Time-dependent AGN disc winds – I. X-ray irradiation

Abstract

We study active galactic nucleus (AGN) line-driven disc winds using time-dependent radiation hydrodynamics. The key criterion for determining wind launching is the coupling strength of the ultraviolet radiation field via the spectral lines of the gas. The strength of these lines in turn relies crucially on the gas ionization state, determined by the local X-ray intensity. We consider a suite of models where the central ionizing radiation is affected by scattering, absorption, and re-emission by the intervening gas. In a pure attenuation model, the disc launches an episodic wind, as previous studies have shown. Including scattering or re-emission tends to weaken the wind, lowering the mass flux and outflow velocity and, if sufficiently dominant, suppressing the outflow entirely. However, the exponential nature of radiative attenuation means that only a modest, factor of a few, increase in the absorption cross-section can overcome the wind suppression due to scattering and re-emission. We find mass outflow rates of ∼20 per cent or more of the assumed inflow rate through the disc, indicating that radiation-driven winds may significantly alter the structure of the accretion flow. The winds also supply a large, time-varying column of material above the nominal constant disc scale height, which will determine the geometry of reprocessed emission from the central source. Our results suggest the need for accurate photoionization modelling, radiation transport, and accretion disc physics, to study their effects on the AGN disc winds.