Magnetically driven accretion disc winds: the role of gas thermodynamics and comparison to ultra-fast outflows

Winds are commonly observed in luminous active galactic nuclei. A plausible model of those winds is magnetohydrodynamic (MHD) disc winds. In the case of disc winds from a thin accretion disc, isothermal or adiabatic assumption is usually adopted in such MHD models. In this work, we perform two-dimensional MHD simulations implementing different thermal treatments (isothermal, adiabatic, and radiative) to study their effects on winds from a thin accretion disc. We find that both the isothermal model and the adiabatic model overestimate the temperature, underestimate the power of disc winds, and cannot predict the local structure of the winds, compared to the results obtained by solving the energy equation with radiative cooling and heating. Based on the model with radiative cooling and heating, the ionization parameter, the column density, and the velocity of the disc winds have been compared to the observed ultra-fast outflows (UFOs). We find that in our simulations the UFOs can only be produced inside hundreds of Schwarzschild radius. At much larger radii, no UFOs are found. Thus, the pure MHD winds cannot interpret all the observed UFOs.

Automated summary

This study demonstrates through MHD simulations with different thermal treatments that the isothermal and adiabatic models overestimate the temperature and underestimate the power of disc winds, and cannot predict the local structure of the winds. Compared to the model with radiative cooling and heating, the pure MHD winds cannot interpret all the observed ultra-fast outflows.