X-ray polarimetric signatures induced by spectral variability in the framework of the receding torus model

TL;DR

This study uses Monte Carlo simulations to predict X-ray polarization signatures in AGN, testing the receding torus model and its variability with viewing angle and spectral properties, with implications for future X-ray polarimetry observations.

Contribution

It introduces a detailed simulation framework for X-ray polarization in AGN based on the receding torus model, providing testable predictions for upcoming polarimetry missions.

Findings

X-ray polarization fraction varies with viewing angle, from less than a few percent to tens of percent.

Polarization variability differs between static and receding torus scenarios near the type-1/type-2 transition.

Future X-ray polarimetry can distinguish between these models and test the receding torus hypothesis.

Abstract

Obscuring circumnuclear dust is a well-established constituent of active galactic nuclei (AGN). Traditionally referred to as the receding dusty torus, its inner radius and angular extension should depend on the photo-ionizing luminosity of the central source. Using a Monte Carlo approach, we simulate the radiative transfer between the multiple components of an AGN adopting model constraints from the bright Seyfert galaxy NGC 4151. We compare our model results to the observed near-IR to UV polarization of the source and predict its X-ray polarization. We find that the 2-8 keV polarization fraction of a standard AGN model varies from less then a few percent along polar viewing angles up to tens of percent at equatorial inclinations. At viewing angles around the type-1/type-2 transition the X-ray polarization variability differs between a static or a receding torus scenario. In the former case, the expected 2-8 keV polarization of NGC 4151 is found to be 1.21% +/- 0.34% with a constant polarization position angle, while in the later scenario it varies from 0.1% to 6% depending on the photon index of the primary radiation. Additionally, an orthogonal rotation of the polarization position angle with photon energy appears for very soft primary spectra. Future X-ray polarimetry missions will be able to test if the receding model is valid for Seyfert galaxies seen at a viewing angle close to the torus horizon. The overall stability of the polarization position angle for photon indexes softer than {\Gamma} = 1.5 ensures that reliable measurements of X-ray polarization are possible. We derive a long-term observational strategy for NGC 4151 assuming observations with a small to medium-size X-ray polarimetry satellite.