The clumpy torus around type II AGN as revealed by X-ray fluorescent lines

TL;DR

This study uses X-ray fluorescent line ratios from Chandra observations to demonstrate that the torus around type II AGN is clumpy rather than smooth, providing insights into the structure of these obscuring regions.

Contribution

The paper presents observational evidence from X-ray line ratios that supports the clumpy torus model around AGN, contrasting with previous smooth torus assumptions.

Findings

Fe Ka/Si Ka ratios are much lower than smooth torus predictions.

Ratios suggest a small number of clumps along the line of sight.

Clumpy torus models better explain the observed line ratios.

Abstract

The reflection spectrum of the torus around AGN is characterized by X-ray fluorescent lines, which are most prominent for type II AGN. A clumpy torus allows photons reflected from the back-side of the torus to leak through the front free-of-obscuration regions. Therefore, the observed X-ray fluorescent lines are sensitive to the clumpiness of the torus. We analyse a sample of type II AGN observed with Chandra HETGS, and measure the fluxes for the Si Ka and Fe Ka lines. The measured Fe Ka/Si Ka ratios, spanning a range between $5-60$, are far smaller than the ratios predicted from simulations of smooth tori, indicating that the tori of the studied sources have clumpy distributions rather than smooth ones. Compared with simulation results of clumpy tori with a half-opening angle of 60$^{\circ}$, the Circinus galaxy has a Fe Ka/Si Ka ratio of $\sim60$, which is close to the simulation results for $N=5$, where $N$ is the average number of clumps along the line of sight. The Fe Ka/Si Ka ratios of the other sources are all below the simulation results for $N=2$. Overall, it shows that the non-Fe fluorescent lines in the soft X-ray band are a potentially powerful probe of the clumpiness of the torus around AGN.