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Title:
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
Authors:
Yaqoob, T.; McKernan, B.; Ptak, A.; Nandra, K.; Serlemitsos, P. J.
Publication:
Astrophysical Journal Letters v.490, p.L25 (ApJL Homepage)
Publication Date:
11/1997
Origin:
APJ
ApJ Keywords:
METHODS: DATA ANALYSIS, GALAXIES: ACTIVE, GALAXIES: INDIVIDUAL ALPHANUMERIC: MCG -6-30-15, X-RAYS: GALAXIES
DOI:
10.1086/311014
Bibliographic Code:
1997ApJ...490L..25Y

Abstract

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- 5x10^{-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 1.6x10^{-2} Hz at greater than 90% confidence. The power-law index of the power spectrum in this range is consistent with ~-1.4. At 2.1x10^{-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 5x10^{-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 5x105 Msolar, and therefore super-Eddington luminosities.
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