A New Sample of Buried Active Galactic Nuclei Selected from the Second XMM-Newton Serendipitous Source Catalogue

TL;DR

This study analyzes X-ray spectra of 22 AGNs to identify buried active nuclei with small scattering fractions, revealing many are heavily obscured and likely embedded in thick tori, advancing understanding of AGN structure.

Contribution

It introduces a new sample of buried AGNs selected via hardness ratios from the XMM-Newton catalogue, highlighting their unique X-ray and optical properties.

Findings

Most objects have scattering fractions below 3%, typical for buried AGNs.

Eight objects have scattering fractions less than 0.5%, indicating very thick obscuring tori.

Some AGNs show smaller [O III] luminosities relative to X-ray luminosities, suggesting smaller narrow line regions.

Abstract

We present the results of X-ray spectral analysis of 22 active galactic nuclei (AGNs) with a small scattering fraction selected from the Second XMM-Newton Serendipitous Source Catalogue using hardness ratios. They are candidates of buried AGNs, since a scattering fraction, which is a fraction of scattered emission by the circumnuclear photoionized gas with respect to direct emission, can be used to estimate the size of the opening part of an obscuring torus. Their X-ray spectra are modeled by a combination of a power law with a photon index of 1.5-2 absorbed by a column density of 10^23-24 cm^-2, an unabsorbed power law, narrow Gaussian lines, and some additional soft components. We find that scattering fractions of 20 among 22 objects are less than a typical value (3%) for Seyfert2s observed so far. In particular, those of eight objects are smaller than 0.5%, which are in the range for buried AGNs found in recent hard X-ray surveys. Moreover, [O III] lambda5007 luminosities at given X-ray luminosities for some objects are smaller than those for Seyfert2s previously known. This fact could be interpreted as a smaller size of optical narrow emission line regions produced in the opening direction of the obscuring torus. These results indicate that they are strong candidates for the AGN buried in a very geometrically thick torus.