The X-ray variability of the narrow-line type 1 Seyfert galaxy IRAS 13224-3809 from an XMM-Newton observation

We report on the XMM-Newton timing properties of the most X-ray variable, radio-quiet, narrow-line type 1 Seyfert galaxy IRAS 13224-3809. IRAS 13224-3809 continues to display the extremely variable behaviour 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-min segments, we found almost all of the segments to be variable at >3σ. 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 (the 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 with the soft component. We also detect a delay between the 0.3-1.5 keV count rate and the 0.8-1.5 to 0.3-0.8 keV hardness ratio, implying flux-induced spectral variability. Such delays and asymmetries in the cross-correlation functions could suggest reprocessing of soft and hard photons. In general, much of the timing behaviour can be attributed to erratic eclipsing behaviour associated with the partial covering phenomenon, in addition to intrinsic variability in the source. The variability behaviour of IRAS 13224-3809 suggests a complicated combination of effects which we have started to disentangle with this present analysis.