We present the analysis of an XMM observation of the Seyfert galaxy NGC 2992. The source was found in its highest level of X-ray activity yet detected, a factor $sim 23.5$ higher in 2--10 keV flux than the historical minimum. NGC 2992 is known to exh
ibit X-ray flaring activity on timescales of days to weeks, and the XMM data provide at least factor of $sim 3$ better spectral resolution in the Fe K band than any previously measured flaring X-ray state. We find that there is a broad feature in the sim 5-7 keV band which could be interpreted as a relativistic Fe K$alpha$ emission line. Its flux appears to have increased in tandem with the 2--10 keV continuum when compared to a previous Suzaku observation when the continuum was a factor of $sim 8$ lower than that during the XMM observation. The XMM data are consistent with the general picture that increased X-ray activity and corresponding changes in the Fe K$alpha$ line emission occur in the innermost regions of the putative accretion disk. This behavior contrasts with the behavior of other AGN in which the Fe K$alpha$ line does not respond to variability in the X-ray.
The absolute luminosity of the Fe Kalpha emission line from matter illuminated by X-rays in astrophysical sources is nontrivial to calculate except when the line-emitting medium is optically-thin to absorption and scattering. We characterize the Fe K
alpha line flux using a dimensionless efficiency, defined as the fraction of continuum photons above the Fe K shell absorption edge threshold energy that appear in the line. The optically-thin approximation begins to break down even for column densities as small as 2 x 10^22 cm^-2. We show how to obtain reliable estimates of the Fe Kalpha line efficiency in the case of cold, neutral matter, even for the Compton-thick regime. We find that, regardless of geometry and covering factor, the largest Fe Kalpha line efficiency is attained well before the medium becomes Compton-thick. For cosmic elemental abundances it is difficult to achieve an efficiency higher than a few percent under the most favorable conditions and lines of sight. For a given geometry, Compton-thick lines-of-sight may have Fe Kalpha line efficiencies that are orders of magnitude less than the maximum possible for that geometry. Configurations that allow unobscured views of a Compton-thick reflecting surface are capable of yielding the highest efficiencies. Our results can be used to estimate the predicted flux of the narrow Fe Kalpha line at ~6.4 keV from absorption models in AGN. In particular we show that contrary to a recent claim in the literature, absorption dominated models for the relativistic Fe Kalpha emission line in MCG -6-30-15 do not over-predict the narrow Fe Kalpha line for any column density or covering factor.
