46 research outputs found

### Epicyclic oscillations of fluid bodies Paper II. Strong gravity

Fluids in external gravity may oscillate with frequencies characteristic of
the epicyclic motions of test particles. We explicitly demonstrate that global
oscillations of a slender, perfect fluid torus around a Kerr black hole admit
incompressible vertical and radial epicyclic modes. Our results may be directly
relevant to one of the most puzzling astrophysical phenomena -- high (hundreds
of hertz) frequency quasiperiodic oscillations (QPOs) detected in X-ray fluxes
from several black hole sources. Such QPOs are pairs of stable frequencies in
the 3/2 ratio. It seems that they originate a few gravitational radii away from
the black hole and thus observations of them have the potential to become an
accurate probe of super-strong gravity.Comment: submitted to Classical and Quantum Gravit

### 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 thirty 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

### Quasi-Periodic Oscillations from Magnetorotational Turbulence

Quasi-periodic oscillations (QPOs) in the X-ray lightcurves of accreting
neutron star and black hole binaries have been widely interpreted as being due
to standing wave modes in accretion disks. These disks are thought to be highly
turbulent due to the magnetorotational instability (MRI). We study wave
excitation by MRI turbulence in the shearing box geometry. We demonstrate that
axisymmetric sound waves and radial epicyclic motions driven by MRI turbulence
give rise to narrow, distinct peaks in the temporal power spectrum. Inertial
waves, on the other hand, do not give rise to distinct peaks which rise
significantly above the continuum noise spectrum set by MRI turbulence, even
when the fluid motions are projected onto the eigenfunctions of the modes. This
is a serious problem for QPO models based on inertial waves.Comment: 4 pages, 2 figures. submitted to ap

### The UV Continuum of Quasars: Models and SDSS Spectral Slopes

We measure long (2200-4000 ang) and short (1450-2200 ang) wavelength spectral
slopes \alpha (F_\nu proportional to \nu^\alpha) for quasar spectra from the
Sloan Digital Sky Survey. The long and short wavelength slopes are computed
from 3646 and 2706 quasars with redshifts in the z=0.76-1.26 and z=1.67-2.07
ranges, respectively. We calculate mean slopes after binning the data by
monochromatic luminosity at 2200 ang and virial mass estimates based on
measurements of the MgII line width and 3000 ang continuum luminosity. We find
little evidence for mass dependent variations in the mean slopes, but a
significant luminosity dependent trend in the near UV spectral slopes is
observed with larger (bluer) slopes at higher luminosities. The far UV slopes
show no clear variation with luminosity and are generally lower (redder) than
the near UV slopes at comparable luminosities, suggesting a slightly concave
quasar continuum shape. We compare these results with Monte Carlo distributions
of slopes computed from models of thin accretion disks, accounting for
uncertainties in the mass estimates. The model slopes produce mass dependent
trends which are larger than observed, though this conclusion is sensitive to
the assumed uncertainties in the mass estimates. The model slopes are also
generally bluer than observed, and we argue that reddening by dust intrinsic to
the source or host galaxy may account for much of the discrepancy.Comment: To be published in ApJ, 18 pages, 10 figure

### Excitation of Trapped Waves in Simulations of Tilted Black Hole Accretion Disks with Magnetorotational Turbulence

We analyze the time dependence of fluid variables in general relativistic,
magnetohydrodynamic simulations of accretion flows onto a black hole with
dimensionless spin parameter a/M=0.9. We consider both the case where the
angular momentum of the accretion material is aligned with the black hole spin
axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted
flow). In comparison to the untilted simulation, the tilted simulation exhibits
a clear excess of inertial variability, that is, variability at frequencies
below the local radial epicyclic frequency. We further study the radial
structure of this inertial-like power by focusing on a radially extended band
at 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables.
The three dimensional density structure at this frequency suggests that the
power is a composite oscillation whose dominant components are an over dense
clump corotating with the background flow, a low order inertial wave, and a low
order inertial-acoustic wave. Our results provide preliminary confirmation of
earlier suggestions that disk tilt can be an important excitation mechanism for
inertial waves.Comment: 8 Pages, 6 Figures, accepted for publication in Ap

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

We present the results of one and three-dimensional radiative transfer
calculations of polarized spectra emerging from snapshots of radiation
magnetohydrodynamical simulations of the local vertical structure of black hole
accretion disks. The simulations cover a wide range of physical regimes
relevant for the high/soft state of black hole X-ray binaries. We constrain the
uncertainties in theoretical spectral color correction factors due to the
presence of magnetic support of the disk surface layers and strong density
inhomogeneities. For the radiation dominated simulation, magnetic support
increases the color correction factor by about ten percent, but this is largely
compensated by a ten percent softening due to inhomogeneities. We also compute
the effects of inhomogeneities and Faraday rotation on the resulting
polarization. Magnetic fields in the simulations are just strong enough to
produce significant Faraday depolarization near the spectral peak of the
radiation field. X-ray polarimetry may therefore be a valuable diagnostic of
accretion disk magnetic fields, being able to directly test simulations of
magnetorotational turbulence.Comment: 18 pages, accepted for publication in Ap

### 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. 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 which are motivated by recent
radiation MHD simulations of magnetorotational turbulence. Bound-free metal
opacity generally produces significantly less spectral hardening than previous
models which only considered Compton scattering and free-free opacity. It also
tends to keep the effective photosphere near the surface, resulting in spectra
which are remarkably independent of the stress prescription and vertical
dissipation profile, provided little dissipation occurs above the effective
photosphere. 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.
(abridged)Comment: 30 pages, 18 figures, submitted to ApJ; corrected reference