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.