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D.-S. Moon, S. S. Eikenberry (Department of Astronomy, Cornell University)
We report the discovery of milli-hertz (mHz) quasi-periodic oscillations (QPOs) and QPO-modulated pulsations during large X-ray flares from the X-ray binary pulsar LMC X-4 and during pre-eclipse dips from Her X-1 using data from the Rossi X-Ray Timing Explorer. The X-ray lightcurves show that, in addition to coherent pulsations of ~0.074 (LMC X-4) and ~0.81 Hz (Her X-1), there exist significant time-varying components at lower frequency. These relatively long-term structures appear in the Leahy-normalized power density spectra (PDS) as both strong mHz QPOs and well-developed sidebands around the coherent pulse frequencies. One interesting feature is that the power distributions of the sidebands are almost identical to those of the mHz QPOs, indicating that the amplitude of the coherent pulsation is modulated by the amplitude of the mHz QPOs. We explain these unusual phenomena using a combination of the beat-frequency-model and a modified version of the Keplerian-frequency-model for QPOs for the case of LMC X-4. In Her X-1, we attribute the origin of the mHz QPOs and the sidebands to the variation of obscuration due to an inhomogeneous distribution of absorbing material in the accretion disk. We discuss the accretion disks around the two sources, and we also discuss the origin of ~13-mHz X-ray QPOs in the normal state of Her X-1. These results present some of the best and most extreme examples of coupling between the periodic and aperiodic variability of X-ray binary pulsars.
DSM is supported by NSF grant AST-9986898. SSE is supported in part by an NSF Faculty Early Careeer Development (CAREER) award (NSF-9983830).
The author(s) of this abstract have provided an email address for comments about the abstract: moon@astrosun.tn.cornell.edu