# Inferring Compton-thick AGN candidates at z>2 with Chandra using the >8 keV restframe spectral curvature

## TL;DR

This paper develops and applies a spectral curvature method to identify Compton-thick AGN at high redshift using Chandra data, providing insights into their population and evolution.

## Contribution

The study introduces a new spectral curvature technique for detecting high-redshift Compton-thick AGN and applies it to deep field data, offering a novel approach for future X-ray surveys.

## Key findings

- Measured CT AGN fractions of 17% and 15-32% in different surveys.
- Found no significant redshift evolution in CT AGN fraction at high redshift.
- Provided spectral curvature equations applicable to future X-ray missions.

## Abstract

To fully understand cosmic black hole growth we need to constrain the population of heavily obscured active galactic nuclei (AGN) at the peak of cosmic black hole growth ($z\sim$1-3). Sources with obscuring column densities higher than $\mathrm{10^{24}}$ atoms $\mathrm{cm^{-2}}$, called Compton-thick (CT) AGN, can be identified by excess X-ray emission at $\sim$20-30 keV, called the "Compton hump". We apply the recently developed Spectral Curvature (SC) method to high-redshift AGN (2<z<5) detected with Chandra. This method parametrizes the characteristic "Compton hump" feature cosmologically redshifted into the X-ray band at observed energies <10 keV. We find good agreement in CT AGN found using the SC method and bright sources fit using their full spectrum with X-ray spectroscopy. In the Chandra deep field south, we measure a CT fraction of $\mathrm{17^{+19}_{-11}\%}$ (3/17) for sources with observed luminosity $\mathrm{>5\times 10^{43}}$ erg $\mathrm{s^{-1}}$. In the Cosmological evolution survey (COSMOS), we find an observed CT fraction of $\mathrm{15^{+4}_{-3}\%}$ (40/272) or $\mathrm{32\pm11 \%}$ when corrected for the survey sensitivity. When comparing to low redshift AGN with similar X-ray luminosities, our results imply the CT AGN fraction is consistent with having no redshift evolution. Finally, we provide SC equations that can be used to find high-redshift CT AGN (z>1) for current (XMM-Newton) and future (eROSITA and ATHENA) X-ray missions.