Vertical Structure of Gas Pressure-Dominated Accretion Disks with Local Dissipation of Turbulence and Radiative Transport

(shortened) We calculate the vertical structure of a local patch of an accretion disk in which heating by dissipation of MRI-driven MHD turbulence is balanced by radiative cooling. Heating, radiative transport, and cooling are computed self-consistently with the structure by solving the equations of radiation MHD in the shearing-box approximation. Using a fully 3-d and energy-conserving code, we compute the structure of this disk segment over a span of more than five cooling times. After a brief relaxation period, a statistically steady-state develops. Measuring height above the midplane in units of the scale-height H predicted by a Shakura-Sunyaev model, we find that the disk atmosphere stretches upward, with the photosphere rising to about 7H, in contrast to the approximately 3H predicted by conventional analytic models. This more extended structure, as well as fluctuations in the height of the photosphere, may lead to departures from Planckian form in the emergent spectra. Dissipation is distributed across the region within roughly 3H of the midplane, but is very weak at greater altitudes. Because fluctuations in the dissipation are particularly strong away from the midplane, the emergent radiation flux can track dissipation fluctuations with a lag that is only 0.1--0.2 times the mean cooling time of the disk. Long timescale asymmetries in the dissipation distribution can also cause significant asymmetry in the flux emerging from the top and bottom surfaces of the disk. Radiative diffusion dominates Poynting flux in the vertical energy flow throughout the disk.Comment: accepted by Ap

Magnetic pressure support and accretion disk spectra

Stellar atmosphere models of ionized accretion disks have generally neglected the contribution of magnetic fields to the vertical hydrostatic support, although magnetic fields are widely believed to play a critical role in the transport of angular momentum. Simulations of magnetorotational turbulence in a vertically stratified shearing box geometry show that magnetic pressure support can be dominant in the upper layers of the disk. We present calculations of accretion disk spectra that include this magnetic pressure support, as well as a vertical dissipation profile based on simulation. Magnetic pressure support generically produces a more vertically extended disk atmosphere with a larger density scale height. This acts to harden the spectrum compared to models that neglect magnetic pressure support. We estimate the significance of this effect on disk-integrated spectra by calculating an illustrative disk model for a stellar mass black hole, assuming that similar magnetic pressure support exists at all radii.Comment: submitted to Ap

The Velocity Field of Quasar Broad Emission Line Gas

In this Letter, the broad emission line (BEL) profiles of superluminal quasars with apparent jet velocities, $\beta_{a}>10$, (ultraluminal QSOs, or ULQSOs hereafter) are studied as a diagnostic of the velocity field of the BEL emitting gas in quasars. The ULQSOs are useful because they satisfy a very strict kinematical constraint, their parsec scale jets must be propagating within $12^{\circ}$ of the line of sight. We know the orientation of these objects with great certainty. The large BEL FWHM, $\sim 3,000 \mathrm{km/s} - 6,000 \mathrm{km/s}$, in ULQSOs tend to indicate that the BEL gas has a larger component of axial velocity (either random or in a wind) along the jet direction than previously thought.Comment: To appear in ApJ Letter

The black hole mass versus velocity dispersion relation in QSOs/Active Galactic Nuclei: observational appearance and black hole growth

Studies of massive black holes (BHs) in nearby galactic centers have revealed a tight correlation between BH mass and galactic velocity dispersion. In this paper we investigate how the BH mass versus velocity dispersion relation and the nuclear luminosity versus velocity dispersion relation in QSOs/active galactic nuclei (AGNs) are connected with the BH mass versus velocity dispersion relation in local galaxies, through the nuclear luminosity evolution of individual QSOs/AGNs and the mass growth of individual BHs. In the study we ignore the effects of BH mergers and assume that the velocity dispersion does not change significantly during and after the nuclear activity phase. Using the observed correlation in local galaxies and an assumed form of the QSO/AGN luminosity evolution and BH growth, we obtain the simulated observational appearance of the BH mass versus velocity dispersion relation in QSOs/AGNs. The simulation results illustrate how the BH accretion history (e.g., the lifetime of nuclear activity and the possibility that QSOs/AGNs accrete at a super-Eddington accretion rate at the early evolutionary stage) can be inferred from the difference between the relation in QSOs/AGNs and that in local galaxies. We also show how the difference may be weakened by the flux limit of telescopes. We expect that a large complete sample of QSOs/AGNs with accurate BH mass and velocity dispersion measurements will help to quantitatively constrain QSO/AGN luminosity evolution and BH growth models.Comment: 20 pages, including 4 figures; revised to match the published versio

General Relativistic Magnetohydrodynamic Simulations of Black Hole Accretion Disks: Results and Observational Implications

A selection of results from the general relativistic MHD accretion simulations described in the previous talk are presented. We find that the magnetic field strength increases sharply with decreasing radius and is also enhanced near rapidly-spinning black holes. The greater magnetic field strength associated with rapid black hole rotation leads to a large outward electromagnetic angular momentum flux that substantially reduces both the mean accretion rate and the net accreted angular momentum per unit rest-mass. This electromagnetic stress strongly violates the traditional guess that the accretion stress vanishes at and inside the marginally stable orbit. Possible observational consequences include a constraint on the maximum spin of black holes, enhancement to the radiative efficiency, and concentration of fluorescent Fe Kalpha to the innermost part of the accretion disk.Comment: invited review at the conference "Stellar-mass, Intermediate-mass, and Supermassive Black Holes", held in Kyoto, Japan, Octorber 28-31, 2003, to be published in Progress of Theoretical Physics Supplemen

Virial Masses of Black Holes from Single Epoch Spectra of AGN

We describe the general problem of estimating black hole masses of AGN by calculating the conditional probability distribution of M_BH given some set of observables. Special attention is given to the case where one uses the AGN continuum luminosity and emission line widths to estimate M_BH, and we outline how to set up the conditional probability distribution of M_BH given the observed luminosity, line width, and redshift. We show how to combine the broad line estimates of M_BH with information from an intrinsic correlation between M_BH and L, and from the intrinsic distribution of M_BH, in a manner that improves the estimates of M_BH. Simulation was used to assess how the distribution of M_BH inferred from the broad line mass estimates differs from the intrinsic distribution, and we find that this can lead to an inferred distribution that is too broad. We use these results and a sample of 25 sources that have recent reverberation mapping estimates of AGN black hole masses to investigate the effectiveness of using the C IV emission line to estimate M_BH and to indirectly probe the C IV region size--luminosity (R--L) relationship. We estimated M_BH from both C IV and H-Beta for a sample of 100 sources, including new spectra of 29 quasars. We find that the two emission lines give consistent estimates if one assumes R \propto L^{1/2}_{UV} for both lines.Comment: 38 pages, 6 figures, accepted by Ap