Different behaviors of wavelet results for type-B and type-C QPOs of MAXI J1535-571 based on NICER data

Abstract

Wavelet analysis, in addition to power density spectra, is another method to study the quasi-periodic signals in the light curves, but has been rarely used in black hole X-ray transients. We performed wavelet analysis of X-ray timing features and quasi-periodic oscillations (QPOs) based on NICER observations of the black hole candidate MAXI J1535-571 in this paper. Separating the light curves by the confidence level of wavelet results, we find significant differences exist in the PDS, hardness ratio and mean count between light curve segments above and below the confidence level. The S-factor, which is defined as the ratio of the effective oscillation time and the total time, demonstrates distinct values between type-C and type-B QPOs. Based on our results, the S-factor for type-B QPO is very close or equal to 0, no matter the confidence level is set as 95% or 68%, while the S-factor of type-C QPO is significantly higher, especially in the 68% confidence level case. We discuss the implications of the wavelet results on resolving type-B and type-C QPOs in black hole X-ray binaries.