# Observations of X-ray reverberation around black holes

## TL;DR

This paper discusses the observation of X-ray reverberation lags in black hole systems, which help map the inner accretion flow geometry and reveal differences between active galactic nuclei and X-ray binaries.

## Contribution

It provides new insights into the geometry of accretion flows around black holes through analysis of X-ray reverberation lags in different systems.

## Key findings

- Reverberation lags scale with black hole mass in AGN.
- Lags in X-ray binaries suggest different accretion geometries.
- Evidence of evolution of accretion flow with luminosity.

## 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 disc 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.