The obscured active nucleus of NGC 7172 as seen by NuSTAR

TL;DR

This study analyzes NuSTAR X-ray data of Seyfert 2 galaxy NGC 7172, revealing a stable Compton-thin obscuration, a moderate reflection component, and long-term variability in intrinsic luminosity and FeKα line strength, indicating a distant reprocessing torus.

Contribution

First detailed NuSTAR analysis of NGC 7172 confirming its Compton-thin nature and estimating torus geometry, with evidence of long-term intrinsic luminosity variability and FeKα line origin in a distant torus.

Findings

Consistent Compton-thin obscuration over 30 years.

Detection of a moderate reflection component with R~0.44.

Long-term variability in intrinsic luminosity and FeKα EW.

Abstract

We analyse the properties of the X-ray emission from Seyfert 2 NGC 7172 which are based on the 32 ks observational data from NuSTAR in 2014. This source showed a constant Compton-thin obscuration $N_{H}\approx 8\cdot 10^{22} cm^{-2}$, which is similar to that observed by XMM-Newton, Suzaku, ASCA and BeppoSAX over past 30 years. We revealed the presence of a moderate reflection component with <R>~0.44 and a narrow FeK{\alpha} line with $67^{+13}_{-14}$ eV. The applied numerical models of torus confirmed the Compton-thin type of the Seyfert nucleus and allowed us to obtain an estimate of the torus opening angle ${\Theta}_{t}\sim59\deg$ and inclination ${\Theta}_{i}\sim61\deg$. Using the observation data of previous observation, we demonstrate also the long-term variability of intrinsic luminosity $L_{intr}$(2-10 keV) almost by order and EW $FeK_{\alpha}$ by factor $\sim4$ on a timescale of $\sim12$ years, that coincides to the distance of $d\sim$3.7 pc between central source and reprocessing medium. Such changes of intrinsic luminosity without changes in $N_{H}$ value indicate the variability of a central source. In the same time the spectral analysis results with the presence of the lag between variability of intrinsic luminosity $L_{intr}$(2-10 keV) and EW $FeK_{\alpha}$, as well as a behaviour of intensity $I_{FeK_{\alpha}}$ of the line are in agreement with the scenario, where observed $FeK_{\alpha}$ line is generated in a distant gas-dust torus.