  We report on the XMM-Newton timing properties of the most X-ray variable,
radio-quiet, NLS1 IRAS 13224-3809. IRAS 13224-3809 continues to display the
extremely variable behavior that was previously observed with ROSAT and ASCA;
however, no giant, rapid flaring events are observed. We detect variations by a
factor as high as ~8 during the 64 ks observation, and the variability is
persistent throughout the light curve. Dividing the light curve into 9 minute
segments we found almost all of the segments to be variable at > 3 sigma. When
the time-averaged cross-correlation function is calculated for the 0.3-0.8 keV
band with the 3-10 keV band, the cross-correlation profile is skewed indicating
a possible smearing of the signal to longer times (soft band leading the hard).
A correlation between count rate and hardness ratio is detected in four energy
bands. In three cases the correlation is consistent with spectral hardening at
lower count rates which can be explained in terms of a partial-covering model.
The other band displays the reverse effect, showing spectral hardening at
higher count rates. We can explain this trend as a more variable power-law
component compared to the soft component. We also detect a delay between the
0.3-1.5 keV count rate and the 0.8-1.5 keV to 0.3-0.8 keV hardness ratio,
implying flux induced spectral variability. Such delays and asymmetries in the
cross correlation functions could be suggesting reprocessing of soft and hard
photons. In general, much of the timing behavior can be attributed to erratic
eclipsing behavior associated with the partial covering phenomenon, in addition
to intrinsic variability in the source. The variability behavior of IRAS
13224-3809 suggests a complicated combination of effects which we have started
to disentangle with this present analysis.
