Compton Thick AGN in the 70 Month Swift-BAT All-Sky Hard X-ray Survey: a Bayesian approach

TL;DR

This study identifies and characterizes Compton-thick AGN in the Swift-BAT survey using a Bayesian approach, revealing their luminosity function and estimating their fraction among AGN.

Contribution

It introduces a Bayesian Markov chain method to accurately estimate column densities and derives the first parametric luminosity function for Compton-thick AGN.

Findings

Approximately 7% of AGN are Compton-thick.

First parametric luminosity function for Compton-thick AGN.

Double power-law shape with a break at 2 x 10^{42} erg/s.

Abstract

The 70-month Swift/BAT catalogue provides a sensitive view of the extragalactic X-ray sky at hard energies (>10 keV) containing about 800 Active Galactic Nuclei. We explore its content in heavily obscured, Compton-thick AGN by combining the BAT (14-195 keV) with the lower energy XRT (0.3-10 keV) data. We apply a Bayesian methodology using Markov chains to estimate the exact probability distribution of the column density for each source. We find 53 possible Compton-thick sources (with probability 3 to 100%) translating to a ~7% fraction of the AGN in our sample. We derive the first parametric luminosity function of Compton-thick AGN. The unabsorbed luminosity function can be represented by a double power-law with a break at $L_{\star} 2 \times 10^{42}$ $\rm ergs~s^{-1}$ in the 20-40 keV band.