  We present a spectral variability analysis of the X-ray emission of the
Narrow Line Seyfert 1 galaxy Ark 564 using the data from a
  ~100 ks XMM-Newton observation. Taking advantage of the high sensitivity of
this long observation and the simple spectral shape of Ark 564, we determine
accurately the spectral variability patterns in the source.
  We use standard cross-correlation methods to investigate the correlations
between the soft and hard energy band light curves. We also generated 200
energy spectra from data stretches of 500 s duration each and fitted each one
of them with a power law plus a bremsstrahlung component (for the soft excess)
and we investigated the correlations between the various best fit model
parameter values. The ``power law plus bremsstrahlung'' model describes the
spectrum well at all times. The iron line and the absorption features, which
are found in the time-averaged spectrum of the source are too weak to effect
the results of the time resolved spectral fits. We find that the power law and
the soft excess flux are variable, on all measured time scales. The power law
slope is also variable, and leads the flux variations of both the power law and
the bremsstrahlung components. Our results can be explained in the framework of
time-dependent Comptonization models. They are consistent with a picture where
instabilities propagate through an extended X-ray source, affecting first the
soft and then the hard photons producing regions. The soft excess could
correspond to ionized disc reflection emission, in which case it responds fast
to the primary continuum variations. The time scales are such that light travel
times might additionally influence the observed variability structure.
