Line driven winds and the UV turnover in AGN accretion discs

Abstract

AGN SEDs generally show a turnover at lambda 1000A, implying a maximal accretion disc (AD) temperature of T_max~50,000K. Massive O stars display a similar T_max, associated with a sharp rise in a line driven mass loss Mdot_wind with increasing surface temperature. AGN AD are also characterized by similar surface gravity to massive O stars. The Mdot_wind of O stars reaches ~10^-5 Msun/year. Since the surface area of AGN AD can be 10^6 larger, the implied Mdot_wind in AGN AD can reach the accretion rate Mdot. A rise to Mdot_wind Mdot towards the AD center may therefore set a similar cap of T_max~50,000K. To explore this idea, we solve the radial structure of an AD with a mass loss term, and calculate the implied AD emission using the mass loss term derived from observations of O stars. We find that Mdot_wind becomes comparable to Mdot typically at a few 10s of GM/c^2. Thus, the standard thin AD solution is effectively truncated well outside the innermost stable orbit. The calculated AD SED shows the observed turnover at lambda~1000A, which is weakly dependent on the AGN luminosity and black hole mass. The AD SED is generally independent of the black hole spin, due to the large truncation radius. However, a cold AD (low Mdot, high black hole mass) is predicted to be windless, and thus its SED should be sensitive to the black hole spin. The accreted gas may form a hot thick disc with a low radiative efficiency inside the truncation radius, or a strong line driven outflow, depending on its ionization state.