An Electron-Scattering Time Delay in Black Hole Accretion Disks

Reverberation mapping 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 and a thermal disk. We show that the timescale for coronal 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.