arXiv subjects

Omer Blaes

Publications and source records attributed to Omer Blaes.

69 records · Page 4Linked to original sources

arXiv2003

Local Radiative Hydrodynamic and Magnetohydrodynamic Instabilities in Optically Thick Media

We examine the local conditions for radiative damping and driving of short wavelength, propagating hydrodynamic and magnetohydrodynamic (MHD) waves in static, optically thick, stratified equilibria. We show that so-called strange modes in stellar oscillation theory and magnetic photon bubbles are intimately related and are both fundamentally driven by the background radiation flux acting on compressible waves. We identify the necessary criteria for unstable driving of these waves, and show that this driving can exist in both gas and radiation pressure dominated media, as well as pure Thomson scattering media in the MHD case. The equilibrium flux acting on opacity fluctuations can drive both hydrodynamic acoustic waves and magnetosonic waves unstable. In addition, magnetosonic waves can be driven unstable by a combination of the equilibrium flux acting on density fluctuations and changes in the background radiation pressure along fluid displacements. We briefly describe the conditions under which these instabilities might be manifested in both main sequence stellar envelopes and accretion disks.

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arXiv2002

A First Close Look at the Balmer-edge Behavior of the Quasar Big Blue Bump

We have found for the first time a Balmer edge feature in the Big Blue Bump emission of a quasar. The feature is seen in the polarized flux spectrum of the quasar, where all the emissions from outside the nucleus are scraped off and removed. The existence of the Balmer-edge absorption feature directly indicates that the Big Blue Bump is indeed thermal and optically-thick.

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arXiv2002

Using Multiwavelength Observations to Determine the Black Hole Mass and Accretion Rate in the Type 1 Seyfert Galaxy NGC 5548

We model the spectral energy distribution of the type 1 Seyfert galaxy NGC 5548, fitting data from simultaneous optical, UV, and X-ray monitoring observations. We assume a geometry consisting of a hot central Comptonizing region surrounded by a thin accretion disk. The properties of the disk and the hot central region are determined by the feedback occurring between the hot Comptonizing region and thermal reprocessing in the disk that, along with viscous dissipation, provides the seed photons for the Comptonization process. The constraints imposed upon this model by the multiwavelength data allow us to derive limits on the central black hole mass, $M \la 2 \times 10^7$ \Msun, the accretion rate, $\Mdot \la 2.5\times 10^5$ M$_\odot^2$yr$^{-1}/M$, and the radius of the transition region between the thin outer disk and the geometrically thick, hot inner region, $\sim 2$--5$\times 10^{14}$ cm.

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arXiv2002

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 ten in over fifty percent 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 also accelerate black hole mergers.

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arXiv2001

Non-LTE, Relativistic Accretion Disk Fits to 3C~273 and the Origin of the Lyman Limit Spectral Break

We fit general relativistic, geometrically thin accretion disk models with non-LTE atmospheres to near simultaneous multiwavelength data of 3C~273, extending from the optical to the far ultraviolet. Our model fits show no flux discontinuity associated with a hydrogen Lyman edge, but they do exhibit a spectral break which qualitatively resembles that seen in the data. This break arises from relativistic smearing of Lyman emission edges which are produced locally at tens of gravitational radii in the disk. We discuss the possible effects of metal line blanketing on the model spectra, as well as the substantial Comptonization required to explain the observed soft X-ray excess. Our best fit accretion disk model underpredicts the near ultraviolet emission in this source, and also has an optical spectrum which is too red. We discuss some of the remaining physical uncertainties, and suggest in particular that an extension of our models to the slim disk regime and/or including nonzero magnetic torques across the innermost stable circular orbit may help resolve these discrepancies.

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arXiv2001

Thermal Comptonization and Disk Thermal Reprocessing in NGC 3516

We present an application of the thermal Comptonization/disk reprocessing model recently proposed by Zdziarski, Lubiński, & Smith. We show that the absence of strong optical variations in the presence of strong concurrent X-ray variations, similar to those found by HST/RXTE monitoring observations of NGC 3516, can be explained by changing the geometry of the Comptonizing plasma rather than the accretion disk itself. The total X-ray luminosity of the Comptonizing plasma must decrease as its spatial extent increases. In contrast, the disk inner radius must be roughly fixed in order not to produce optical/ultraviolet color variations stronger than observed. By including emission due to internal viscous dissipation in the disk, we can roughly match the optical and X-ray flux levels and variability amplitudes seen from NGC 3516 during the HST/RXTE campaign.

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arXiv2001

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/L_Edd = 0.3) and central masses of 10^6, 10^7, and 10^8 M_sun. We also consider two values of the Shakura-Sunyaev alpha parameter, 0.1 and 0.01. Although it has little effect when M > 10^8 M_sun, 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 EUV/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. 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.

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arXiv2000

Local Dynamical Instabilities in Magnetized, Radiation Pressure Supported Accretion Disks

We present a general linear dispersion relation which describes the coupled behavior of magnetorotational, photon bubble, and convective instabilities in weakly magnetized, differentially rotating accretion disks. We presume the accretion disks to be geometrically thin and supported vertically by radiation pressure. We fully incorporate the effects of a nonzero radiative diffusion length on the linear modes. In an equilibrium with purely vertical magnetic field, the vertical magnetorotational modes are completely unaffected by compressibility, stratification, and radiative diffusion. However, in the presence of azimuthal fields, which are expected in differentially rotating flows, the growth rate of all magnetorotational modes can be reduced substantially below the orbital frequency. This occurs if diffusion destroys radiation sound waves on the length scale of the instability, and the magnetic energy density of the azimuthal component exceeds the non-radiative thermal energy density. While sluggish in this case, the magnetorotational instability still persists and will still tap the free energy of the differential rotation. Photon bubble instabilities are generically present in radiation pressure dominated flows where diffusion is present. We show that their growth rates are limited to a maximum value which is reached at short wavelengths where the modes may be viewed as unstable slow magnetosonic waves. We also find that vertical radiation pressure destabilizes upward propagating fast waves, and that Alfvén waves can be unstable. These instabilities typically have smaller growth rates than the photon bubble/slow modes. We discuss how all these modes behave in various regimes of interest, and speculate how they may affect the dynamics of real accretion disk flows.

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arXiv2000

The Puzzle of the Lyman Continuum Polarization of QSOs

Recent spectropolarimetry of QSOs has revealed a surprising rise in polarization in the Lyman continuum of several objects. We discuss several recent attempts to interpret this feature, including the role of Lyman limit absorption in PG 1222+228. We present new theoretical results involving electron scattering in a hot corona or wind above an accretion disk, and polarization resulting from the relativistic returning radiation. These mechanisms can lead to potentially observable polarization at short wavelengths, but neither has quantitative success in fitting the observed Lyman continuum polarization rises. A renewed capability for ultraviolet spectropolarimetry from space is urgently needed to provide further clues to the nature of this phenomenon.

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arXiv2000

Keck Mid-Infrared Imaging of the QSO 2237+0305

Using the Long Wavelength Spectrometer on Keck I, we have imaged the gravitationally lensed radio quiet quasi-stellar object (QSO) 2237+0305 at 8.9 and 11.7 microns for the first time. The mid-infrared flux ratios are inconsistent with the optical flux ratios, but agree with the radio flux ratios and with some published gravitational lens models. These flux ratios indicate that the infrared emission is not affected by microlensing, which rules out the synchrotron emission model. The infrared emission is likely produced by hot dust extended on a length scale of more than 0.03 parsecs. The spectral energy distribution further implies a narrow range of dust temperatures, suggesting that the dust may be located in a shell extending between ~1 to 3 parsecs from the nucleus, and intercepting about half of the QSO luminosity.

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arXiv1999

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 black hole mass and mass accretion rate, for several values of viscosity parameter alpha, and for two extreme values of the black hole spin: the maximum-rotation Kerr black hole, and the Schwarzschild (non-rotating) black hole. Our procedure calculates self-consistently the vertical structure of all disk annuli together with the radiation field, 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 and 32 billion solar masses with a two-fold increase of mass for each subsequent value; and eleven values of the mass accretion rate, each a power of 2 times 1 solar mass/year. The highest value of the accretion rate corresponds to 0.3 Eddington. We show the vertical structure of individual annuli within the set of accretion disk models, along with their local emergent flux, 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 properties of the models, such as optical/ultraviolet colors, the behavior of the hydrogen Lyman limit region, polarization, and number of ionizing photons. Our calculations are far from definitive in terms of the input physics, but generally we find that our models exhibit rather red optical/UV colors. Flux discontinuities 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.

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arXiv1997

Optical/Ultraviolet Continuum Emission Theory in Radio Quiet Quasars and Active Galactic Nuclei

Accretion disk models still do not provide a satisfactory explanation of the optical/ultraviolet continuum observed in Seyferts and quasars. Substantial theoretical progress has been made in understanding one aspect of the problem: the dearth of spectral features at the Lyman limit. Promising solutions have also been proposed to explain the surprising observations of large polarization in the Lyman continuum observed in some sources. I review the recent progress in this field, and try to point out future research directions which would be fruitful in trying to obtain a complete, self-consistent model of the continuum emitting regions.

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arXiv1997

Continuum Spectra of Quasar Accretion Disk Models

We have calculated the spectrum and polarization of a standard thin accretion disk with parameters appropriate for a bright quasar. This model improves upon previous work by including ultraviolet metal line opacities, assumed for now to be in LTE. Though not yet fully self-consistent, our calculations demonstrate that metal lines can change the spectral slope, reduce the polarization, and reduce the Lyman edge feature in accretion disk spectra. Some observational differences between quasar spectra and accretion disk models might be reconciled with the inclusion of metal lines.

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arXiv1997

Compton scattering of polarized radiation in two phase accretion disks in active galactic nuclei

We present results of radiative transfer calculations of Compton scattering of polarized radiation in a corona above an accretion disk. The corona has a plane-parallel geometry and is assumed to be homogeneous with a fixed temperature. We employ a Feautrier method to calculate the radiative transfer, incorporating the full relativistic, polarized Compton redistribution matrix. This allows us to explore a broad range of parameter space, in particular large optical depths and/or low corona temperatures, for which the iterative scattering method is very inefficient. Compared to Thomson scattering, Compton scattering generally produces smaller polarizations for featureless spectra. Distinct frequency-dependent patterns in polarization are produced near lines and edges, which can be used as diagnostics for the corona. We discuss the physics which gives rise to these results. Compton scattering can smear out Lyman edges in total flux, and at the same time produce large increases in polarization. This might help explain the steep rises in polarization observed near the Lyman limit in some quasars.

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arXiv1996

Polarization from magnetized accretion discs: II. The effects of absorption opacity on Faraday rotation

Equipartition magnetic fields can dramatically affect the polarization of radiation emerging from accretion disk atmospheres in active galactic nuclei. We extend our previous work on this subject by exploring the interaction between Faraday rotation and absorption opacity in local, plane-parallel atmospheres with parameters appropriate for accretion discs. Faraday rotation in pure scattering atmospheres acts to depolarize the radiation field by rotating the polarization planes of photons after last scattering. Absorption opacity in an unmagnetized atmosphere can increase or decrease the polarization compared to the pure scattering case, depending on the thermal source function gradient. Combining both Faraday rotation and absorption opacity, we find the following results. If absorption opacity is much larger than scattering opacity throughout the atmosphere, then Faraday rotation generally has only a small effect on the emerging polarization because of the small electron column density along a photon mean free path. However, if the absorption opacity is not too large and it acts alone to increase the polarization, then the effects of Faraday rotation can be enhanced over those in a pure scattering atmosphere. Finally, while Faraday rotation often depolarizes the radiation field, it can in some cases increase the polarization when the thermal source function does not rise too steeply with optical depth. We confirm the correctness of the Silant'ev (1979) analytic calculation of the high magnetic field limit of the pure scattering atmosphere, which we incorrectly disputed in our previous paper.

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