# The contribution of bulk Comptonization to the soft X-ray excess in AGN

## TL;DR

This paper models how bulk Comptonization, caused by turbulence and shear in radiation pressure dominated accretion flows, contributes to the soft X-ray excess in AGN, aligning with observations of specific NLS1 galaxies.

## Contribution

It introduces a detailed model of bulk Comptonization's role in AGN soft X-ray excess, incorporating turbulence and shear effects from radiation MHD simulations.

## Key findings

- Bulk Comptonization shifts the Wien tail to higher energies.
- It broadens the spectrum and lowers the gas temperature.
- Results agree with observed properties of RE1034+396.

## Abstract

Bulk velocities exceed thermal velocities for sufficiently radiation pressure dominated accretion flows. We model the contribution of bulk Comptonization to the soft X-ray excess in AGN. Bulk Comptonization is due to both turbulence and the background shear. We calculate spectra both taking into account and not taking into account bulk velocities using scaled data from radiation magnetohydrodynamic (MHD) shearing box simulations. We characterize our results with temperatures and optical depths to make contact with other warm Comptonization models of the soft excess. We chose our fiducial mass, $M = 2 \times 10^6 M_{\odot}$, and accretion rate, $L/L_{\rm Edd} = 2.5$, to correspond to those fit to the super-Eddington narrow line Seyfert 1 (NLS1) RE1034+396. The temperatures, optical depths, and Compton $y$ parameters we find broadly agree with those fit to RE1034+396. The effect of bulk Comptonization is to shift the Wien tail to higher energy and lower the gas temperature, broadening the spectrum. Observations of the soft excess in NLS1s can constrain the properties of disc turbulence if the bulk Comptonization contribution can be separated out from contributions from other physical effects, such as reflection and absorption.