Wavelet analysis of MAXI J1535-571 with Insight-HXMT

TL;DR

This study employs wavelet analysis to investigate spectral-timing properties of MAXI J1535-571, revealing state-dependent spectral differences associated with QPOs and proposing a precession-based explanation.

Contribution

It introduces wavelet analysis to distinguish QPO and non-QPO spectral states and links spectral variations to Lense-Thirring precession and jet activity.

Findings

QPO spectra are softer than non-QPO in HIMS.

QPO spectra are slightly harder in SIMS.

Disk temperature varies between states and regimes.

Abstract

In this paper, wavelet analysis is used to study spectral-timing properties of MAXI J1535-571 observed by Insight-HXMT. The low-frequency quasi-periodic oscillations (QPOs) are detected in nine observations. Based on wavelet analysis, the time intervals with QPO and non-QPO are isolated separately, and the corresponding spectra with QPO and non-QPO are analyzed. We find that the spectra with QPO (hereafter QPO spectra) are softer than those without QPO (hereafter non-QPO spectra) in the hard intermediate state (HIMS). While in the soft intermediate state (SIMS), the QPO spectra are slightly harder. The disk temperature of QPO regime is slightly lower during HIMS, but becomes higher during SIMS. The cutoff energies of QPO spectra and non-QPO spectra do not show significant differences. The flux ratio of the disk to total flux is higher for the time intervals with non-QPO than that of QPO regime. We propose that these differences in the spectral properties between QPO and non-QPO regimes could be explained in the scenario of Lense-Thirring precession, and the reversal of the QPO/non-QPO behavior between HIMS and SIMS may be associated with appearance/disappearance of a type-B QPO which might origin from the precession of the jet.