An Electron-Scattering Time Delay in Black Hole Accretion Disks

Reverberation mapping of X-ray binaries is a promising technique to map out the structures closest to a black hole that cannot be imaged. These X-ray-emitting structures roughly divide into two components: a non-thermal corona that is geometrically compact, and a thermal disk that is geometrically thin. During the intermediate state, the response time for coronal variability to get thermalized by the disk is much longer than current reverberation models predict, based on considering only light-travel time delays. We show that the timescale for irradiation to thermalize inside the electron-scattering-dominated accretion disk atmosphere can plausibly rival or exceed the light-travel timescale, potentially explaining the anomalously long-duration thermal reverberation lags in X-ray binaries. Our analytic toy model cannot fully explain the observed lags, but it motivates the need for higher-fidelity models that include time delay effects from electron scattering.