On the Relationship between the Periodic and Aperiodic Variability of Accreting X‐Ray Pulsars

Besides the narrow peaks originating from the periodic signal, the powerspectra of accreting X-ray pulsars display continuum components usuallyincreasing towards low frequencies; these arise from the source aperiodicvariability. Most studies so far adopted the view that the periodic andaperiodic variations are independent. However any aperiodic variability in theemission from the accretion column(s) towards the magnetic neutron star shouldbe modulated at the X-ray pulsar period, by virtue of the same rotation-inducedgeometric effects which give rise to the periodic signal. We develop here asimple shot noise model to test the presence of a coupling between the periodicand aperiodic variability of X-ray pulsars. The model power spectrum is fit tothe power spectra of three X-ray pulsars, Vela X-1, 4U 1145-62 and Cen X-3observed with EXOSAT. In the first two cases we find that a highly significantcoupling is present, as testified by a substantialbroadening in the wings ofthe power spectrum peaks due to the periodic modulation. We find also thatthese wings can mimick the presence of a knee in the continuum power spectrumcomponents around the fundamental of the periodic modulation, thereforequestioning the correlation reported by Takeshima (1992) between the X-raypulsar frequency and the knee frequency, beyond which the continuum powerspectral component steepens.Comment: 19 pages + 5 figures, Accepted for publication in the Astrophysical Journa