A simple new method for analysing gapped time-series: Power spectrum of MCG–6–30–15

Abstract

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 10−3 Hz to >5×10−2 Hz, thus extending the high-frequency coverage by a factor of over 50 compared to previous studies. The ASCA data are consistent with a power-law power spectrum with index ∼−1.4, implying a simple extrapolation from the power spectrum measured by EXOSAT at lower frequencies. 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 5×105M⊙, and therefore super-Eddington luminosities. Researchers are encouraged to use the new method on their ASCA, BeppoSAX and archival data. We also propose a new interpretation of the deep minimum in intensity at the end of the long observation of MCG–6–30–15 in terms of an occultation by optically thick matter. If this interpretation is correct, the data allow for the first time, a crude spatial resolution of the X-ray source and possibly of the innermost regions of the putative accretion disk.