Non‐LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei. IV. Effects of Compton Scattering and Metal Opacities

Abstract

We extend our models of the vertical structure and emergent radiation field of accretion disks around supermassive black holes described in previous papers of this series. Our models now include both a self-consistent treatment of Compton scattering and the effects of continuum opacities of the most important metal species (C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni). With these new effects incorporated, we compute the predicted spectrum from black holes accreting at nearly the Eddington luminosity (L/L Edd ≈ 0.3) and central masses of 10 6 , 10 7 , and 10 8 M ⊙ . We also consider two values of the Shakura-Sunyaev α parameter, 0.1 and 0.01, but in contrast to our previous papers, we consider a kinematic viscosity which is independent of depth.Although it has little effect when M > 10 8 M ⊙ , Comptonization grows in importance as the central mass decreases and the central temperature rises. It generally produces an increase in temperature with height in the uppermost layers of hot atmospheres. Compared to models with coherent electron scattering, Comptonized models have enhanced extreme ultraviolet/soft X-ray emission, but they also have a more sharply declining spectrum at very high frequencies.Comptonization also smears the hydrogen and the He II Lyman edges. The effects of metals on the overall spectral energy distribution are smaller than the effects of Comptonization for these parameters. Compared to pure hydrogen-helium models, models with metal continuum opacities have reduced flux in the high frequency tail, except at the highest frequencies, where the flux is very low. Metal photoionization edges are not present in the overall disk-integrated model spectra.The viscosity parameter α has a more dramatic effect on the emergent spectrum than do metal continuum opacities. As α increases (and therefore the disk column density decreases), the flux at both the high and low frequency extremes of the spectrum increases, while the flux near the peak decreases. Multitemperature blackbodies are a very poor approximation to accretion disk spectra in the soft X-ray region, and such crude modeling may greatly overestimate the accretion luminosity required to explain observed soft X-ray excesses in active galactic nuclei. In addition to our new grid of models, we also present a simple analytic prescription for the vertical temperature structure of the disk in the presence of Comptonization, and show under what conditions a hot outer layer (a corona) is formed.

The Quasar Accretion Disk Size-Black Hole Mass Relation

“…Whether we use the microlensing or flux scales for the disk, the optical emission is mostly radiated far from the scale expected for the inner edges of disk (∼60r g rather than a few r g = GM BH /c 2 ). Dai et al (2010) examined this problem for RXJ 1131-1231 using the full relativistic Hubeny et al (2001) models and found few changes from our simple standard model. We illustrate this here by adding an inner edge to the disk with R in = 0.1R λ , which is larger than we would expect from the sizes and black hole masses of these sources.…”

“…Despite nearly 40 years of work on accretion disk physics, the simple Shakura & Sunyaev (1973) thin-disk model, its relativistic cousins (e.g., Page & Thorne 1974;Hubeny & Hubeny 1997;Hubeny et al 2001;Li et al 2005) and more sophisticated implementations (e.g., Narayan et al 1997;De Villiers et al 2003;Blaes 2007) remain the standard model despite some observational reservations (see Francis et al 1991;Koratkar & Blaes 1999;Collin et al 2002). Quasar accretion disks cannot be spatially resolved with ordinary telescopes, so we have been forced to test accretion physics through time variability (e.g., Vanden Berk et al 2004;Sergeev et al 2005;Cackett et al 2007) and spectral modeling (e.g., Sun & Malkan 1989;Collin et al 2002;Bonning et al 2007).…”

The near-to-mid infrared spectrum of quasars

“…However, both theoretical considerations (e.g. Hubeny et al 2001) and measurements of polarised light (Kishimoto et al 2008) favour a power-law extension of the optical spectrum to the NIR, though the actual spectral shape remains unclear. At the same time, and while the geometry and composition of the dust surrounding the nucleus are still a matter of debate (e.g.…”

Magnetic Pressure Support and Accretion Disk Spectra

“…In black hole accretion disks, for example, the most detailed of such models so far are those of Hubeny et al (2001) for supermassive black holes, Davis et al (2005) for stellar mass black holes, and Hui et al (2005) for intermediate mass black holes. The models fully account for relativistic effects, and include a detailed non-LTE treatment of level populations of hydrogen, helium, and abundant metals.…”