Lifting the Veil on Obscured Accretion: Active Galactic Nuclei Number Counts and Survey Strategies for Imaging Hard X-ray Missions

TL;DR

This paper predicts active galactic nuclei counts in hard X-ray bands for upcoming missions, emphasizing the importance of survey strategies to constrain obscured AGN populations and their evolution.

Contribution

It provides detailed predictions of AGN number counts across different X-ray bands and models, guiding future hard X-ray survey strategies and constraining obscured AGN evolution.

Findings

Majority of counts are Compton thin AGNs.

Over 10x increase in Compton thick counts from 6-10 keV to 10-30 keV.

Survey strategies can effectively constrain Compton thick evolution.

Abstract

Finding and characterizing the population of active galactic nuclei (AGNs) that produces the X-ray background (XRB) is necessary to connect the history of accretion to observations of galaxy evolution at longer wavelengths. The year 2012 will see the deployment of the first hard X-ray imaging telescope that, through deep extragalactic surveys, will be able to measure the AGN population at the energies where the XRB peaks (~20-30 keV). Here, we present predictions of AGN number counts in three hard X-ray bandpasses: 6-10 keV, 10-30 keV and 30-60 keV. Separate predictions are presented for the number counts of Compton thick AGNs, the most heavily obscured active galaxies. The number counts are calculated for five different models of the XRB that differ in the assumed hard X-ray luminosity function, the evolution of the Compton thick AGNs, and the underlying AGN spectral model. The majority of the hard X-ray number counts will be Compton thin AGNs, but there is a >10x increase in the Compton thick number counts from the 6-10 keV to the 10-30 keV band. The Compton thick population show enough variation that a hard X-ray number counts measurement will constrain the models. The computed number counts are used to consider various survey strategies for the NuSTAR mission, assuming a total exposure time of 6.2 Ms. We find that multiple surveys will allow a measurement of Compton thick evolution. The predictions presented here should be useful for all future imaging hard X-ray missions.