The Seyfert Galaxy NGC 6814 -- a highly variable X-ray source
  The Seyfert galaxy NGC 6814 is a highly variable X-ray source despite the fact that it has recently been shown not to be the source of periodic variability. The 1.5 year monitoring by ROSAT has revealed a long term downward trend of the X-ray flux and an episode of high and rapidly varying flux (e.g. by a factor of about 3 in 8 hours) during the October 1992 PSPC observation. Temporal analysis of this data using both Fourier and autoregressive techniques have shown that the variability timescales are larger than a few hundred seconds. The behavior at higher frequencies can be described by white noise. 
Michael Koenig (1), Susanne Friedrich (1), Ruediger Staubert (1), Jens   Timmer (2) ((1) Institute for Astronomy and Astroph...