A simple new method for analyzing gapped time series: Search for a high-frequency cutoff in the x-ray power spectrum of the Seyfert galaxy MCG-6-30-15

We present a new, simple method for analyzing gapped time series that is particularly suited to probe the Xray power spectra of active galactic nuclei (AGNs). The method is applied to a 4 day observation of the Seyfert 1 galaxy MCG -6-30-15 with ASCA (Advanced Satellite for Cosmology and Astrophysics). MCG -6-30-15 is well known for rapid, large-amplitude X-ray variability and was one of the first AGNs to show evidence of relativistic effects in its Fe-K line profile. In this source, our method probes the power spectrum in the range 10(-3)-5 x 10(-2) Hz, thus extending the high-frequency coverage by a factor of over 50 compared to previous studies. The ASCA data rule out a cessation of variability up to 1.6 x 10(-2) Hz at greater than 90% confidence. The power-law index of the power spectrum in this range is consistent with similar to-1.4. At 2.1 x 10(-2) 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 5 x 10(-2) 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 5 x 10(5) M., and therefore super-Eddington luminosities.