Clumpy tori around type II active galactic nuclei as revealed by X-ray fluorescent lines

The reflection spectrum of a torus around an active galactic nucleus (AGN) is characterized by X-ray fluorescent lines, which are most prominent for type II AGNs. A clumpy torus allows photons reflected from the back-side of the torus to leak through the front regions that are free of obscuration. The observed X-ray fluorescent lines are therefore sensitive to the clumpiness of the torus. We analysed a sample of type II AGNs observed with the Chandra High Energy Transmission Grating Spectrometer (HETGS), and measured the fluxes for the Si K alpha and Fe K alpha lines. The measured FeK alpha/Si K alpha ratios, spanning a range between 5 and 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. We compared the measured Fe K alpha/Si K alpha ratios with simulation results of clumpy tori. The Circinus galaxy has a FeK alpha/Si K alpha ratio of similar to 60, which is close to the simulation results for N = 5, where N is the average number of clumps along the line of sight. The FeK alpha/Si K alpha ratios of the other sources are all below the simulation results for N = 2. Overall, this shows that the non-Fe fluorescent lines in the soft X-ray band are a potentially powerful probe of the clumpiness of tori around AGNs.