A Simple New Method For Analysing Gapped Time-Series: Search For a High Frequency Cut-off In The X-ray Power Spectrum of MCG -6-30-15

We present a new, simple method for analysing gapped time-series which is particularly suited to probe the X-ray power spectra of active galactic nuclei (AGN).The method is applied to a four-day observation of the Seyfert 1 galaxy MCG -6-30-15 with ASCA. MCG -6-30-15 is well-known for rapid, large amplitude X-ray variability. In this source, our method probes the power spectrum in the range 0.001 Hz to 0.05 Hz, thus extending the high-frequency coverage by a factor of over 50 compared to previous studies. The ASCA data out a cessation of variability up to 0.016 Hz at greater than 90% confidence. The power-law index of the power spectrum is consistent with -1.4.At 0.021 Hz we can place an upper limit of 7% on the intrinsic fluctuations of the source luminosity at greater than 99% confidence. Above this frequency Poisson noise becomes dominant but we can place an upper limit of 10% on intrinsic fluctuations up to 0.05 Hz at 98.9% confidence. The data argue against orbital modulation as the origin of the variability since it would imply upper limits on the mass of the putative black hole of less than half a million solar solar masses and therefore super-Eddington luminosities.