Large X-ray flares from LMC X-4: Discovery of millihertz quasi-periodic oscillations and quasi-periodic oscillation-modulate pulsations

We report the discovery of millihertz (mHz) quasi-periodic oscillations (QP Os) and QPO-modulated pulsations during large X-ray flares from the high-ma ss X-ray binary pulsar LMC X-4 using data from the Rossi X-Ray Timing Explo rer. The light curves of flares show that, in addition to similar to 74 mHz coherent pulsations, there exist two more time-varying temporal structures at frequencies of similar to0.65-1.35 and similar to2-20 mHz. These relati vely long term structures appear in the power density spectra as mHz QPOs a nd also as well-developed sidebands around the coherent pulse frequency, in dicating that the amplitudes of the coherent pulsation are modulated by tho se of the mHz QPOs. One interesting feature is that, while the first flare shows symmetric sidebands around the coherent pulse frequency, the second f lare shows significant excess emission in the lower frequency sidebands due to the similar to2-20 mHz QPOs. We discuss the origin of the QPOs using a combination of the beat-frequency model and a modified version of the Keple rian-frequency model. According to our discussion, it seems possible to att ribute the origin of the similar to0.65-1.35 and similar to2-20 mHz QPOs to the beating between the rotational frequency of the neutron star and the K eplerian frequency of large accreting clumps near the corotation radius and to the orbital motion of clumps at Keplerian radii of (2-10) x 10(9) cm, r espectively.