Observations of the X-Ray Millihertz Quasiperiodic Oscillations in Hercules X-1

Abstract

With a systematic timing investigation of the persistent X-ray binary pulsar Her X-1 based on a large number of Insight-HXMT observations between 2017 and 2019, we confirm the presence of X-ray millihertz quasiperiodic oscillations (mHz QPOs) at ∼0.01 Hz. By applying wavelet analysis in our data analysis procedures, we first identify ∼0.005–0.009 Hz QPOs coexisting with the ∼0.01 Hz QPOs. Wavelet analysis suggests that these QPO features show transient behaviors, that the frequencies of mHz QPOs evolve in short timescales. There exists a positive relation between QPO centroid frequency (from ∼0.005 to 0.009 Hz) and X-ray luminosity, while the 10 mHz QPO frequencies remain nearly constant for different luminosities, which suggests different physical mechanisms for the two types of mHz QPOs. The 10 mHz QPOs in both X-ray and UV bands would have the same origin related to the beat frequency where the Alfvén radius is close to the corotation radius, and the 5 mHz QPOs may originate from magnetic disk precession.