A New Sample of Obscured AGNs Selected from the XMM-Newton and AKARI Surveys

TL;DR

This study introduces a new sample of obscured AGNs identified through combined X-ray and infrared data, demonstrating high success in detecting obscured and elusive AGNs, including Compton-thick types, using specific selection criteria.

Contribution

The paper presents a novel selection method for obscured AGNs based on X-ray and infrared flux ratios, significantly increasing detection efficiency and identifying new elusive AGNs.

Findings

26 out of 48 candidates show X-ray evidence of obscuration.

Six are classified as Compton-thick AGNs.

Success rate of 92% in identifying obscured AGNs using the criteria.

Abstract

We report a new sample of obscured active galactic nuclei (AGNs) selected from the XMM serendipitous source and AKARI point-source catalogs. We match X-ray sources with infrared (18 and 90 micron) sources located at |b|>10 deg to create a sample consisting of 173 objects. Their optical classifications and absorption column densities measured by X-ray spectra are compiled and study efficient selection criteria to find obscured AGNs. We apply the criteria (1) X-ray hardness ratio defined by using the 2-4.5 keV and 4.5-12 keV bands >-0.1 and (2) EPIC-PN count rate (CR) in the 0.2-12 keV to infrared flux ratio CR/F90<0.1 or CR/F18<1, where F18 and F90 are infrared fluxes at 18 and 90 micron in Jy, respectively, to search for obscured AGNs. X-ray spectra of 48 candidates, for which no X-ray results have been published, are analyzed and X-ray evidence for the presence of obscured AGNs such as a convex shape X-ray spectrum indicative of absorption of NH~10^{22-24} cm^-2, a very flat continuum, or a strong Fe-K emission line with an equivalent width of >700 eV is found in 26 objects. Six among them are classified as Compton-thick AGNs, and four are represented by either Compton-thin or Compton-thick spectral models. The success rate of finding obscured AGNs combining our analysis and the literature is 92% if the 18 micron condition is used. Of the 26 objects, 4 are optically classified as an HII nucleus and are new "elusive AGNs" in which star formation activity likely overwhelms AGN emission in the optical and infrared bands.