too weak to explain the large difference in narrow Fe Kalpha line luminosity between type 1 and type 2 AGNs. This is the first observational evidence showing the narrow Fe Kalpha line emission in AGNs is anisotropic. The observed difference is consistent with theoretical calculations assuming a smoothly distributed obscuring torus, and could provide independent constraints on the clumpiness of the torus.