Speaker
Description
Although low frequency quasi-periodic oscillations (LFQPOs) are commonly detected in the X-ray light curves of accreting black hole X-ray binaries (BHXRBs), their origin still remains elusive. We use Insight-HXMT observations to conduct phase-resolved spectroscopy in a broad energy band for LFQPOs from MAXI J1820+070. By employing the Hilbert-Huang Transform method, we extract the intrinsic QPO variability, and obtain the corresponding instantaneous amplitude, phase, and frequency functions for each data point. With well-defined phases, we construct QPO waveforms and phase-resolved spectra. By comparing the phase-folded waveform with that obtained from the Fourier method, we find that phase-folding on the phase of the QPO fundamental frequency leads to a slight reduction in the contribution of the harmonic component. This suggests that the phase difference between QPO harmonics exhibits time variability. Phase-resolved spectral analysis reveals strong concurrent modulations of the spectral index and flux across the bright hard state. The modulation of the spectral index could potentially be explained by both the corona and jet precession models, with the latter requiring efficient acceleration within the jet. Furthermore, significant modulations in the reflection fraction are detected exclusively during the later stages of the bright hard state. These findings provide support for the geometric origin of LFQPOs and offer valuable insights into the evolution of the accretion geometry during the outburst in MAXI J1820+070.
