Irradiation of accretion discs in active galactic nuclei due to the presence of a warm absorber

The presence of a warm absorber of considerable optical depth is seen in many AGN. We show that this medium may significantly affect the optical/UV spectrum of an AGN by backscattering a fraction of the total radiation flux towards the disc surface. We consider in detail the case in which the disc extends down to a marginally stable orbit, all the emission comes from the disc surface, and the scattering medium forms a cone around the symmetry axis. Disc irradiation results in a much flatter optical/UV continuum than is predicted by standard disc models. The effect depends both on the total optical depth of the warm absorber and on the specific density distribution of this medium, so analysis of the optical/UV continuum allows constraints to be obtained for the warm absorber that are complementary to those obtained from the soft X-ray data analysis. We give results for two example sources-RE J1034+396 and PG 1211+143, and for the bluest composite quasar spectrum of Richards et al. obtained from the SDSS.