Beyond Disk Truncation: X-ray Reverberation Signatures of an Outflowing Corona

Abstract

The corona in black hole X-ray binaries (BHXRBs) is likely dynamic during outbursts. A mildly relativistic outflowing corona has been proposed to explain hard spectra, weak disk reflection, and the higher than expected polarization degree observed in the hard state of X-ray binaries. In this work, we investigate the spectral and timing effects of coronal outflow using Monte Carlo radiative-transfer simulations. The model consists of a geometrically thin, optically thick truncated disk and an ellipsoidal corona with a prescribed bulk outflow velocity. We calculate the Comptonized continuum, the disk reflection component, and the lag-frequency spectra for different outflow velocities and disk truncation radii. We find that increasing the outflow velocity reduces the reflection fraction through relativistic beaming, because fewer Comptonized photons irradiate the disk. For $β\lesssim 0.5$, the high-frequency soft lag is only weakly affected by the outflow velocity, whereas increasing the disk truncation radius shifts the zero-crossing frequency($ν_0$) to substantially lower frequencies. Thus, timing properties can help distinguish the coronal-outflow scenario from the disk-truncation scenario. We further apply the model to MAXI J1820+070 and find that its unusual $R-Γ$ anti-correlation during the plateau phase is qualitatively consistent with a contracting corona accompanied by increasing bulk velocity. Disk recession alone would predict the opposite evolution of $ν_0$ under the assumptions of our model. Future polarization calculations will provide an additional test of extended outflowing-corona models.