Intermittent QPO properties of MAXI J1820+070 revealed by Insight-HXMT

TL;DR

This study analyzes the intermittent nature of low-frequency QPOs in MAXI J1820+070 during its 2018 outburst using wavelet analysis, revealing that QPOs are transient with average lifetimes of about 5 cycles, and explores possible physical causes.

Contribution

It introduces a wavelet-based method to characterize QPO intermittency and discusses potential physical mechanisms like jet precession and speed variations for the observed phenomena.

Findings

QPOs are intermittent with an average lifetime of ~5 cycles.

No significant spectral differences between intervals with strong and weak QPOs.

Intermittency may be explained by jet precession or changes in jet speed.

Abstract

We investigate the dynamical properties of low frequency quasi-periodic oscillations (QPOs) observed from the black hole X-ray binary MAXI J1820+070 during the early part of its 2018 outburst, when the system was in a bright hard state. To this aim, we use a series of observations from the Hard X-ray Modulation Telescope Insight-HXMT, and apply a wavelet decomposition (weighted wavelet Z-transforms) to the X-ray light-curve. We find that the QPO phenomenon is intermittent within each individual observation, with some sub-intervals where the oscillation is strongly detected (high root-mean-square amplitude) and others where it is weak or absent. The average life time of individual QPO segments is ~ 5 oscillation cycles, with a 3 sigma tail up to ~ 20 cycles. There is no substantial difference between the energy spectra during intervals with strong and weak/absent QPOs. We discuss two possible reasons for the intermittent QPO strength, within the precessing jet model previously proposed for MAXI J1820+070. In the rigid precession model, intermittent QPOs are predicted to occur with a coherence Q ~ a few when the disk alignment time-scale is only a few times the precession time-scale. Alternatively, we suggest that changes in oscillation amplitude can be caused by changes in the jet speed. We discuss a possible reason for the intermittent QPO strength, within the precessing jet model previously proposed for MAXI J1820+070: we suggest that changes in oscillation amplitude are caused by changes in the jet speed. We argue that a misaligned, precessing jet scenario is also consistent with other recent observational findings that suggest an oscillation of the Compton reflection component in phase with the QPOs.