X-ray Reverberation Observational Modelling in Active Galactic Nuclei

TL;DR

This paper reviews X-ray reverberation in Active Galactic Nuclei, highlighting the limitations of simple models and advocating for extended corona models to better explain observed time lags.

Contribution

It provides observational evidence supporting the need for extended corona models over simple lamp-post geometries in AGN reverberation studies.

Findings

Detection of residuals indicating model inadequacy

Progress in developing extended corona models

Support for extended corona in explaining time lags

Abstract

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.