On the effect of coronal outflow on spectra formation in galactic black hole systems

We present the results of both analytical and numerical calculations of the amplitude of the reflection component in X-ray spectra of galactic black hole systems. We take into account the anisotropy of Compton scattering and the systematic relativistic bulk motion of the hot plasma. In the case of the single scattering approximation, the reflection from the disc surface is significantly enhanced owing to the anisotropy of Compton scattering. On the other hand, the calculations of multiple scattering obtained using the Monte Carlo method show that the anisotropy effect is much weaker in that case. Therefore, the enhanced back-scattered flux may affect the observed spectra only if the disc surface is highly ionized, which reduces the absorption in the energy band corresponding to the first Compton scattering.     

Small-scale inviscid accretion discs around black holes

Gas falling quasi-spherically on to a black hole forms an inner accretion disc if its specific angular momentum l exceeds l ∗∼ r _g c , where r _g is the Schwarzschild radius. The standard disc model assumes l ≫ l ∗. We argue that, in many black hole sources, accretion flows have angular momenta just above the threshold for disc formation, l ≳ l ∗, and assess the accretion mechanism in this regime. In a range l ∗< l < l _cr, a small-scale disc forms in which gas spirals fast into the black hole without any help from horizontal viscous stresses. Such an 'inviscid' disc, however, interacts inelastically with the feeding infall. The disc–infall interaction determines the dynamics and luminosity of the accretion flow. The inviscid disc radius can be as large as 14 r _g, and the energy release peaks at 2 r _g. The disc emits a Comptonized X-ray spectrum with a break at ∼100 keV. This accretion regime is likely to take place in wind-fed X-ray binaries and is also possible in active galactic nuclei.     

Mini-discs around spinning black holes

Accretion on to black holes in wind-fed binaries and in collapsars forms small rotating discs with peculiar properties. Such 'mini-discs' accrete on the free-fall time without the help of viscosity and nevertheless can have a high radiative efficiency. The inviscid mini-disc model was previously constructed for a non-rotating black hole. We extend the model to the case of a spinning black hole, calculate the structure and radiative efficiency of the disc and find their dependence on the black hole spin. If the angular momenta of the disc and the black hole are anti-aligned, a hydrodynamic analogue of Penrose process takes place.     

Emission-line profiles from self-gravitating toroids around black holes

We have computed line profiles from self-gravitating toroids around black holes. The specific angular momentum of the toroids is assumed to be constant in space. The images of the toroids show peculiar features in the far sides of the black holes. Concerning the line profiles, the red wing extends to the very low frequency region because the location of the inner edge is rather near the event horizon of the black hole and consequently the velocity of the inner edges of the toroids can be greater than that of the Kepler discs.     

Warp and eccentricity propagation in discs around black holes

We consider the inward propagation of warping and eccentric disturbances in discs around black holes under a wide variety of conditions. In our calculations, we use secular theories of warped and eccentric discs and assume the deformations to be stationary and propagating in a disc model similar to regions (a) and (b) of Shakura & Sunyaev discs. We find that the propagation of deformations to the innermost regions of the disc is facilitated for low viscous damping and high accretion rate. We relate our results to the possible excitation of trapped inertial modes, and to the observations of high-frequency quasi-periodic oscillations (QPOs) in black hole systems in the very high spectral state.     

On the interpretation of the multicolour disc model for black hole candidates

We present a critical analysis of the usual interpretation of the multicolour disc model parameters for black hole candidates in terms of the inner radius and temperature of the accretion disc. Using a self-consistent model for the radiative transfer and the vertical temperature structure in a Shakura–Sunyaev disc, we simulate the observed disc spectra, taking into account Doppler blurring and gravitational redshift, and fit them with multicolour models. We show not only that such a model systematically underestimates the value of the inner-disc radius, but that when the accretion rate and/or the energy dissipated in the corona are allowed to change, the inner edge of the disc, as inferred from the multicolour model, appears to move even when it is in fact fixed at the innermost stable orbit.     

On an excitation mechanism for trapped inertial waves in discs around black holes

According to one model, high-frequency quasi-periodic oscillations (QPOs) can be identified with inertial waves, trapped in the inner regions of accretion discs around black holes due to relativistic effects. In order to be detected, their amplitudes need to reach large enough values via some excitation mechanism. We work out in detail a non-linear coupling mechanism suggested by Kato, in which a global warping or eccentricity of the disc has a fundamental role. These large-scale deformations combine with trapped modes to generate 'intermediate' waves of negative energy that are damped as they approach either their corotation resonance or the inner edge of the disc, resulting in amplification of the trapped waves. We determine the growth rates of the inertial modes, as well as their dependence on the spin of the black hole and the properties of the disc. Our results indicate that this coupling mechanism can provide an efficient excitation of trapped inertial waves, provided the global deformations reach the inner part of the disc with non-negligible amplitude.     

Kinematics of hypervelocity stars in the triaxial halo of the Milky Way

Hypervelocity stars (HVSs) ejected by the massive black hole at the Galactic Centre have unique kinematic properties compared to other halo stars. Their trajectories will deviate from being exactly radial because of the asymmetry of the Milky Way potential produced by the flattened disc and the triaxial dark matter halo, causing a change of angular momentum that can be much larger than the initial small value at injection. We study the kinematics of HVSs and propose an estimator of dark halo triaxiality that is determined only by instantaneous position and velocity vectors of HVSs at large Galactocentric distances ( r ≳ 50 kpc). We show that, in the case of a substantially triaxial halo, the distribution of deflection angles (the angle between the stellar position and velocity vector) for HVSs on bound orbits is spread uniformly over the range 10°–180°. Future astrometric and deep wide-field surveys should measure the positions and velocities of a significant number of HVSs, and provide useful constraints on the shape of the Galactic dark matter halo.