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

Abstract

ABSTRACT The circumnuclear material around active galactic nuclei (AGNs) 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 broad-band X-ray spectral modelling of a nearby Compton-thick AGN in Circinus galaxy. We utilize all the available hard X-ray (>10 keV) observations taken from different telescopes, i.e. BeppoSAX, Suzaku, NuSTAR, and AstroSat, at 10 different epochs across 22 yr from 1998 to 2020. The 3.0–79 keV broad-band 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 time-scales ranging from 1 d to 1 week to a few years, suggesting a clumpy circumnuclear material located at subparsec to tens of parsec scales.