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

Samantha Creech, Francesca Civano, Daniel R. Wik, Ross Silver, Xiurui Zhao, Rafael Ortiz, Tonima Ananna, Normal A. Grogin, Rolf Jansen, Anton M. Koekemoer, Christopher N. A. Willmer, Rogier A. Windhorst

First listed 2025-10-30 | Last updated 2025-10-30

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.

Short digest

NuSTAR Cycles 5+6+8+9 plus simultaneous XMM-Newton enable the first full spectral analysis of all 60 hard X-ray sources in the JWST–NEP TDF, yielding the sample’s N_H distribution. The team finds an observed Compton-thick fraction of 0.13_{-0.04}^{+0.15} to an 8–24 keV flux limit of 6.0×10^-14 erg s^-1 cm^-2, which rises to 0.32_{-0.08}^{+0.23} after correcting for absorption bias. These results, derived from the deepest NuSTAR survey to date, align with AGN population-synthesis expectations and prior surveys, sharpening constraints on obscured growth contributing to the CXB. The work provides a clean, hard-band-selected benchmark for CT demographics in the NEP field.

Key figures to inspect

• Figure 1 — Redshift and 3–24 keV luminosity distributions: verify the z–L_X range of the NEP sample and how quasars/galaxies/unclassified sources populate it, setting context for where CT candidates sit in parameter space.
• Figure 2 — Net counts histogram (NuSTAR vs XMM-Newton): assess spectral leverage per source and which subset (high-count vs low-count) dominates the N_H constraints and CT identification.
• Figure 3 — Baseline spectral model schematic: track how absorbed and scattered components plus Galactic emission map onto XMM (soft) and NuSTAR (hard) bands, clarifying where CT sensitivity originates.
• Figure 4 — N_H from baseline vs clumpy models: check robustness of CT classifications and identify sources that shift across the CT threshold; note red-square points lacking redshifts as a potential systematic.