Self-consistent population synthesis of active galactic nuclei from observational constraints in the X-rays

¹ Department of Astronomy, University of Geneva, 1290 Versoix, Switzerland
² Instituto de Estudios Astrofísicos, Universidad Diego Portales, Chile
³ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China

^⋆ Corresponding author: dimitra.gerolymatou@unige.ch

Received: 24 January 2025
Accepted: 16 June 2025

Abstract

The cosmic X-ray background (CXB) is produced by the emission of unresolved active galactic nuclei (AGNs), thus providing key information about the properties of the primary and reprocessed X-ray emission components of the AGN population. Equally important are studies of individual sources that provide additional constraints on the properties of AGNs, such as their luminosity and obscuration. Until now, these constraints have not been self-consistently addressed by intrinsically linking emission, absorption, and reflection. Here, we performed numerical simulations with the ray-tracing code REFLEX, which allows us to self-consistently model the X-ray emission of AGNs with flexible geometries for the circumnuclear medium. Using the REFLEX-simulated emission of an AGN population, we attempted to simultaneously reproduce the CXB and absorption properties measured in the X-rays, namely the observed fraction of N_H in bins of log(N_H) and the fraction of absorbed AGNs, including their redshift and luminosity evolution. We sampled an intrinsic X-ray-luminosity function and constructed gradually more complex, physically motivated geometrical models. We examined how well each model can match all observational constraints using a simulation-based inference (SBI) approach. We find that, while the simple unification model can reproduce the CXB, a luminosity-dependent dusty torus is needed to reproduce the absorption properties. When adding an accretion disc, the model best matches all constraints simultaneously. Our synthetic population is able to reproduce the dependence of the covering factor on luminosity, the AGN number counts from several surveys, and the observed correlation between reflection and obscuration. Finally, we derived an intrinsic Compton-thick fraction of 21 ± 7%, which is consistent with local observations.