54 2 v 1 3 0 Ja n 20 01 X - ray Power Density Spectrum of the Narrow Line Seyfert 1 Galaxy Akn 564

Beginning in 1999 January, the bright, strongly variable Narrow-Line Seyfert 1 (NLS1) galaxy Akn 564 has been observed by RXTE once every ∼4.3 days. It was also monitored every ∼3.2 hr throughout 2000 July. These evenly-sampled observations have allowed the first quantitative comparison of long and short time-scale X-ray variability in an NLS1 and the derivation of an X-ray Power Density Spectrum (PDS). The variability amplitude in the short time-scale light curve is very similar to that in the long time-scale light curve, in marked contrast to the stronger variability on longer timescales which is characteristic of “normal” broad-line Seyfert 1s (BLS1s). Furthermore, the Akn 564 PDS power law cuts off at a frequency of 8.7 × 10 Hz corresponding to a timescale of ∼13 d, significantly shorter than that seen in the PDS of NGC 3516, a BLS1 of comparable luminosity. This result is consistent with NLS1s showing faster (as opposed to larger amplitude) variations than BLS1s, providing further evidence that NLS1s harbour lower mass black holes than BLS1s of similar luminosity, accreting at a correspondingly higher relative rate. Subject headings: galaxies: active — galaxies: individual (Akn 564) — galaxies: Seyfert — X-rays: galaxies