Thermally driven wind as the origin of warm absorbers in AGN

Abstract

Warm absorbers are present in many Active Galactic Nuclei (AGN), seen as mildly ionised gas outflowing with velocities of a few hundred to a few thousand kilometres per second. These slow velocities imply a large launch radius, pointing to the broad line region and/or torus as the origin of this material. Thermal driving was originally suggested as a plausible mechanism for launching this material but recent work has focused instead on magnetic winds, unifying these slow, mildly ionised winds with the more highly ionised ultra-fast outflows. Here we use the recently developed quantitative models for thermal winds in black hole binary systems to predict the column density, velocity and ionisation state from AGN. Thermal winds are sensitive to the spectral energy distribution (SED), so we use realistic models for SEDs which change as a function of mass and mass accretion rate, becoming X-ray weaker (and hence more disc dominated) at higher Eddington ratio. These models allow us to predict the launch radius, velocity, column density and ionisation state of thermal winds as well as the mass loss rate and energetics. While these match well to some of the observed properties of warm absorbers, the data point to the presence of additional wind material, most likely from dust driving.