Kendrah D. Murphy

Monte Carlo simulations of the nickel Kα fluorescent emission line in a toroidal geometry

We present new results from Monte Carlo calculations of the flux and equivalent width (EW) of the Ni Kα fluorescent emission line in the toroidal X-ray reprocessor model of Murphy & Yaqoob. In the Compton-thin regime, the EW of the Ni Kα line is a factor of ∼22 less than that of the Fe Kα line but this factor can be as low as ∼6 in the Compton-thick regime. We show that the optically thin limit for this ratio depends only on the Fe to Ni abundance ratio, it being independent of the geometry and covering factor of the reprocessor, and also independent of the shape of the incident X-ray continuum. We give some useful analytic expressions for the absolute flux and the EW of the Ni Kα line in the optically thin limit. When the reprocessor is Compton thick and the incident continuum is a power law with a photon index of 1.9, the Ni Kα EW has a maximum value of ∼3 and ∼250 eV for non-intercepting and intercepting lines of sight, respectively. Larger EWs are obtained for flatter continua. We have also studied the Compton shoulder of the Ni Kα line and find that the ratio of scattered to unscattered flux in the line has a maximum value of 0.26, less than the corresponding maximum for the Fe Kα line. However, we find that the shape of the Compton shoulder profile for a given column density and inclination angle of the torus is similar to the corresponding profile for the Fe Kα line. Our results will be useful for interpreting X-ray spectra of active galactic nuclei (AGNs) and X-ray binary systems in which the system parameters are favourable for the Ni Kα line to be detected.

The energy budget for X‐ray to infrared reprocessing in Compton‐thin and Compton‐thick active galaxies

Heavily obscured active galactic nuclei (AGNs) play an important role in contributing to the cosmic X-ray background (CXRB). However, the AGNs found in deep X-ray surveys are often too weak to allow direct measurement of the column density of obscuring matter. One method adopted in recent years to identify heavily obscured, Compton-thick AGNs under such circumstances is to use the observed mid-infrared-to-X-ray luminosity ratio as a proxy for the column density. This is based on the supposition that the amount of energy lost by the illuminating X-ray continuum to the obscuring matter and reprocessed into infrared emission is directly related to the column density and that the proxy is not sensitive to other physical parameters of the system (aside from contamination by dust emission from, for example, star-forming regions). Using Monte Carlo simulations, we find that the energy losses experienced by the illuminating X-ray continuum in the obscuring matter are far more sensitive to the shape of the X-ray continuum and to the covering factor of the X-ray reprocessor than they are to the column density of the material. Specifically we find that it is possible for the infrared to X-ray luminosity ratio for a Compton-thin source to be just as large as that for a Compton-thick source even without any contamination from dust. Since the intrinsic X-ray continuum and covering factor of the reprocessor are poorly constrained from deep X-ray survey data, we conclude that the mid-infrared-to-X-ray luminosity ratio is not a reliable proxy for the column density of obscuring matter in AGNs even when there is no other contribution to the mid-infrared luminosity aside from X-ray reprocessing. This conclusion is independent of the geometry of the obscuring matter.

A Chandra-HETG view of MCG +8-11-11

United States. National Aeronautics and Space Administration (Chandra X-ray Observatory (U.S.) Award GO1-12147X)