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 ∼ 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 ∼ 88 mHz is detected and the ∼ 5 Hz and ∼ 7
Hz LFQPOs are almost overlapping. In the third group, namely behavior C, the
QRM frequency below ∼ 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 Fpowerlaw​ / Ftotal​. Finally, we give
our understanding for this mHz QRM phenomena.Comment: 14pages, 15 figure