Are optical and X-ray AGN mostly disjoint?

TL;DR

This study clarifies the relationship between optical and X-ray AGN populations, showing they are largely overlapping, contrary to previous claims of disjointness, through re-analysis and comparison of different samples.

Contribution

The paper demonstrates that optical and X-ray AGN are not mostly disjoint by re-analyzing data and correcting prior misinterpretations, providing a clearer understanding of AGN populations.

Findings

Most X-ray AGN are also optical AGN, contradicting previous claims.

Re-analysis of data shows significant overlap between optical and X-ray AGN.

Previous studies underestimated the overlap due to sampling and criteria differences.

Abstract

The relationship between the populations of optically and X-ray selected Active Galactic Nuclei (AGN) has been unclear due to divergent results from different studies. Arnold et al. (2009) claim that X-ray AGN are almost entirely disjoint from optical AGN, while the Swift-BAT 70-month hard X-ray survey reported that 553 of their 711 X-ray AGN are optical. In this work, we set out to understand this difference by cross-checking between these studies and examining their sampling and AGN-selection criteria. We also re-analyze the X-ray and optical AGN in 16 groups and clusters reported by Arnold et al. using our own optical spectrum fitting techniques. We find that 6 of the 8 X-ray AGN in the Arnold et al. sample are also optical AGN, contrary to Arnold et al.'s report that only 1 of the 8 X-ray AGN is also an optical AGN, thereby falsifies their conclusion that optical and X-ray AGN are nearly disjoint sets.