Discovery of coupling between periodic and aperiodic variability and X-rayquasi-periodic oscillations from Hercules X-1

We report the discovery of coupling between periodic and aperiodic variabil ity and 12 mHz X-ray quasi-periodic oscillations (QPOs) from the X-ray bina ry pulsar Hercules X-1 using data from the Rossi X-Ray Timing Explorer. We found two different couplings, one during the preeclipse dips and the other during the normal state of the source, using a method that directly compar es the low-frequency power-density spectra (PDSs) with those of the sideban ds around the coherent pulse frequency. The preeclipse dip light curves sho w significant time variation of photon counts, and this variation appears i n the PDSs as both strong millihertz powers and well-developed sidebands ar ound the coherent pulse frequency. The linear correlation coefficients betw een the millihertz PDSs and the sideband PDSs obtained from two preeclipse dip data segments are 0.880 +/- 0.003 and 0.982 +/- 0.001, respectively. Th is very strong coupling demonstrates that the amplitudes of the coherent pu lsations are almost exactly modulated by the aperiodic variabilities, sugge sting that both the periodic and aperiodic variabilities are related to tim e variation of obscuration of X-rays from the central pulsar by an accretio n disk during preeclipse dips. We also found weak coupling during the norma l state of the source, together with 12 mHz QPOs. The normal state coupling seems to reconcile with the prediction that a significant fraction of the aperiodic variabilities from X-ray binary pulsars are due to time-varying a ccretion flows onto the pulsar's magnetic poles. We discuss the possible or igin of the 12 mHz QPOs.