HARMONIC COUPLING OF THE RED NOISE IN X-RAY PULSARS

The power spectra of X-ray pulsars often show the presence of a red-no ise component. This noise is produced by aperiodic variability believe d to be associated with instabilities that seem to occur in accretion hows onto compact objects. In this paper we discuss how, independently of the details of the physical processes that generate these instabil ities, a careful analysis of the power spectra can furnish some constr aints on the distance from the stellar surface at which the sudden ene rgy release associated with the instabilities occurs. In particular, a ny aperiodic variability coming from the accretion flow funneled towar d the magnetic poles should be modulated at the pulsar spin period (co upling). We show how, in the power spectra, this coupling results in a broadening at the base of the harmonics. To investigate this effect, we have adopted a mathematical description of the noise in order to pr oduce simulated light curves and the resulting power spectra. A compar ison of power spectra from simulations with real data allows the detec tion or exclusion of the broadening effect. As an application of this method we have compared simulated power spectra with one obtained from a Ginga observation of the X-ray pulsar SMC X-1. For this source the coupling effect is evident.