Three years Swift-BAT Survey of AGN: reconciling theory and observations?

TL;DR

This study analyzes Swift-BAT data over three years to determine the true fraction of Compton-thick AGN, revealing a higher intrinsic fraction than observed and exploring the luminosity dependence of AGN absorption.

Contribution

First to correct for detection bias and estimate the intrinsic Compton-thick AGN fraction using Swift-BAT data, and to demonstrate the luminosity dependence of AGN absorption.

Findings

Intrinsic Compton-thick AGN fraction is approximately 20%.

Absorbed AGN are generally less luminous than unabsorbed ones.

The fraction of obscured AGN decreases at very low luminosities.

Abstract

It is well accepted that unabsorbed as well as absorbed AGN are needed to explain the nature and the shape of the Cosmic X-ray background, even if the fraction of highly absorbed objects (dubbed Compton-thick sources) substantially still escapes detection. We derive and analyze the absorption distribution using a complete sample of AGN detected by Swift-BAT in the first three years of the survey. The fraction of Compton-thick AGN represents only 4.6% of the total AGN population detected by Swift-BAT. However, we show that once corrected for the bias against the detection of very absorbed sources the real intrinsic fraction of Compton-thick AGN is 20$^{+9}_{-6}$%. We proved for the first time (also in the BAT band) that the anti-correlation of the fraction of absorbed AGN and luminosity it tightly connected to the different behavior of the luminosity functions (XLFs) of absorbed and unabsorbed AGN. This points towards a difference between the two subsamples of objects with absorbed AGN being, on average, intrinsically less luminous than unobscured ones. Moreover the XLFs show that the fraction of obscured AGN might also decrease at very low luminosity. This can be successfully interpreted in the framework of a disk cloud outflow scenario as the disappearance of the obscuring region below a critical luminosity. Our results are discussed in the framework of population synthesis models and the origin of the Cosmic X-ray Background.