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A New Source of X-ray Time Lags in Accreting Black Holes

Presentation #110.41 in the session “Stellar/Compact (Poster)”.

Published onApr 01, 2022
A New Source of X-ray Time Lags in Accreting Black Holes

Accreting black holes such as those in X-ray binaries and active galactic nuclei, often show signatures of time lags between different X-ray bands. Hard lags are typically associated with fluctuations in the accretion flow, while soft lags are explained by the light-crossing time between the corona and the disk (reverberation). The radiation component reprocessed in the disk, which shows characteristic reflection features such as the iron K emission, is commonly assumed to respond instantaneously to any variability in the corona. Here we explore a new source of soft lags caused by the time that reflected photons take to diffuse in the hot layers of the illuminated accretion disk. By only considering electron scattering, we estimate the time spent by the random walk of photons in the disk atmosphere. We show that this process, which is a function of the gas density, increases the X-ray lags and can produce an enhancement of the reverberation signal in the lag-energy spectrum. We argue that this process is responsible for the apparent discrepancy observed in the strength of the reflection component required by current models to reproduce both the lag-energy and flux-energy spectra.

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