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Abstract:
The X‐ray emission from accreting black hole (BH) systems displays strong variability. Short reverberation lags are expected between the primary hard X‐ray continuum and the reprocessed disk emission. These lags depend on light‐travel distances, thus offering the opportunity to map the geometry of the innermost accretion flow. X‐ray reverberation lags have been observed in several BH accreting systems. In radio‐quiet active galactic nuclei (AGN), these lags scale with BH mass and point to a reprocessing region located close to the comptonizing X‐ray corona. On the other hand, reverberation lags detected in the hard state of some BH X‐ray binaries (BHXRB) suggest a different accretion flow geometry than in AGN, showing evidence of evolution as a function of luminosity.