  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.
