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We analyse eight XMM-Newton observations of the bright Narrow-Line Seyfert 1galaxy Arakelian 564 (Ark 564). These observations, separated by ~6 days, allowus to look for correlations between the simultaneous UV emission (from theOptical Monitor) with not only the X-ray flux but also with the different X-rayspectral parameters. The X-ray spectra from all the observations are found tobe adequately fitted by a double Comptonization model where the soft excess andthe hard X-ray power law are represented by thermal Comptonization in a lowtemperature plasma and hot corona, respectively. Apart from the fluxes of eachcomponent, the hard X-ray power law index is found to be variable. Theseresults suggest that the variability is associated with changes in the geometryof the inner region. The UV emission is found to be variable and wellcorrelated with the high energy index while the correlations with the fluxes ofeach component are found to be weaker. Using viscous time-scale arguments werule out the possibility that the UV variation is due to fluctuating accretionrate in the outer disc. If the UV variation is driven by X-ray reprocessing,then our results indicate that the strength of the X-ray reprocessing dependsmore on the geometry of the X-ray producing inner region rather than on theX-ray luminosity alone.

UV and X-Ray Variability of the Narrow-Line Seyfert 1 Galaxy Ark 564