The origin of the lag spectra observed in AGN: Reverberation and the propagation of X-ray source fluctuations

Understand

The X-ray emission from active galactic nuclei (AGN) is highly variable.

• Measurements of time lags (characterised by lag spectra) between variability in the light curves in energy bands corresponding to directly observed continuum emission from the corona around the black hole and to X-rays reflected from the accretion disc adds a further dimension to studies of the structure and energetics of these systems.
• We seek to understand these measurements in terms of the physical parameters of the X-ray source (its location, extent, etc.) through the calculation of theoretical lag spectra for a range of source parameters in general relativistic ray tracing simulations, combined with knowledge of the observed variability of the X-ray emission from AGN.
• Due to the proximity of the emission to the central black hole, Shapiro delays are important and the effects of general relativity should be considered when interpreting the lags as the light travel time between the source and reflector.