2023MNRAS.524.3215Z

We present the results of a detailed timing and spectral analysis of the quasi-regular modulation (QRM) phenomenon in the black hole X-ray binary 4U 1630-47 during its 1998 outburst observed by the Rossi X-ray Timing Explorer (RXTE). We find that the ∼50-110 mHz QRM is flux dependent, and the QRM is detected with simultaneous low-frequency quasi-periodic oscillations (LFQPOs). According to the behaviour of the power density spectrum, we divide the observations into four groups. In the first group, namely behaviour A, LFQPOs are detected, but no mHz QRM. The second group, namely behaviour B, a QRM with frequency above ∼88 mHz is detected and the ∼5 and ∼7 Hz LFQPOs are almost overlapping. In the third group, namely behaviour C, the QRM frequency below ∼88 mHz is detected and the LFQPOs are significantly separated. In the fourth group, namely behaviour D, neither QRM nor LFQPOs are detected. We study the energy dependence of the fractional rms, centroid frequency, and phase lag of QRM and LFQPOs for behaviour B and C. We then study the evolution of QRM and find that the frequency of QRM increases with hardness, while its rms decreases with hardness. We also analyse the spectra of each observation, and find that the QRM rms of behaviour B has a positive correlation with $F_{\rm power\ law}$/$F_{\rm total}$. Finally, we give our understanding for this mHz QRM phenomenon.