Bulk Comptonization by Turbulence in Accretion Disks

TL;DR

This paper investigates how turbulence-induced bulk Comptonization influences the X-ray spectra of radiation pressure dominated accretion disks, highlighting the roles of turbulence divergence and optical depth in spectral formation.

Contribution

It introduces a comprehensive framework for modeling bulk Comptonization by turbulence, including divergence effects and optical depth regimes, applicable to radiation MHD simulations.

Findings

Bulk Comptonization can dominate thermal Comptonization in turbulent accretion disks.

Divergenceless turbulence is treated as thermal Comptonization via radiation viscous dissipation.

Optically thick turbulence suppresses viscous dissipation, affecting photon spectrum evolution.

Abstract

Radiation pressure dominated accretion discs around compact objects may have turbulent velocities that greatly exceed the electron thermal velocities within the disc. Bulk Comptonization by the turbulence may therefore dominate over thermal Comptonization in determining the emergent spectrum. Bulk Comptonization by divergenceless turbulence is due to radiation viscous dissipation only. It can be treated as thermal Comptonization by solving the Kompaneets equation with an equivalent "wave" temperature, which is a weighted sum over the power present at each scale in the turbulent cascade. Bulk Comptonization by turbulence with non-zero divergence is due to both pressure work and radiation viscous dissipation. Pressure work has negligible effect on photon spectra in the limit of optically thin turbulence, and in this limit radiation viscous dissipation alone can be treated as thermal Comptonization with a temperature equivalent to the full turbulent power. In the limit of extremely optically thick turbulence, radiation viscous dissipation is suppressed, and the evolution of local photon spectra can be understood in terms of compression and expansion of the strongly coupled photon and gas fluids. We discuss the consequences of these effects for self-consistently resolving and interpreting turbulent Comptonization in spectral calculations in radiation MHD simulations of high luminosity accretion flows.