Radiation-ionization hydrodynamic simulations of AGN line-driven winds lead to transient shielding and BAL/UFO signatures

ABSTRACT Disc winds from active galactic nuclei (AGNs) can be launched by radiation pressure acting on spectral lines. However, launching a line-driven wind in the X-ray-rich environment of AGNs is challenging, as the wind easily gets overionized. Previous simulations suggested that X-ray self-shielding could enable line-driving, though it remained unclear whether this relied on simplified treatments of radiation and ionization. Here, we revisit the X-ray shielding scenario using the first multifrequency multidirectional Monte Carlo radiative photoionization hydrodynamical simulations of AGN line-driven winds. We find that sustaining a steady wind with mass-loss rates of $\approx 20 {{\ \rm per\ cent}}$ of the accretion rate requires an unrealistically weak X-ray flux ($\alpha _{\rm OX}\lt -3$). For stronger X-ray emission ($-3\lt \alpha _{\rm OX}\lt -1$), self-shielding is only transient, leading to episodic ejections with mass-loss rates approaching the accretion rate. Our steady winds naturally produce FeLoBAL, HiBAL, and broad emission-line signatures, depending on the disc spectral energy distribution and the observer’s inclination. At moderate X-ray luminosities ($\alpha _{\rm OX}\sim -3$), transient winds can generate short-lived BAL and ultra-fast outflow (UFO) features. At the highest X-ray luminosities ($\alpha _{\rm OX}\sim -1$), the winds are too ionized to form BALs, but still produce UFOs. These results imply that additional physics is required to explain BAL outflows at realistic X-ray levels and to drive winds strong enough for AGN feedback. None the less, our simulations provide a new framework for interpreting the observed diversity of AGN outflow signatures with fully coupled radiation and dynamics.