The $N_H$ Distribution of Hard X-ray Selected AGN in the NEP Field

TL;DR

This study analyzes the distribution of hydrogen column densities in hard X-ray selected active galactic nuclei (AGN) within the NEP field, providing insights into the fraction of obscured, Compton Thick AGN using deep NuSTAR and XMM-Newton data.

Contribution

First spectroscopic analysis of a 60-source AGN sample in the NEP field, measuring the $N_H$ distribution and obscured AGN fraction with bias correction.

Findings

Observed Compton Thick fraction of 13%

Corrected underlying CT fraction of 32%

Results align with population synthesis models

Abstract

X-ray surveys are one of the most unbiased methods for detecting Compton Thick (CT; $N_{{\mathrm{H}}} \geq 10^{24}$ cm$^{-2}$) AGN, which are thought to comprise up to $60\%$ of AGN within $z \lesssim 1.0$. These CT AGN are often difficult to detect with current instruments, but the X-ray data within the JWST-North Ecliptic Pole (NEP) Time Domain Field (TDF) present a unique opportunity to study faint and obscured AGN. The NEP contains the deepest NuSTAR survey to date, and Zhao et al. (2024) detected 60 hard X-ray sources from the combined exposure of NuSTAR's Cycle 5 and 6 observations. In this work, we utilize the NuSTAR Cycle 5+6+8+9 data and simultaneous XMM-Newton observations in order to perform the first spectroscopic analysis of the 60-source catalog. We present this analysis and measure the $N_{{\mathrm{H}}}$ distribution of the sample. We measure an observed CT fraction of $0.13_{-0.04}^{+0.15}$ down to an observed $8-24$ keV flux of $6.0 \times 10^{-14}$ erg/s/cm$^{2}$, and we correct our analysis for absorption bias to estimate an underlying CT fraction of $0.32_{-0.08}^{+0.23}$. The derived obscuration distribution and CT fraction are consistent with population synthesis models and previous surveys.