The Vertical Structure and Ultraviolet Spectrum of Accretion Disks Heated by Internal Dissipation in Active Galactic Nuclei

Abstract

We present an improved calculation of the vertical structure and ultraviolet spectrum of a dissipative accretion disk in an active galactic nucleus (AGN). We calculate model spectra in which the viscous stress is proportional to the total pressure, the gas pressure only, and the geometric mean of the radiation and gas pressures. As a result of a more complete treatment of absorptive opacity, we find greater overall spectral curvature than Laor & Netzer did, as well as larger amplitudes in both the Lyman and He II photoionization edges. The local blackbody approximation is not a good description of the near-UV spectrum. With relativistic corrections (appropriate to nonrotating black holes) included, we find that the near-UV spectrum hardens with increasing /m8 ( is the accretion rate in Eddington units, m8 the black hole mass in units of 108 M☉). The near-UV spectrum is consistent with observations if m−18~10−3, but disks this cold would have absorption features at the Lyman edge, which are never seen in real AGNs. The edge amplitude is reduced when /m8 is larger, but then the near-UV slope is too hard to match observations. We conclude that models in which conventional disks orbit nonrotating black holes do not adequately explain UV continuum production in AGNs.