A Compton-thin Solution for the Suzaku X-ray Spectrum of the Seyfert 2 Galaxy Mkn 3

TL;DR

This study models the Suzaku X-ray spectrum of Mkn 3, revealing a globally Compton-thin medium with a lower average column density than the line-of-sight, challenging previous assumptions of uniform, Compton-thick matter distribution.

Contribution

It introduces a self-consistent model showing Mkn 3's global column density is significantly lower than the line-of-sight value, emphasizing the importance of global properties in AGN X-ray analysis.

Findings

Global column density is about 5 times smaller than line-of-sight density.

A uniform spherical model cannot reproduce the observed spectral features.

Clumpy matter distribution with varying densities is favored.

Abstract

Mkn 3 is a Seyfert 2 galaxy that is widely regarded as an exemplary Compton-thick AGN. We study the Suzaku X-ray spectrum using models of the X-ray reprocessor that self-consistently account for the Fe K$\alpha$ fluorescent emission line and the associated Compton-scattered, or reflection, continuum. We find a solution in which the average global column density, $0.234^{+0.012}_{-0.010} \times 10^{24} \ \rm cm^{-2}$, is very different to the line-of-sight column density, $0.902^{+0.012}_{-0.013} \times 10^{24} \ \rm cm^{-2}$. The global column density is $\sim 5$ times smaller than that required for the matter distribution to be Compton-thick. Our model accounts for the profiles of the Fe K$\alpha$ and Fe K$\beta$ lines, and the Fe K edge remarkably well, with a solar abundance of Fe. The matter distribution could consist of a clumpy medium with a line-of-sight column density higher than the global average. A uniform, spherically-symmetric distribution alone cannot simultaneously produce the correct fluorescent line spectrum and reflection continuum. Previous works on Mkn 3, and other AGN, that assumed a reflection continuum from matter with an infinite column density could therefore lead to erroneous or "puzzling" conclusions if the matter out of the line-of-sight is really Compton-thin. Whereas studies of samples of AGN have generally only probed the line-of-sight column density, with simplistic, one-dimensional models, it is important now to establish the global column densities in AGN. It is the global properties that affect the energy budget in terms of reprocessing of X-rays into infrared emission, and that constrain population synthesis models of the cosmic X-ray background.