Jiri Horak

Epicyclic oscillations of fluid bodies: 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 their observations have the potential to become an accurate probe of super-strong gravity.

The Polish doughnuts revisited I. The angular momentum distribution and equipressure surfaces

We construct a new family of analytic models of black hole accretion disks in dynamical equilibria. Our construction is based on assuming distributions of angular momentum and entropy. For a particular choice of the distribution of angular momentum, we calculate the shapes of equipressure surfaces. The equipressure surfaces we find are similar to those in thick, slim and thin disks, and to those in ADAFs.

The correlations and anticorrelations in QPO data

Double peak kHz QPO frequencies in neutron star sources varies in time by a factor of hundreds Hz while in microquasar sources the frequencies are fixed and located at the line ν2 = 1.5ν1 in the frequency-frequency plot. The crucial question in the theory of twin HFQPOs is whether or not those observed in neutron-star systems are essentially different from those observed in black holes. In black hole systems the twin HFQPOs are known to be in a 3:2 ratio for each source. At first sight, this seems not to be the case for neutron stars. For each individual neutron star, the upper and lower kHz QPO frequencies, ν2 and ν1, are linearly correlated, ν2 = Aν1 + B , with the slope A < 1.5, i.e., the frequencies definitely are not in a 1.5 ratio. In this contribution we show that when considered jointly on a frequency-frequency plot, the data for the twin kHz QPO frequencies in several (as opposed to one) neutron stars uniquely pick out a certain preferred frequency ratio that is equal to 1.5 for the six sources examined so far. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Twin-peak quasiperiodic oscillations as an internal resonance

Two peaks occur in high-frequency power spectra of X-ray lightcurves of several black-hole candidates. We further explore the idea that a non-linear resonance mechanism operating in strong-gravity regime is responsible for these inter-related quasi-periodic oscillations (twin QPOs). By extending the multiple-scales analysis of Rebusco, we construct two-dimensional phase-space sections, which enable us to identify different topologies governing the system and to follow the evolutionary tracks of the twin peaks. This approach suggests that the original (Abramowicz & Kluzniak) parametric-resonance scheme can be viewed as a naive account of the QPOs model with an internal resonance.We show an example of internal resonance in a system with up to two critical points, and describe a general technique that permits other cases to be treated in a systematical manner. A separatrix divides the phase-space sections into regions of different topology. Inside the libration region, the evolutionary tracks bring the observed twin-peak frequencies to an exact rational ratio, whereas the observed frequencies remain off resonance in the circulation region. Our scheme predicts the power should be cyclically exchanged between the two oscillations. Like the high-frequency QPOs in neutron-star binaries, one also expects, as a general property of the non-linear model, that in black-hole sources slight detuning pushes the twin-peak frequencies out of sharp resonance.

Alpha-Viscosity Effects in Slender Tori

We explore effects of the Shakura-Sunyaev alpha-viscosity on the dynamics and oscillations of slender tori. We start with a slow secular evolution of the torus. We show that the angular-momentum profile approaches the Keplerian one on the timescale longer than a dynamical one by a factor of the order of 1/\alpha. Then we focus our attention on the oscillations of the torus. We discuss effects of various angular momentum distributions. Using a perturbation theory, we have found a rather general result that the high-order acoustic modes are damped by the viscosity, while the high-order inertial modes are enhanced. We calculate a viscous growth rates for the lowest-order modes and show that already lowest-order inertial mode is unstable for less steep angular momentum profiles or very close to the central gravitating object.

X-Ray Polarimetry: Polarization from the oscillating magnetized accretion torus

We study oscillations of accretion torus with azimuthal magnetic field. For several lowest-order modes we calculate eigenfrequencies and eigenfunctions and calculate corresponding intensity and polarization light curves using advanced ray-tracing methods.

Polarization of Light from Warm Clouds above an Accretion Disk: Effects of Strong Gravity near a Black Hole

We study polarization from scattering of light on a cloud in radial motion along the symmetry axis of an accretion disk. Radiation drag from the disk and gravitational attraction of the central black hole are taken into account, as well as the effect of the cloud cooling in the radiation field. This provides us with a self-consistent toy-model for predicted lightcurves, including the linear polarization that arises from the scattering. Strong gravitational lensing creates indirect images; these are formed by photons that originate from the disk, get backscattered onto the photon circular orbit and eventually redirected towards an observer. Under suitable geometrical conditions the indirect photons may visibly influence the resulting magnitude of polarization and light-curve profiles. Relevant targets are black holes in active galactic nuclei and stellar-mass Galactic black-holes exhibiting episodic accretion/ejection events.