LONG-TERM X-RAY VARIABILITY OF NGC-4151

We present a new method of estimating the power spectrum in the case of unevenly sampled data, together with its application to the long X-ray light curve of the Seyfert galaxy NGC 4151. This method is based on the Fourier transform of the observed autocovariance function. The method is simple to use and widely applicable. The long-term power spectrum of NGC4151, covering the range 10(-6) to 10(-8) Hz, is significantly flatter than the short-time-scale (10(-3) to 10(-5) Hz) power spectrum derived from EXOSAT observations, which has a slope of - 2. The long-term power spectrum can be fitted equally well either by a single power law of slope - 1.1 or, at the high frequencies, by a power law of slope similar to that of the EXOSAT power spectrum which then flattens to a slope of zero below similar to 10(-7) Hz, corresponding to a time-scale of similar to 2 weeks.