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 X-ray 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 10-3-5×10−2 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 1.6×10−2 Hz at greater than 90% confidence. The power-law index of the power spectrum in this range is consistent with ~-1.4. At 2.1×10−2 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 5×10−2 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 5×105 M☉, and therefore super-Eddington luminosities.