An Insight-HXMT view of the mHz quasi-regular modulation phenomenon in the black hole X-ray binary 4U 1630-47

TL;DR

This study analyzes the spectral-timing properties of the 60 mHz quasi-regular modulation in the black hole X-ray binary 4U 1630-47 during its 2021 outburst, revealing energy dependence and potential accretion instability origins.

Contribution

First detailed energy-dependent analysis of mHz QRM in 4U 1630-47 using HXMT data, linking QRM behavior to accretion processes and corona instability.

Findings

QRM fractional rms increases with photon energy.

QRM exhibits soft time lags decreasing with energy.

QRM recurrence occurs on sub-hour timescales.

Abstract

Here we report the spectral-timing results of the black hole X-ray binary 4U 1630-47 during its 2021 outburst using observations from the Hard X-ray Modulation Telescope. Type-C quasi-periodic oscillations (QPOs) in 1.6--4.2 Hz and quasi-regular modulation (QRM) near 60 mHz are detected during the outburst. The mHz QRM has a fractional rms of 10%--16% in the 8--35 keV energy band with a Q factor (frequency/width) of 2--4. Benefiting from the broad energy band of hxmt, we study the energy dependence of the 60 mHz QRM in 1--100 keV for the first time. We find that the fractional rms of the mHz QRM increases with photon energy, while the time lags of the mHz QRM are soft and decrease with photon energy. Fast recurrence of the mHz QRM, in a timescale of less than one hour, has been observed during the outburst. During this period, the corresponding energy spectra moderately change when the source transitions from the QRM state to the non-QRM state. The QRM phenomena also shows a dependence with the accretion rate. We suggest that the QRM could be caused by an unknown accretion instability aroused from the corona.