The vertical structure and ultraviolet spectrum of X-ray irradiated accretion disks in active galactic nuclei

Abstract

Motivated by recent work indicating that the UV continuum in active galactic nuclei 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 nonrotating 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 H I, He I, and He II 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: (1) a radiation pressure-supported region in which the UV flux is created, and, immediately above this layer, (2) 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 that emerges in the UV falls with increasing ${u{m}{705F}}$ --> (the accretion rate in Eddington units). At frequencies below the Lyman edge, the slope of the continuum (d ln L?/d ln ?) varies from -1.6 to 0.8 as ${u{m}{705F}}$ -->/m -->8 increases from 0.001 to 1. Here m8 is the mass of the central black hole in units of 108 M?. In all cases examined (0.003?${u{m}{705F}}$ -->?0.3 and 0.27 ? m8 ? 27), the Lyman edge appears in emission. The amplitude of the Lyman edge feature increases with m8 but is relatively independent of ${u{m}{705F}}$ -->. The amplitude of the Lyman edge emission feature increases with disk inclination. Compton scattering in disk coronae can smooth the Lyman edge feature only if ?c 0.5, where ?c is the Thomson depth of the coronae. While the overall spectral shape predicted by X-ray irradiation may be compatible with observations, the Lyman edge emission feature it predicts is not. This finding raises questions for many otherwise plausible models in which X-ray irradiation plays a major role.