Observational hints on the torus obscuring gas behaviour through X-rays with NuSTAR data

TL;DR

This study uses NuSTAR X-ray data to investigate the presence and behavior of the torus obscuring gas in active galactic nuclei, revealing that the reflection component varies with black hole properties and Eddington ratio.

Contribution

It provides observational evidence linking the disappearance of the torus to specific AGN properties and Eddington ratios, expanding understanding of AGN structure.

Findings

13% of AGNs lack reflection signatures, indicating possible torus absence.

Most sources with reflection are highly obscured, with 20% being Compton-thick.

An increase in unobscured sources and FeKα line ratio is observed at low Eddington ratios.

Abstract

According to theory, the torus of active galactic nuclei (AGN) is sustained from a wind coming off the accretion disk, and for low efficient AGN, it has been proposed that such structure disappears. However, the exact conditions for its disappearance remain unclear. This can be studied throughout the reflection component at X-rays, which is associated with distant and neutral material at the inner walls of the torus in obscured AGN. We select a sample of 81 AGNs observed with NuSTAR with a distance limit of D< 200\,Mpc and Eddington rate $\rm{\lambda_{Edd} \equiv L_{bol}/L_{Edd}<10^{-3}}$. We fit the 3-70\,keV spectra using a model accounting for a partial-covering absorber plus a reflection component from neutral material. We find that the existence of the reflection component spans in a wide range of black-hole mass and bolometric luminosities, with only $\sim$13\% of our sample (11 sources) lacking of any reflection signatures. These sources fall in the region in which the torus may be lacking in the L-MBH diagram. For the sources with a detected reflection component, we find that the vast majority of them are highly obscured ($\rm{\log \ N_H > 23}$), with $\rm{\sim 20\%}$ being Compton-thick. We also find an increase on the number of unobscured sources and a tentative increase on the ratio between $\rm{FeK\alpha}$ emission line and Compton-hump luminosities toward $\rm{\lambda_{Edd}=10^{-5}}$, suggesting that the contribution of the $\rm{FeK\alpha}$ line changes with Eddington ratio.