Multi-epoch hard X-ray view of Compton-thick AGN Circinus Galaxy

Abstract

The circumnuclear material around Active Galactic Nuclei (AGN) is one of the essential components of the obscuration-based unification model. However, our understanding of the circumnuclear material in terms of its geometrical shape, structure and its dependence on accretion rate is still debated. In this paper, we present the multi-epoch broadband X-ray spectral modelling of a nearby Compton-thick AGN in Circinus galaxy. We utilise all the available hard X-ray (> 10 keV) observations taken from different telescopes, i.e. BeppoSAX, Suzaku, NuSTAR and AstroSat, at ten different epochs across 22 years from 1998 to 2020. The 3.0–79 keV broadband X-ray spectral modelling using physically-motivated models, namely MYTorus, borus02 and UXCLUMPY, infers the presence of a torus with a low covering factor of 0.28, an inclination angle of 77○–81○ and Compton-thick line-of-sight column densities (NH, LOS = 4.13 − 9.26 × 1024 cm−2) in all the epochs. The joint multi-epoch spectral modelling suggests that the overall structure of the torus is likely to remain unchanged. However, we find tentative evidence for the variable line-of-sight column density on timescales ranging from one day to one week to a few years, suggesting a clumpy circumnuclear material located at sub-parsec to tens of parsec scales.

A Simple Method for Predicting N H Variability in Active Galactic Nuclei

“…Perhaps an even better way is to use a physically motivated torus model (e.g., Murphy & Yaqoob 2009;Baloković et al 2018;Buchner et al 2019). Some recent examples include Marchesi et al (2022), Silver et al (2022), Pizzetti et al (2022), Lefkir et al (2023), Kayal et al (2023), andTorres-Albà et al (2023). However, these methods are time consuming when applied to sources with multiple observations, and thus are not practical for a very large sample of blindly selected sources, especially when over half the sources in these samples tend to show no variability (e.g., Markowitz et al 2014;Hernández-García et al 2015;Laha et al 2020).…”