X-Ray Power Density Spectrum of the Narrow-Line Seyfert 1 Galaxy Arakelian 564

Beginning in 1999 January, the bright, strongly variable Narrow-Line Seyfert1 (NLS1) galaxy Akn 564 has been observed by RXTE once every ~4.3 days. It wasalso monitored every ~3.2 hr throughout 2000 July. These evenly-sampledobservations have allowed the first quantitative comparison of long and shorttime-scale X-ray variability in an NLS1 and the derivation of an X-ray PowerDensity Spectrum (PDS). The variability amplitude in the short time-scale lightcurve is very similar to that in the long time-scale light curve, in markedcontrast to the stronger variability on longer time-scales which ischaracteristic of "normal" broad-line Seyfert 1s (BLS1s). Furthermore, the Akn564 PDS power law cuts off at a frequency of 8.7x10^-7 Hz corresponding to atimescale of ~13 d, significantly shorter than that seen in the PDS of NGC3516, a BLS1 of comparable luminosity. This result is consistent with NLS1s showing faster (as opposed to largeramplitude) variations than BLS1s, providing further evidence that NLS1s harbourlower mass black holes than BLS1s of similar luminosity, accreting at acorrespondingly higher relative rate.Comment: Accepted for publication in Ap