The X-ray view of optically selected dual AGN

TL;DR

This study uses multi-wavelength data to analyze optically selected dual AGN, revealing higher obscuration and luminosity trends related to merging stages, and demonstrating X-ray observations as effective for confirming dual AGN systems.

Contribution

It provides a comprehensive multi-wavelength analysis of dual AGN, highlighting the relationship between obscuration, luminosity, and merging stage, and emphasizes the effectiveness of X-ray observations for dual AGN confirmation.

Findings

Over 80% of X-ray detected pairs host confirmed AGN.

AGN luminosity increases as the separation decreases.

Pairs show higher obscuration than isolated AGN.

Abstract

We present a study of optically selected dual AGN with projected separations of 3--97~kpc. Using multi-wavelength (MWL) information (optical, X-rays, mid-IR), we characterized the intrinsic nuclear properties of this sample and compared them with those of isolated systems. Among the 124 X-ray detected AGN candidates, 52 appear in pairs and 72 as single X-ray sources. Through MWL analysis, we confirmed the presence of the AGN in a fraction >80\% of the detected targets in pairs (42 over 52). X-ray spectral analysis confirms the trend of increasing AGN luminosity with decreasing separation, suggesting that mergers may have contributed in triggering more luminous AGN. Through X/mid-IR ratio $vs$ X-ray colors, we estimated a fraction of Compton-thin AGN (with 10$^{22}$ cm$^{-2}$ $<$ N$_{\rm H} <$10$^{24}$ cm$^{-2}$) of about 80\%, while about 16\% are Compton thick (CT, with N$_{\rm H}>$10$^{24}$ cm$^{-2}$) sources. These fractions of obscured sources are larger than those found in samples of isolated AGN, confirming that pairs of AGN show higher obscuration. This trend is further confirmed by comparing the de-reddened [O\ III] emission with the observed X-ray luminosity. However, the derived fraction of Compton-thick sources in this sample at early stage of merging is lower than reported for late-merging dual-AGN samples. Comparing N$_{\rm H}$ from X-rays with that derived from E(B-V) from Narrow Line Regions, we find that the absorbing material is likely associated with the torus or the Broad Line Regions. We also explored the X-ray detection efficiency of dual-AGN candidates, finding that, when observed properly (at on-axis positions and with long exposures), X-ray data represent a powerful way to confirm and investigate dual-AGN systems.