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.