NuSTAR view of heavily absorbed AGN: The $R-N_\text{H}$ correlation

TL;DR

This study uses NuSTAR X-ray observations to explore the relationship between reflection features and absorption in heavily obscured AGN, supporting a clumpy torus model for their outer structure.

Contribution

It provides new evidence linking reflection strength and absorption in AGN, supporting a clumpy torus model with variable filling factors.

Findings

Reflection and Fe Kα line strength increase with absorption.

Clumpy torus models reproduce the observed correlation.

Reflection features are present even in Compton-thick AGN.

Abstract

The nature of the putative torus and the outer geometry of active galactic nuclei (AGN) are still rather unknown and the subject of active research. Improving our understanding of them is crucial for developing a physical picture for the structure of AGN. The main goal of this work is to investigate the outer geometry of AGN by studying the observed hard X-ray spectrum of obscured sources. We primarily aim at researching the reflected emission in these sources. To that end, we analysed archived NuSTAR observations of a sample of nearby AGN, whose X-ray emission has been found to be heavily absorbed, with $10^{23}<N_\text{H}<2.5\cdot 10^{23}\text{ cm}^{-2}$; the upper limit on $N_\text{H}$ was necessary due to the analysis we followed and the data quality. Fitting their emission with both a phenomelogical and a physical model, we investigated the relation between reflection and absorption. The strength of reflected emission, as well as the equivalent width of the Fe K$\alpha$ line, correlates with the absorption column density, which can be explained with a clumpy torus origin for the reflection in these sources. The shape of the observed correlation is found to be well reproduced when the effects of a clumpy torus with a variable filling factor are simulated. A similar increase in reflection seems to be featured even by sources with larger absorption, reaching the Compton thick ($N_\text{H}>1.5\cdot 10^{24}\text{ cm}^{-2}$) regime.