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 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 (cf. Laor & Netzer 1989: LN89). As a result of a more complete treatment of absorptive opacity, we find greater overall spectral curvature than did LN89, as well as larger amplitudes in both the Lyman and HeII photoionization edges. The local black body approximation is not a good description of the near UV spectrum. With relativistic corrections (appropriate to non-rotating black holes) included, we find that the near UV spectrum hardens with increasingṁ/m 8 (ṁ is the accretion rate in Eddington units, m 8 the black hole mass in units of 10 8 M ⊙ ). The near UV spectrum is consistent with observations ifṁm −1 8 ∼ 10 −3 , but disks this cold would have large, and unobserved, absorption features at the Lyman edge. The edge amplitude is reduced whenṁ/m 8 is larger, but then the near-UV slope is too hard to match observations. We conclude that models in which conventional disks orbit non-rotating black holes do not adequately explain UV continuum production in AGN.

Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research

“…More complex and realistic models have been explored since then, and include relativistic effects in inclined disks surrounding both Kerr and Schwarzschild SBHs (Sun & Malkan 1989), modification to the black body spectrum due to electron scattering, comptonization and irradiation of the surface of the disk by an external X-ray source (Ross, Fabian & Mineshige 1992;Siemiginowska et al 1995), and the presence of an inner shock front inside which the flow becomes supersonic, and the disk gives way to a hot, ionized, pressure supported torus. Geometrically thick disks able to support accretion rates comparable to the Eddington rate have also been studied (Szuszkiewicz, Malkan & Abramowicz 1996;Sincell & Krolik 1998;Wang et al 1999;Hubeny et al 2000). Figure 24 (from Hubeny et al 2000) represents recent simulations showing the effects of varying SBH mass and accretion rate on the emerging integrated spectral energy distribution, for a disk seen nearly face-on.…”

“…Furthermore, the models rarely do a good job at fitting the available data, and often the SBH mass and other parameters enter the models in a degenerate way. For instance, Szuszkiewicz, Malkan & Abramowicz (1996) and Sincell & Krolik (1998) shows that the shape of the spectrum is determined mainly by the ratio of the SBH mass to the accretion rate, rather than by the two quantities independently.…”

The Effects of Magnetic Fields and Inhomogeneities on Accretion Disk Spectra and Polarization

“…Considerable theoretical effort has been devoted to calculating spectral models of such disks for comparison with observations (e.g., Kolykhalov & Sunyaev 1984;Laor & Netzer 1989;Shimura & Takahara 1995;Sincell & Krolik 1998;Hubeny et al 2001;Davis et al 2005;Hui et al 2005;Davis & Hubeny 2006). These models have been compared with observed colors and spectra of active galactic nuclei (e.g., Bonning et al 2007;Davis et al 2007), ultraluminous X-ray sources (Hui & Krolik 2008), and black hole X-ray binaries Done & Davis 2008).…”

Hubble Space TelescopeFaint Object Camera Spectroscopy of the Narrow‐Line Region of NGC 4151. I. Gas Kinematics

“…If such a correlation is found to hold for other nuclei, the two methods above would provide totally independent estimates for each of the quantities. However, viscous dissipation timescales are too long to reconcile with the observed continuum lags, which forces consideration of XrayÈirradiated or composite models (Sincell & Krolik 1998 ;Collier et al 1999). Therefore, with an independent measure of the central object mass, obtaining would be dependent M 0 on further development of the accretion disk theory itself, and to obtain it using AGN continuum variability would make it necessary to carry on high sampling rate, simultaneous multiwavelength campaigns on other nearby active galaxies.…”