Infrared and hard X-ray diagnostics of AGN identification from the Swift/BAT and AKARI all-sky surveys

TL;DR

This study combines Swift/BAT and AKARI survey data to analyze the correlation between infrared and hard X-ray emissions in local AGN, proposing diagnostics for classifying AGN types and identifying Compton-thick sources.

Contribution

It introduces new photometric diagnostics based on infrared and X-ray data for classifying AGN and distinguishing Compton-thick sources in all-sky surveys.

Findings

Strong correlation between infrared and hard X-ray luminosities in AGN.

Radio galaxies and Seyfert 1s show similar emission processes.

Diagnostics effectively identify Compton-thick AGN and differentiate them from starbursts.

Abstract

We combine data from two all-sky surveys in order to study the connection between the infrared and hard X-ray (>10keV) properties for local active galactic nuclei (AGN). The Swift/Burst Alert Telescope all-sky survey provides an unbiased, flux-limited selection of hard X-ray detected AGN. Cross-correlating the 22-month hard X-ray survey with the AKARI all-sky survey, we studied 158 AGN detected by the AKARI instruments. We find a strong correlation for most AGN between the infrared (9, 18, and 90 micron) and hard X-ray (14-195 keV) luminosities, and quantify the correlation for various subsamples of AGN. Partial correlation analysis confirms the intrinsic correlation after removing the redshift contribution. The correlation for radio galaxies has a slope and normalization identical to that for Seyfert 1s, implying similar hard X-ray/infrared emission processes in both. In contrast, Compton-thick sources show a large deficit in the hard X-ray band, because high gas column densities diminish even their hard X-ray luminosities. We propose two photometric diagnostics for source classification: one is an X-ray luminosity vs. infrared color diagram, in which type 1 radio-loud AGN are well isolated from the others in the sample. The other uses the X-ray vs. infrared color as a useful redshift-independent indicator for identifying Compton-thick AGN. Importantly, Compton-thick AGN and starburst galaxies in composite systems can also be differentiated in this plane based upon their hard X-ray fluxes and dust temperatures. This diagram may be useful as a new indicator to classify objects in new and upcoming surveys such as WISE and NuSTAR.