Monte Carlo simulations of the Nickel K$\alpha$ fluorescent emission line in a toroidal geometry

TL;DR

This study uses Monte Carlo simulations to analyze the flux, equivalent width, and spectral features of the Ni Kalpha fluorescent line in a toroidal X-ray reprocessor model, providing insights for interpreting AGN and X-ray binary spectra.

Contribution

It offers new Monte Carlo simulation results for Ni Kalpha line properties in toroidal geometries, including analytic expressions and comparisons with Fe Kalpha line features.

Findings

Ni Kalpha EW is ~22 times less than Fe Kalpha in Compton-thin regime

Maximum Ni Kalpha EW is ~3 eV (non-intercepting) and ~250 eV (intercepting) in Compton-thick cases

The shape of the Compton shoulder for Ni Kalpha is similar to Fe Kalpha

Abstract

We present new results from Monte Carlo calculations of the flux and equivalent width (EW) of the Ni Kalpha fluorescent emission line in the toroidal X-ray reprocessor model of Murphy & Yaqoob (2009, MNRAS, 397, 1549). In the Compton-thin regime, the EW of the Ni Kalpha line is a factor of ~22 less than that of the Fe Kalpha 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 Kalpha 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 Kalpha line EW has a maximum value of ~3 eV 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 Ka 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 Kalpha 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 Ka 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 favorable for the Ni Kalpha line to be detected.