Discovery of Coupling between Periodic and Aperiodic Variability and X-ray Quasi-periodic Oscillations from Her X-1

Abstract

We report the discovery of coupling between periodic and aperiodic variability and ~12-mHz X-ray quasi-periodic oscillations (QPOs) from the X-ray binary pulsar Her X-1 using data from the Rossi X-Ray Timing Explorer. We found two different couplings, one during the pre-eclipse dips and the other during the normal state of the source, using a method which directly compares the low-frequency power-density spectra (PDS) with those of the sidebands around the coherent pulse frequency. The pre-eclipse dip lightcurves show significant time variation of photon counts, and this variation appears in the PDS as both strong mHz powers and well-developed sidebands around the coherent pulse frequency. The linear correlation coefficients between the mHz PDS and the sideband PDS obtained from two pre-eclipse dip data segments are 0.880 +- 0.003 and 0.982 +- 0.001, respectively. This very strong coupling demonstrates that the amplitudes of the coherent pulsations are almost exactly modulated by the aperiodic variabilities, suggesting that both the periodic and aperiodic variabilities are related to time variation of obscuration of X-rays from the central pulsar by an accretion disk during pre-eclipse dips. We also found weak coupling during the normal state of the source, together with ~12-mHz QPOs. The normal state coupling seems to reconcile with the prediction that the aperiodic variabilities from X-ray binary pulsars are due to time-varying accretion flows onto the pulsar's magnetic poles. If the ~12-mHz QPOs are due to global-normal disk oscillations caused by the gravitational interactions between the central pulsar and the accretion disk, the inferred inner-disk radius is roughly comparable to the magnetospheric radius, ~1 10^8 cm.