Detection of transient periodicity in an EXOSAT observation of NGC4151

Abstract

Fourier analysis of an EXOSAT (ME) 30000 s observation of the Seyfert 1 galaxy NGC 4151 revealed a transient periodical phenomenon (period 131 +/- 1 s and duration of a few thousand seconds) with a probability of a chance occurrence of the peak in the power spectrum of 2 10^-3. Analysis of short data segments provides further evidence of this periodicity and indicates that the energy spectrum of this component becomes softer during the phenomenon. A careful analysis of possible instrumental effects does not appear to offer an explanation of the observed phenomenon.