Omer Blaes

Local shear instabilities in weakly ionized, weakly magnetized disks

We extend the analysis of axisymmetric magnetic shear instabilities from ideal magnetohydrodynamic (MHD) flows to weakly ionized plasmas with coupling between ions and neutrals caused by collisions, ionization, and recombination. As part of the analysis, we derive the single-fluid MHD dispersion relation without invoking the Boussinesq approximation. This work expands the range of applications of these instabilities from fully ionized accretion disks to molecular disks in galaxies and, with somewhat more uncertainty, to protostellar disks. Instability generally requires the angular velocity to decrease outward, the magnetic field strengths to be subthermal, and the ions and neutrals to be sufficiently well coupled. If ionization and recombination processes can be neglected on an orbital timescale, adequate coupling is achieved when the collision frequency of a given neutral with the ions exceeds the local epicyclic freqency. When ionization equilibrium is maintained on an orbital timescale, a new feature is present in the disk dynamics: in contrast to a single-fluid system, subthermal azimuthal fields can affect the axisymmetric stability of weakly ionized two-fluid systems. We discuss the underlying causes for this behavior. Azimuthal fields tend to be stabilizing under these circumstances, and good coupling between the neutrals and ions requires the collision frequency to exceed the epicyclic frequency by a potentially large secant factor related to the magnetic field geometry. When the instability is present, subthermal azimuthal fields may also reduce the growth rate unless the collision frequency is high, but this is important only if the field strengths are very subthermal and/or the azimuthal field is the dominant field component. We briefly discuss our results in the context of the Galactic center circumnuclear disk, and suggest that the shear instability might be present there, and be responsible for the observed turbulent motions.

Global General Relativistic Magnetohydrodynamic Simulation of a Tilted Black Hole Accretion Disk

This paper presents a continuation of our efforts to numerically study accretion disks that are misaligned (tilted) with respect to the rotation axis of a Kerr black hole. Here we present results of a global numerical simulation which fully incorporates the effects of the black hole spacetime as well as magnetorotational turbulence that is the primary source of angular momentum transport in the flow. This simulation shows dramatic differences from comparable simulations of untilted disks. Accretion onto the hole occurs predominantly through two opposing plunging streams that start from high latitudes with respect to both the black hole and disk midplanes. This is due to the aspherical nature of the gravitational spacetime around the rotating black hole. These plunging streams start from a larger radius than would be expected for an untilted disk. In this regard, the tilted black hole effectively acts like an untilted black hole of lesser spin. Throughout the duration of the simulation, the main body of the disk remains tilted with respect to the symmetry plane of the black hole; thus, there is no indication of a Bardeen-Petterson effect in the disk at large. The torque of the black hole instead principally causes a global precession of the main disk body. In this simulation, the precession has a frequency of 3(M☉/M) Hz, a value consistent with many observed low-frequency quasi-periodic oscillations. However, this value is strongly dependent on the size of the disk, so this frequency can be expected to vary over a large range.

The Ultraviolet and Optical Continuum Emission in Active Galactic Nuclei: The Status of Accretion Disks

A fundamental component of models of active galactic nuclei (AGNs) is an accretion disk around a supermassive black hole. However, the nature of this accretion disk is not well understood, and current models do not provide a satisfactory explanation of the optical/UV continuum observed in AGNs. In this paper we review the substantial theoretical and observational progress made in the field. We also try to point out future research directions that would be fruitful in trying to obtain a complete, self-consistent model of the continuum-emitting regions.

Relativistic accretion disk models of high-state black hole X-ray binary spectra

We present calculations of non-LTE, relativistic accretion disk models applicable to the high/soft state of black hole X-ray binaries. We include the effects of thermal Comptonization and bound-free and free-free opacities of all abundant ion species. Taking into account the relativistic propagation of photons from the local disk surface to an observer at infinity, we present spectra calculated for a variety of accretion rates, black hole spin parameters, disk inclinations, and stress prescriptions. We also consider nonzero inner torques on the disk and explore different vertical dissipation profiles, including some that are motivated by recent radiation magnetohydrodynamic (MHD) simulations of magnetorotational turbulence. Bound-free metal opacity generally produces significantly less spectral hardening than previous models that only considered Compton scattering and free-free opacity. We find that the resulting effective photosphere usually lies at a small fraction of the total column depth, producing spectra that are remarkably independent of the stress prescription and vertical structure assumptions. We provide detailed comparisons between our models and the widely used multicolor disk model. Frequency-dependent discrepancies exist that may affect the parameters of other spectral components when this simpler disk model is used to fit modern X-ray data. For a given source, our models predict that the luminosity in the high/soft state should approximately scale with the fourth power of the empirically inferred maximum temperature, but with a slight hardening at high luminosities. This is in good agreement with observations.

RADIATION-DOMINATED DISKS ARE THERMALLY STABLE

When the accretion rate is more than a small fraction of Eddington, the inner regions of accretion disks around black holes are expected to be radiation dominated. However, in the α-model, these regions are also expected to be thermally unstable. In this paper, we report two three-dimensional radiation magnetohydrodynamic simulations of a vertically stratified shearing box in which the ratio of radiation to gas pressure is ~10, and yet no thermal runaway occurs over a timespan 40 cooling times. Where the time-averaged dissipation rate is greater than the critical dissipation rate that creates hydrostatic equilibrium by diffusive radiation flux, the time-averaged radiation flux is held to the critical value, with the excess dissipated energy transported by radiative advection. Although the stress and total pressure are well correlated as predicted by the α-model, we show that stress fluctuations precede pressure fluctuations, contrary to the usual supposition that the pressure controls the saturation level of the magnetic energy. This fact explains the thermal stability. Using a simple toy model, we show that independently generated magnetic fluctuations can drive radiation pressure fluctuations, creating a correlation between the two while maintaining thermal stability.

The kozai mechanism and the evolution of binary supermassive black holes

We consider the dynamical evolution of bound, hierarchical triples of supermassive black holes that might be formed in the nuclei of galaxies undergoing sequential mergers. The tidal force of the outer black hole on the inner binary produces eccentricity oscillations through the Kozai mechanism, and this can substantially reduce the gravitational wave merger time of the inner binary. We numerically calculate the merger time for a wide range of initial conditions and black hole mass ratios, including the effects of octupole interactions in the triple as well as general relativistic periastron precession in the inner binary. The semimajor axes and the mutual inclination of the inner and outer binaries are the most important factors affecting the merger time. We find that for a random distribution of inclination angles and approximately equal mass black holes, it is possible to reduce the merger time of a near circular inner binary by more than a factor of 10 in over 50% of all cases. We estimate that a typical exterior quadrupole moment from surrounding matter in the galaxy may also be sufficient to excite eccentricity oscillations in supermassive black hole binaries and to accelerate black hole mergers.

Testing Accretion Disk Theory in Black Hole X-Ray Binaries

WepresentresultsfromspectralmodelingofthreeblackholeX-raybinaries:LMCX-3,GROJ1655� 40,andXTE J1550� 564. Using a sample of disk-dominated observations, we fit the data with a range of spectral models that in- cludes a simple multitemperature blackbody (DISKBB), a relativistic accretion disk model based on color-corrected blackbodies (KERRBB), and a relativistic model based on non-LTE atmosphere models within anprescription (BHSPEC). BHSPEC provides the best fit for a BeppoSAXobservation of LMC X-3, which has the broadest energy coverage of our sample. It also provides the best fit for multiple epochs of Rossi X-Ray Timing Explorer (RXTE) data in this source, except at the very highest luminosity (L/LEdd k0:7), where additional physics must be coming into play.BHSPECisalsothebest-fitmodelformultiepochRXTEobservationsofGROJ1655� 40andXTEJ1550� 564, although the best-fit inclination of the inner disk differs from the binary inclination. All our fits prefer � ¼ 0:01 to � ¼ 0:1, in apparent disagreement with the large stresses inferred from the rapid rise times observed in outbursts of these two sources. In all three sources our fits imply moderate black hole spins (a� � 0:1 0:8), but this is sensitive to the reliability of independent measurements of these system parameters and to the physical assumptions that un- derly our spectral models. Subject headingg accretion, accretion disks — black hole physics — X-rays: binaries

Simultaneous EUVE/ASCA/RXTE Observations of NGC 5548

We present simultaneous observations by EUVE, ASCA, and RXTE of the type?1 Seyfert galaxy NGC?5548. These data indicate that variations in the EUV emission (at ~0.2 keV) appear to lead similar modulations in higher energy (1 keV) X-rays by ~10-30 ks. This is contrary to popular models which attribute the correlated variability of the EUV, UV and optical emission in type?1 Seyferts to reprocessing of higher energy radiation. This behavior instead suggests that the variability of the optical through EUV emission is an important driver for the variability of the harder X-rays which are likely produced by thermal Comptonization. We also compute the power density spectra at the various energy bands probed by these observations. Over 10-300 ks timescales, the emission in EUV shows about a factor of two greater rms variability than that of the 2-20 keV RXTE-PCA band?18. ? 1.4% versus 7.4 ? 0.6%. On longer timescales, we construct a PDS from 1-12 keV RXTE-ASM data which shows evidence for a break at about 6 ? 10-8 Hz. Furthermore, we find that the combined RXTE-ASM/PCA power spectrum is remarkably similar in shape to PDSs found for the low/hard states of Galactic black hole candidates such as Cygnus X-1. The implied scaling factor of ~106 is comparable to the expected mass ratio for these two objects. In addition, we investigate the spectral characteristics of the fluorescent iron K? line and Compton reflection emission. In contrast to prior measurements of these spectral features, we find that the iron K? line has a relatively small equivalent width (WK? ~ 100 eV) and that the reflection component is consistent with a covering factor which is significantly less than unity (?/2? ~ 0.4-0.5). Notably, although the 2-10?keV X-ray flux varies by ~?25% and the derived reflection fraction appears to be constant throughout our observations, the flux in the iron K? line is also constant. This behavior is difficult to reconcile in the context of standard Compton reflection models.

Non-LTE models and theoretical spectra of accretion disks in active galactic nuclei. IV. Effects of Compton scattering and metal opacities

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/LEdd ≈ 0.3) and central masses of 106, 107, and 108 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 that is independent of depth. Although it has little effect when M > 108 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.

NON-LTE MODELS AND THEORETICAL SPECTRA OF ACCRETION DISKS IN ACTIVE GALACTIC NUCLEI. III. INTEGRATED SPECTRA FOR HYDROGEN-HELIUM DISKS

We have constructed a grid of non-LTE disk models for a wide range of values of black hole mass and mass accretion rate, for several values of the viscosity parameter a, and for two extreme values of the black hole spin: the maximum-rotation Kerr black hole, and the Schwarzschild (nonrotating) black hole. Our procedure calculates self-consistently the vertical structure of all disk annuli together with the radiation —eld, without any approximations imposed on the optical thickness of the disk, and without any ad hoc approximations to the behavior of the radiation intensity. The total spectrum of a disk is computed by summing the spectra of the individual annuli, taking into account the general relativistic transfer function. The grid covers nine values of the black hole mass between M \ 1/8 ) 109 and 32 ) 109 with a M _ twofold increase of mass for each subsequent value; and eleven values of the mass accretion rate, each a power of 2 times 1 yr~1. The highest value of the accretion rate corresponds to the total luminosity M _ We show the vertical structure of individual annuli within the set of accretion disk models, L /L Edd B 0.3. along with their local emergent —ux, and discuss the internal physical self-consistency of the models. We then present the full disk-integrated spectra and discuss a number of observationally interesting proper- ties of the models, such as optical/ultraviolet colors, the behavior of the hydrogen Lyman limit region, polarization, and the number of ionizing photons. Our calculations are far from de—nitive in terms of the input physics, but generally we —nd that our models exhibit rather red optical/UV colors. Flux discon- tinuities in the region of the hydrogen Lyman limit are only present in cool, low-luminosity models, while hotter models exhibit blueshifted changes in spectral slope. Subject headings: accretion, accretion disksgalaxies: activegalaxies: nuclei