Rms variability properties of the iron K alpha line in Seyfert galaxies
  We present an analysis of the rms variability spectra of a sample of 18
observations of 14 Seyfert galaxies observed by XMM-Newton, which exhibit
sufficient variability and signal-to-noise ratio to examine the variations in
the iron K-band. The narrow core of the K alpha line at 6.4 keV, seen
universally in Seyferts, shows minimal evidence for variability and is always
less variable than the continuum, supporting an origin in distant material such
as the torus. At least half the observations do show evidence for variations in
the wider iron K-band, however, and in at least 5 cases the excess line
variations appear to be broad. The simplest prediction -- that the broad
emission line is as variable as the continuum -- is generally not confirmed as
only two observations show this type of behaviour. In four cases, the red wing
of the line is more variable than the power-law continuum and extends down to
energies of ~ 5 keV. Three observations show strong variability blueward of the
line core that could also be from the disk, but alternatively might be due to
emission or absorption by other hot or photoionised gas close to the nucleus.
In cases where this excess blue variability is present, it is not always seen
in the time-averaged spectrum. Six observations possess a broad iron line in
the time-averaged spectra but with an invariant red wing, and three of these
six show no variability across the entire iron line region. This suggests a
decoupling of the continuum and reflection component, perhaps due to light
bending or other anisotropic effects as has been suggested for MCG-6-30-15 and
other narrow-line Seyfert 1s. A key result is that the rms spectra of objects
such as NGC 3516 do not agree with complex absorption effects mimicking the
broad red wing.
