The mHz quasi-regular modulations of 4U 1630--47 during its 1998 outburst

TL;DR

This paper investigates the mHz quasi-regular modulation in the black hole binary 4U 1630--47 during its 1998 outburst, revealing flux dependence, correlations with spectral states, and detailed timing behavior.

Contribution

It provides a detailed analysis of the timing and spectral properties of mHz QRM, identifying distinct behaviors and their relation to spectral states, which advances understanding of accretion processes in black hole binaries.

Findings

QRM frequency is flux dependent and varies with spectral hardness.

Different behaviors of QRM and LFQPOs are classified based on power density spectrum analysis.

QRM rms correlates positively with the power-law flux fraction.

Abstract

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 Rossi X-ray Timing Explore (RXTE). We find that the $\sim$ 50-110 mHz QRM is flux dependent, and the QRM is detected with simultaneous low frequency quasi-periodic oscillations (LFQPOs). According to the behavior of the power density spectrum, we divide the observations into four groups. In the first group, namely behavior A, LFQPOs are detected, but no mHz QRM. The second group, namely behavior B, a QRM with frequency above $\sim$ 88 mHz is detected and the $\sim$ 5 Hz and $\sim$ 7 Hz LFQPOs are almost overlapping. In the third group, namely behavior C, the QRM frequency below $\sim$ 88 mHz is detected and the LFQPOs are significantly separated. In the forth group, namely behavior 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 behavior 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 analyze the spectra of each observation, and find that the QRM rms of behavior B has a positive correlation with $\rm F_{\rm powerlaw}$ / $\rm F_{\rm total}$. Finally, we give our understanding for this mHz QRM phenomena.