A Phase-resolved View of the Low-frequency Quasiperiodic Oscillations from the Black Hole Binary MAXI J1820+070

Abstract

Although low-frequency quasiperiodic oscillations (LFQPOs) are commonly detected in the X-ray light curves of accreting black hole X-ray binaries, their origin still remains elusive. In this study, we conduct phase-resolved spectroscopy in a broad energy band for LFQPOs in MAXI J1820+070 during its 2018 outburst, utilizing Insight-HXMT observations. By employing the Hilbert–Huang transform method, we extract the intrinsic quasiperiodic oscillation (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.

The First Polarimetric View on Quasiperiodic Oscillations in a Black Hole X-Ray Binary

“…We conducted a phase-resolved analysis using the Hilbert-Huang transform technique, which is a powerful tool for examining nonstationary signals (Huang et al 1998;Huang & Wu 2008). This method has been previously applied in the phaseresolved analysis of QPOs (Su et al 2015;Hsieh & Chou 2020;Shui et al 2023;Yu et al 2023). Two essential steps are involved in this approach: mode decomposition and Hilbert transform analysis.…”

“…Two essential steps are involved in this approach: mode decomposition and Hilbert transform analysis. Following the approach described in Shui et al (2023), we employed variational mode decomposition, which represents a more advanced method when compared to traditional decomposition techniques (Dragomiretskiy & Zosso 2014). To perform the mode decomposition, we utilized the open-source code vmdpy v0.2 10 (Carvalho et al 2020).…”

Recovery of High-energy Low-frequency Quasiperiodic Oscillations from Black Hole X-Ray Binary MAXI J1535–571 with a Hilbert–Huang Transform Method

“…Comptonization is widely considered to be the radiative mechanism behind type-C QPOs (e.g., Bellavita et al 2022;Méndez et al 2022) because observational studies show that the factional rms of type-C QPOs generally increases with photon energy (Zhang et al 2017;Huang et al 2018;Kong et al 2020) and no evident disk-like component is detected in their rms spectra (Sobolewska & Życki 2006;Axelsson et al 2014;Axelsson & Done 2016;Shui et al 2023b). Furthermore, Ma et al (2021) reported the discovery of type-C QPOs above 200 keV, further indicating a strong association with Comptonized emission.…”

“…Furthermore, Ma et al (2021) reported the discovery of type-C QPOs above 200 keV, further indicating a strong association with Comptonized emission. Additionally, inclination-dependent amplitudes and time lags of type-C QPOs (see Motta et al 2015;van den Eijnden et al 2017), and a QPO phase-dependent reflection component revealed with phase-resolved spectroscopy (see Ingram & van der Klis 2015;Ingram et al 2016Ingram et al , 2017Nathan et al 2022;Shui et al 2023b) may suggest type-C QPOs originate from some geometrical effects. For recent reviews of observations and theories of LFQPOs, the reader is referred to Ingram & Motta (2019).…”

“…In this Letter, we propose a novel technique based on the Hilbert-Huang transform (HHT) for recovering high-energy QPO signals in MAXI J1535-571, which is one of the brightest BHXRBs observed by Insight-HXMT. The HHT is a powerful tool for analyzing phenomena with nonstationary periodicity proposed by Huang et al (1998) and has been successfully applied in astronomical research (Camp et al 2007;Hu et al 2011Hu et al , 2014Su et al 2015;Hsieh & Chou 2020;Shui et al 2023b;Yu et al 2023b). The HHT allows us to track changes in the frequency of QPOs on short timescales and conduct their phase-resolved spectral analysis.…”