Broadband Variability Study of Maxi J1631-479 in Its Hard-intermediate State Observed with Insight-HXMT

Abstract

Abstract We report the energy-resolved broadband timing analysis of the black hole X-ray transient MAXI J1631-479 during its 2019 outburst from February 11 to April 9, using data from the Insight−Hard X-ray Modulation Telescope (Insight-HXMT), which caught the source from its hard-intermediate state to the soft state. Thanks to the large effective area of Insight-HXMT at high energies, we are able to present the energy dependence of fast variability up to ∼100 keV. Type-C quasi-periodic oscillations (QPOs) with a frequency varying between 4.9 and 6.5 Hz are observed in the 1–100 keV energy band. While the QPO fractional rms increases with photon energy from 1 keV to ∼10 keV and remains more or less constant from ∼10 keV to ∼100 keV, the rms of the flat-top noise first increases from 1 keV to ∼8 keV and then drops to less than 0.1% above ∼30 keV. We suggest that the disappearance of the broadband variability above 30 keV could be caused by the nonthermal acceleration in the Comptonizing plasma. At the same time, the QPOs could be produced by the precession of either a small-scale jet or a hot inner flow model.