Beginning in 1999 January , the bright , strongly variable Narrow-Line Seyfert 1 ( NLS1 ) galaxy Akn 564 has been observed by RXTE once every \sim 4.3 days .
It was also monitored every \sim 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 time-scales 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 \times 10 ^ { -7 } Hz corresponding to a timescale of \sim 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 .