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Surprisingly, the relict cosmological constant has a crucial influence on properties of accretion discs orbiting black holes in quasars and active galactic nuclei. We show it by considering basic properties of both the geometrically thin and thick accretion discs in the Kerr–de Sitter black hole (naked-singularity) spacetimes. Both thin and thick discs must have an outer edge allowing outflow of matter into the outer space, located nearby the so-called static radius, where the gravitational attraction of a black hole is balanced by the cosmological repulsion. Jets produced by thick discs can be significantly collimated after crossing the static radius. Extension of discs in quasars is comparable with extension of the associated galaxies, indicating a possibility that the relict cosmological constant puts an upper limit on extension of galaxies.

We consider the specific case of disc accretion for negligibly low viscosity and infinitely high electric conductivity. The key component in this model is the outflowing magnetized wind from the accretion disc, since this wind effectively carries away angular momentum of the accreting matter. Assuming magnetic field has variable polarity in the disc (to avoid magnetic flux and energy accumulation at the gravitational center), this leads to radiatively inefficient accretion of the disc matter onto the gravitational center. In such a case, the wind forms an outflow, which carries away all the energy and angular momentum of the accreted matter. Interestingly, in this framework, the basic properties of the outflow (as well as angular momentum and energy flux per particle in the outflow) do not depend on the structure of accretion disc. The self-similar solutions obtained prove the existence of such an accreting regime. In the self-similar case, the disc accretion rate (Ṁ) depends on the distance to the gravitational center, r, as , where λ is the dimensionless Alfvenic radius. Thus, the outflow predominantly occurs from the very central part of the disc provided that λ ≫ 1 (it follows from the conservation of matter). The accretion/outflow mechanism provides transformation of the gravitational energy from the accreted matter into the energy of the outflowing wind with efficiency close to 100%. The flow velocity can essentially exceed the Kepler velocity at the site of the wind launch.

Evidence is presented indicating that in the hard state of Cygnus X-1, the coronal magnetic field might be below equipartition with radiation (suggesting that the corona is not powered by magnetic field dissipation) and that the ion temperature in the corona is significantly lower than what predicted by ADAF like models. It is also shown that the current estimates of the jet power set interesting contraints on the jet velocity (which is at least mildly relativistic), the accretion efficiency (which is large in both spectral states), and the nature of the X–ray emitting region (which is unlikely to be the jet).

In the nonlinear Navier–Stokes viscous flow dynamics, physical damping is mathematically accomplished by a braking term in the momentum equation, corresponding to a heating term in the energy equation, both responsible of the conversion of mechanical energy into heat. In such two terms, it is essential the role of the viscous stress tensor, relative to contiguous macroscopic moving flow components, depending on the macroscopic viscosity coefficient ν. A working formulation for ν can always be found analytically, tuning some arbitrary parameters in the current known formulations, according to the geometry, morphology and physics of the flow. Instead, in this paper, we write an alternative hybrid formulation for ν, where molecular parameters are also included. Our expression for ν has a more physical interpretation of the internal damping in dilute gases because the macroscopic viscosity is related to the small scale molecular dissipation, not strictly dependent on the flow morphology, as well as it is free of any arbitrary parameter. Results for some basic 2D tests are shown in the smoothed particle hydrodynamics (SPH) framework. An application to the 3D accretion disc modeling for low mass cataclysmic variables is also discussed. Consequences of the macroscopic viscosity coefficient reformulation in a more strictly physical terms on the thermal conductivity coefficient for dilute gases are also discussed.

Stars are spheres of gas held together by self-gravity. When flying by a black hole, however, the star self-binding force can be overwhelmed by the black hole tides and the star can be torn apart. This is a physically rich and fascinating event which will be described by first introducing the concept of black hole from a mathematical point of view. We will then dive into the physics of the tidal disruption and proceed describing the accompanying electromagnetic flare and gravitational wave burst in the frequency range of the Laser Interferometer Space Antenna. This empowers such events to discover the elusive black holes with mass intermediate between the solar and the million/billion solar masses.

We address the role of large scale magnetic stress in the angular momentum transport, as well as the formation of different kinds of magnetic barrier in geometrically thick, optically thin, vertically averaged 1.5-dimensional advective accretion flows around black holes. The externally generated magnetic fields are captured by the accretion process from the environment, say, companion stars or interstellar medium. This field becomes dynamically dominant near the event horizon of a black hole due to continuous advection of the magnetic flux. In such magnetically dominated accretion flows, the accreting matter either decelerates or faces magnetic barrier in vicinity of the black hole depending on the magnetic field geometry. We find that the accumulated strong poloidal fields along with certain toroidal field geometry help in the formation of magnetic barrier which may knock the matter to infinity. When matter is trying to go back to infinity after getting knocked out by the barrier, in some cases it is prevented being escaped due to cumulative action of strong magnetic tension and gravity, and hence another magnetic barrier. We suggest, this kind of flow may be responsible for the formation of episodic jets in which magnetic field can lock the matter in between these two barriers. We also find that for the toroidally dominated disc, the accreting matter rotates very fast and decelerates towards the central black hole.

In this work we consider the effects of vertical self-gravity on a magnetized neutrino-dominated accretion disk, which is supposed to be a candidate for central engine of gamma-ray bursts (GRBs). We study some of the physical timescales that are considered to play a crucial role in the disk’s late-time activity, such as viscous, cooling, and diffusion timescales. We are also interested to probe the emission of X-ray flares’ probability, observed in GRBs’ extended emission by an investigation on the “magnetic barrier” and “fragmentation”. Our results approve the self-gravity as an amplifier for Blandford–Payne luminosity (BP power) and the magnetic field produced through the accretion process, but a suppressor for neutrino luminosity and magnetic barrier. The latter takes place as a result of the fragmentation enhancement in the outer disk, which is more likely to happen for the higher mass accretion rates.

The evolution of the accretion discs surrounding different compact objects (such as white dwarfs, neutron stars and black holes) is closely connected with the magnetic fields and their features. It is highly likely that the magnetic field generation is connected with the dynamo action. As for the dynamo, we have a lot of specific models which have been constructed for different astrophysical objects. For accretion discs we can take the approaches that were used for galactic discs which have nearly the same shape. There are two main models. The first one is connected with no-z approximation which is based on the fact that the discs is quite thin. RZ-model describes the discs with large half-thickness and can be used for the objects where we should study the vertical structure of the magnetic field. Here we show the results obtained for the accretion discs which used both of these approaches and compare the results.

We consider a static and axially symmetric metric containing two quadrupole parameters. In the present contribution, we study the quadrupole moments constraints on the properties of the relativistic accretion disc models, also explore the relation of oscillatory frequencies of charged particles to the frequencies of the twin high-frequency quasi-periodic oscillations observed in some microquasars. We also compare the results with Schwarzschild and Kerr metrics.

In this paper, I will review the the recent progresses in understanding the nonlinear evolution of gravitationally unstable gaseous discs. Gaseous accretion discs are a fundamental ingredient in the modeling on very diverse physical systems, spanning from the large scale discs that provide the fueling for supermassive black holes (SMBH) in the nuclei of active galaxies (AGN) to the smaller scale discs surrounding young stars, which are thought to be the site where planet formation occurs. Gravitational instabilities (GI) might play an important role in determining the structure and the evolution of such discs in many cases. The advances in numerical techniques have recently made possible to run complex simulations of the non-linear behaviour of such collective phenomena, leading to a deeper understanding of important related aspects, such as fragmentation and angular momentum transport. I will also present one specific example that shows the importance of gravitational instabilities in a system of considerable astrophysical relevance: high-redshift proto-galaxies, where GI might lead to the formation of the seeds of SMBHs.

We investigate the possibility that stellar mass black holes, with masses in the range of 3:8M_⊙ and 6M_⊙, respectively, could be in fact quark stars in the Color-Flavor-Locked (CFL) phase. Depending on the value of the gap parameter, rapidly rotating CFL quark stars can achieve much higher masses than standard neutron stars, thus making them possible stellar mass black hole candidates. Moreover, quark stars have a very low luminosity and a completely absorbing surface - the infalling matter on the surface of the quark star is converted into quark matter. A possibility of distinguishing CFL quark stars from stellar mass black holes could be through the study of thin accretion disks around rapidly rotating quark stars and Kerr type black holes, respectively. Strange stars exhibit a low luminosity, but high temperature bremsstrahlung spectrum, which, in combination with the emission properties of the accretion disk, may be the key signature to differentiate massive strange stars from black hole.

Matter falling onto a black hole is trans-relativistic, transonic, and close to the horizon it is sub-Keplerian. Such a flow shows the existence of multiple critical point, shocks etc. Employing relativistic equation of state and realistic composition, it has been shown that the solution strongly depend on the composition of the uid. Electron-positron fluid is the least relativistic, to the extent that multiple critical points, shocks etc do not form. The most relativistic fluid is the one whose ratio of proton to electron number density is ~ 0.2. Since the solution strongly depend on composition, the emitted radiation should strongly depend on the composition too.

In this contribution we discuss the dynamics of a twisted fully relativistic accretion disc around a slowly rotating black hole. At first, we outline the derivation of basic equations assuming that geometrical thickness of disc, inclination angle of its rings with respect to the black hole equatorial plane and rotation parameter of the black hole are small. Then, we analyse the shapes of the stationary twisted configurations for a particular case of the Novikov-Thorne model of the disc.