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 $77^{\circ}$ $-$ $81^{\circ}$ and Compton-thick line-of-sight column densities ($N_{\rm H,LOS} = 4.13~-~9.26~\times~10^{24}$ 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.