A simple framework for modelling the dependence of bulk Comptonization by turbulence on accretion disc parameters

TL;DR

This paper presents a model linking bulk Comptonization by turbulence in accretion discs to fundamental disc parameters, providing insights into the soft X-ray excess in AGN and connecting observational features to disc turbulence.

Contribution

It introduces a physically motivated framework that relates bulk Comptonization to accretion disc parameters, aiding interpretation of X-ray observations and turbulence effects.

Findings

Bulk Comptonization depends on disc mass, luminosity, radius, spin, boundary conditions, and alpha.

The model can distinguish bulk Comptonization contributions from other mechanisms like absorption.

Observations of the soft X-ray excess can probe disc turbulence via this model.

Abstract

Warm Comptonization models for the soft X-ray excess in AGN do not self-consistently explain the relationship between the Comptonizing medium and the underlying accretion disc. Because of this, they cannot directly connect the fitted Comptonization temperatures and optical depths to accretion disc parameters. Since bulk velocities exceed thermal velocities in highly radiation pressure dominated discs, in these systems bulk Comptonization by turbulence may provide a physical basis in the disc itself for warm Comptonization models. We model the dependence of bulk Comptonization on fundamental accretion disc parameters, such as mass, luminosity, radius, spin, inner boundary condition, and $\alpha$. In addition to constraining warm Comptonization models, our model can help distinguish contributions from bulk Comptonization to the soft X-ray excess from those due to other physical mechanisms, such as absorption and reflection. By linking the time variability of bulk Comptonization to fluctuations in the disc vertical structure due to magnetorotational instability (MRI) turbulence, our results show that observations of the soft X-ray excess can be used to study disc turbulence in the radiation pressure dominated regime. Because our model connects bulk Comptonization to one dimensional vertical structure temperature profiles in a physically intuitive way, it will be useful for understanding this effect in future simulations run in new regimes.