  X-ray reverberation in Active Galactic Nuclei, believed to be the result of
the reprocessing of coronal photons by the underlying accretion disc, has
allowed us to probe the properties of the inner-most regions of the accretion
flow and the central black hole. Our current model (KYNREFREV) computes the
time-dependent reflection spectra of the disc as a response to a flash of
primary power-law radiation from a point source corona located on the axis of
the black hole accretion disc (lamp-post geometry). Full relativistic effects
are taken into account. The ionization of the disc is set for each radius
according to the amount of the incident primary flux and the density of the
accretion disc. We detect wavy residuals around the best-fit reverberation
model time lags at high frequencies. This result suggests that the simple
lamp-post geometry does not fully explain the X-ray source/disc configuration
in Active Galactic Nuclei. There has been a noticeable progress into the
development of codes for extended coronae (Wilkins+16, Chainakun & Young 2017,
Taylor & Reynolds 2018a,b). Indeed, the model from Chainakun & Young (2017),
consisting of two axial point sources illuminating an accretion disc that
produce the reverberation lags is able to reproduce the observed time-lag
versus frequency spectra. The goal of this paper is to observationally justify
the need for an extended corona in order to provide (in the near future) with a
mathematical formulation of a model for an extended corona in its simplest
form.
