A Spectral-timing Study of the Inner Flow Geometry in MAXI J1535-571 with Insight-HXMT and NICER

Abstract

We have performed a spectral-timing analysis of the black hole X-ray binary MAXI J1535-571 during its 2017 outburst, with the aim of exploring the evolution of the inner accretion flow geometry. X-ray reverberation lags are observed in the hard-intermediate state (HIMS) and soft-intermediate state of the outburst. During the HIMS, the characteristic frequency of the reverberation lags ν 0 (the frequency at which the soft lag turns to zero in lag–frequency spectra) increases when the spectrum softens. This reflects a reduction of the spatial distance between the corona and accretion disk, when assuming the measured time lags are associated with the light travel time. We also find a strong correlation between ν 0 and the type-C quasi-periodic oscillation (QPO) centroid frequency ν QPO, which can be well explained by the Lense–Thirring precession model under a truncated disk geometry. Despite the degeneracy in the spectral modeling, our results suggest that the accretion disk is largely truncated in the low hard state, and moves inward as the spectrum softens. Combine the spectral modeling results with the ν 0 – ν QPO evolution, we are inclined to believe that this source probably has a truncated disk geometry in the hard state.