Bulk Comptonization by Turbulence in Black Hole Accretion Discs

TL;DR

This paper explores how turbulence-induced bulk Comptonization in radiation pressure dominated accretion discs affects their spectra, providing a self-consistent model that links turbulence, temperature, and spectral features, with implications for understanding AGN X-ray excess.

Contribution

It introduces a self-consistent method to model bulk Comptonization in accretion discs, connecting turbulence effects to spectral features and enabling comparison with observations.

Findings

Bulk Comptonization shifts the Wien tail to higher energies.

It broadens the spectrum and lowers the gas temperature.

The model depends on fundamental disc parameters like mass, luminosity, and spin.

Abstract

Radiation pressure dominated accretion discs may have turbulent velocities that exceed the electron thermal velocities. Bulk Comptonization by the turbulence may therefore dominate over thermal Comptonization in determining the emergent spectrum. We discuss how to self-consistently resolve and interpret this effect in calculations of spectra of radiation MHD simulations. In particular, we show that this effect is dominated by radiation viscous dissipation and can be treated as thermal Comptonization with an equivalent temperature. We investigate whether bulk Comptonization may provide a physical basis for warm Comptonization models of the soft X-ray excess in AGN. We characterize our results with temperatures and optical depths to make contact with other models of this component. We show that bulk Comptonization shifts the Wien tail to higher energy and lowers the gas temperature, broadening the spectrum. More generally, we model the dependence of this effect on a wide range of fundamental accretion disc parameters, such as mass, luminosity, radius, spin, inner boundary condition, and alpha. 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. We also develop a global Monte Carlo code to study this effect in global radiation MHD simulations. This code can be used more broadly to compare global simulations with observed systems, and in particular to investigate whether magnetically dominated discs can explain why observed high Eddington accretion discs appear to be thermally stable.