Wavelet analysis of low-frequency quasi-periodic oscillations in MAXI J1803−298 observed with <i>Insight-HXMT</i> and <i>NICER</i>

ABSTRACT With data observed by the Hard X-ray Modulation Telescope (Insight-HXMT) and the Neutron star Interior Composition Explorer (NICER), we study low-frequency quasi-periodic oscillations (QPOs) of the black hole candidate MAXI J1803−298 during the 2021 outburst. Based on the hardness–intensity diagram and the difference in the QPOs properties, type-C and type-B QPOs are found in the low-hard state and soft-intermediate state, respectively. After searching for and classifying QPOs in the Fourier domains, we extract the QPO component and study it with wavelet analysis. The QPO and no-QPO time intervals are separated by the confidence level, so that the S-factor, which is defined as the ratio of the QPO time interval to the total length of the good time interval, is calculated. We found S-factors decrease with QPOs frequency for type-C QPOs but stay stable around zero for type-B QPOs. The relation of S-factor of type-C QPOs and photon energy and the correlation of S-factor and counts are also studied. Different correlations between the S-factor and counts for different energy bands indicate different origins of QPOs in high- and low-energy bands, which may be due to a dual-corona scenario.