The X-ray variability of the Seyfert galaxy NGC 6814.
Abstract
We present the results of an accurate temporal analysis of five EXOSAT observations of the Seyfert 1 galaxy NGC 6814 covering about 2 yr. As firstly reported by Mittaz & Branduardi-Raymont (1989), the power spectrum of the longest (~1 d) observation is characterized by two major components: a low frequency noise which can be described, in the 610^-5^ -10^-2^ Hz frequency interval, by a power law with exponent ~ -1.0, and a series of four outstanding peaks at frequencies harmonic of the fundamental at F_a_ = 8.210^-5^ Hz, which can be considered as the signature of a periodic signal. The small number of cycles observed, however, would not allow one to obtain a conclusive information about its structure. The analysis of the other shorter (~1/4 d each) observations shows that a peak is present in their composite power spectrum at 410^-4^ Hz, a frequency which is compatible with the fourth harmonics of F_a_ within the uncertainties due to the coarser frequency resolution, at the confidence level of ~4σ. We analyzed these observations also by means of the non-linear method of Prony, which allows a frequency resolution better than 1/T and found that some harmonics of the periodic signal are present in each single time series; in particular the fourth harmonic is present in all the observations. We are therefore led to conclude that periodic variations might be a stable characteristic of the X-ray emission of this source, at least over two years.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- August 1992
- Bibcode:
- 1992A&A...261..405F
- Keywords:
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- Exosat Satellite;
- Seyfert Galaxies;
- Spaceborne Astronomy;
- X Ray Astronomy;
- Astronomical Models;
- Light Curve;
- Power Spectra;
- Time Series Analysis;
- Astrophysics