The Vertical Structure and Ultraviolet Spectrum of X-ray Irradiated Accretion Disks in Active Galactic Nuclei

Motivated by recent work indicating that the UV continuum in AGN may be produced by reradiation of energy absorbed from X-rays irradiating an accretion disk, we present a calculation of the vertical structures and ultraviolet spectra of X-ray irradiated accretion disks around massive non-rotating black holes. After finding the radial dependence of vertically-integrated quantities for these disks, we solve the equations of hydrostatic equilibrium, energy balance, and frequency-dependent radiation transfer as functions of altitude. To solve the last set of equations, we use a variable Eddington factor method. We include electron scattering, free-free, and HI, HeI, and HeII bound-free opacities and the corresponding continuum cooling processes. The incident X-ray flux heats a thin layer of material 3-4 scale heights above the midplane of the disk. This X-ray heated skin has two layers: a radiation pressure supported region in which the UV flux is created, and, immediately above this layer, a warmer zone, optically thin to UV radiation, formed where the X-ray ionization parameter is large. In the lower layer the gas pressure is nearly independent of altitude but the temperature increases upward. The fraction of the incident hard X-ray flux which emerges in the UV falls with increasing ṁ (the accretion rate in Eddington units). At frequencies below the Lyman edge the slope of the continuum (d lnLν/d ln ν) varies from -1.6 to 0.8 as ṁ/m8 increases from 0.001 to 1. Here m8 is the mass of the central black hole in units of 10M⊙. In all cases examined (0.003 ≤ ṁ ≤ 0.3 and 0.27 ≤ m8 ≤ 27), the Lyman edge appears in emission. The amplitude of the