X-Ray Reverberation From Black Hole Accretion Disks with Realistic Geometric Thickness

TL;DR

This paper investigates how realistic, finite-thickness accretion disks around black holes affect X-ray reverberation signals, using advanced raytracing simulations to improve understanding of the inner accretion flow and corona geometry.

Contribution

It introduces the Fenrir raytracing suite to model reverberation with finite disk thickness, challenging the thin disk assumption and exploring corona-disk spatial configurations.

Findings

Finite disk thickness alters reverberation transfer functions.

Disk-hugging corona models do not match observed signatures.

Flaring corona separated from the disk better explains observations.

Abstract

X-ray reverberation in AGN, believed to be the result of the reprocessing of corona photons by the underlying accretion disk, has allowed us to probe the properties of the inner-most regions of the accretion flow and the central black hole. This process is modeled via raytracing in the Kerr metric, with the disk thickness almost ubiquitously assumed to be negligible (razor-thin) and the corona commonly approximated as a point source located along the polar axis (a lamppost). In this work, we use the new raytracing suite, Fenrir, to explore the effect that accretion disk geometry has on reverberation signatures, assuming a lamppost configuration but allowing for a finite disk scale height. We characterize the signatures of finite disk thickness in the reverberation transfer-function and calculate how they might manifest in observed lag-frequency spectra. We also show that a disk-hugging corona (approximated by off-axis point-like flares) exhibits characteristics that are qualitatively different from observation, thus providing further evidence for a flaring corona that is separated from the underlying disk material.