THE X-RAY VARIABILITY OF THE SEYFERT-GALAXY NGC-6814

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 (approximately 1 d) observation is characterized by two major components:. a low frequency noise which can be described, in the 6 10(-5)-10(-2) Hz frequency interval, by a power law with exponent approximately -1.0, and a series of four outstanding peaks at frequencies harmonic of the fundamental at f(a) = 8.2 10(-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 (approximately 1/4 d each) observations shows that a peak is present in their composite power spectrum at 4 10(-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 approximately 4-sigma. 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-rav emission of this source, at least over two years.