  We present a method to calculate the natural line width and energy dependent
line shape due to fluorescence decay of core excited atoms within a full
relativistic multi-reference configuration interaction theory. The atomic
absorption lines show a deviation from a Lorentzian line-shape due to energy
dependent matrix elements of the localized electronic state coupling to the
photon field. This gives rise to spectral lines with small but visible
asymmetry. One generally finds an excess of spectral weight at the high energy
shoulder of the atomic absorption line. We present the example of nuclear decay
of $^{55}$Fe by electron capture of an inner-shell core electron. We show that
the amount of ionizing radiation in the energy window between 50 and 200 keV is
around one order of magnitude larger due to the energy dependent fluorescence
yield lifetime compared to the value one would obtain if one assumes a constant
fluorescence decay rate. This yields a total change of energy deposited into
ionizing radiation of about 1\textperthousand. Our calculations are in good
agreement with previous calculations and experimental observations where data
is available. Our results can be further validated by high precision
measurements of electron capture nuclear decay spectra using recently developed
experimental methods.
