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.Comment: Accepted for publication in PASJ; 7 pages, 4 figure

Frame-dragging effects on magnetic fields near a rotating black hole

We discuss the role of general relativity frame dragging acting on magnetic field lines near a rotating (Kerr) black hole. Near ergosphere the magnetic structure becomes strongly influenced and magnetic null points can develop. We consider aligned magnetic fields as well as fields inclined with respect to the rotation axis, and the two cases are shown to behave in profoundly different ways. Further, we construct surfaces of equal values of local electric and magnetic intensities, which have not yet been discussed in the full generality of a boosted rotating black hole.Comment: to appear in the proceedings of "The Central Kiloparsec in Galactic Nuclei (AHAR 2011)", Journal of Physics: Conference Series (JPCS), IOP Publishin

Star-disc interactions in a galactic centre and oblateness of the inner stellar cluster

Structure of a quasi-stationary stellar cluster is modelled assuming that it is embedded in the gravitational field of a super-massive black hole. Gradual orbital decay of stellar trajectories is caused by the dissipative interaction with an accretion disc. Gravitational field of the disc is constructed and its effect on the cluster structure is taken into account as an axially symmetric perturbation. Attention is focused on a circumnuclear region (r<10^4 gravitational radii) where the effects of the central black hole and the disc dominate over the influence of an outer galaxy. It is shown how the stellar system becomes gradually flattened towards the disc plane. For certain combinations of the model parameters, a toroidal structure is formed by a fraction of stars. Growing anisotropy of stellar velocities as well as their segregation occur. The mass function of the inner cluster is modified and it progressively departs from the asymptotic form assumed in the outer cluster. A new stationary distribution can be characterized in terms of velocity dispersion of the stellar sample in the central region of the modified cluster.Comment: Accepted for publication in MNRAS; 12 pages, 10 figure

Of NBOs and kHz QPOs: a low-frequency modulation in resonant oscillations of relativistic accretion disks

The origin of quasi periodic modulations of flux in the kilohertz range (kHz QPOs), observed in low-mass X-ray binaries, is usually assumed to be physically distinct from that of the ``normal branch oscillations'' (NBOs) in the Z-sources. We show that a low-frequency modulation of the kHz QPOs is a natural consequence of the non-linear relativistic resonance suggested previously to explain the properties of the high-frequency twin peaks. The theoretical results discussed here are reminiscent of the 6 Hz variations of frequency and amplitude of the kHz QPOs reported by Yu, van der Klis and Jonker (2001).Comment: Accepted for publication in PASJ; 4 pages, 1 figur

Interpreting the High Frequency QPO Power Spectra of Accreting Black Holes

In the context of a relativistic hot spot model, we investigate different physical mechanisms to explain the behavior of quasi-periodic oscillations (QPOs) from accreting black holes. The locations and amplitudes of the QPO peaks are determined by the ray-tracing calculations presented in Schnittman & Bertschinger (2004a): the black hole mass and angular momentum give the geodesic coordinate frequencies, while the disk inclination and the hot spot size, shape, and overbrightness give the amplitudes of the different peaks. In this paper additional features are added to the existing model to explain the broadening of the QPO peaks as well as the damping of higher frequency harmonics in the power spectrum. We present a number of analytic results that closely agree with more detailed numerical calculations. Four primary pieces are developed: the addition of multiple hot spots with random phases, a finite width in the distribution of geodesic orbits, Poisson sampling of the detected photons, and the scattering of photons from the hot spot through a corona of hot electrons around the black hole. Finally, the complete model is used to fit the observed power spectra of both type A and type B QPOs seen in XTE J1550-564, giving confidence limits on each of the model parameters.Comment: 30 pages, 5 figures, submitted to Ap

Twin-peak quasiperiodic oscillations as an internal resonance

Two inter-related 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 quasi-periodic oscillations (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 evolutionary tracks of the twin peaks. This suggests that the original (Abramowicz and Kluzniak) parametric-resonance scheme can be viewed as an ingenuous 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 we describe a general technique that permits to treat other cases 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 in the circulation region the observed frequencies remain off resonance. Our scheme predicts the power should cyclically be exchanged between the two oscillations. Likewise the high-frequency QPOs in neutron-star binaries, also in black-hole sources one expects, as a general property of the non-linear model, that slight detuning pushes the twin-peak frequencies out of sharp resonance.Comment: Accepted for publication in A&A; 11 pages, 6 figure