The effect of radiation stress on gas motions in AGN accretion discs

In this paper we look at one of the effects of irradiation on a warped accr etion disc in the context of active galactic nuclei (AGN). A warp will catc h a substantial amount of the radiation emitted by the central object. We c onsider the fluid motions that may arise inside a warped disc when the surf ace is subject to a radiation stress, and also the net mass flows that resu lt. We find that, to first order, we have a balance of the viscous and Cori olis-type forces. The radial radiation stress causes outward motion of the surface layer, but only the azimuthal Poynting-Robertson drag leads to an i ncrease in the net accretion rate. We investigate the distribution of the v elocity perturbations and find them to be significant in determining the lo cal structure of the disc. An unexpected result is that the picture changes significantly when we take into account the periodic illumination of the warped disc. A type of reson ance at the local Keplerian rotation frequency causes a flow that penetrate s the whole thickness of the disc; these flows are faster than the flows du e to unchanging illumination. They totally dominate the induced flows in te rms of sheer mass, but significant impact on disc structure still occurs on ly near the surface, where velocity perturbations typically go up to some k ilometres per second.