We have calculated a series of models of outflows from the obscuring torus in active galactic nuclei (AGN). Our modeling assumes that the inner face of a rotationally supported torus is illuminated and heated by the intense X-rays from the inner accretion disk and black hole. As a result of such heating a strong biconical outflow is observed in our simulations. We calculate 3-dimensional hydrodynamical models, assuming axial symmetry, and including the effects of X-ray heating, ionization, and radiation pressure. We discuss the behavior of a large family of these models, their velocity fields, mass fluxes and temperature, as functions of the torus properties and X-ray flux. Synthetic warm absorber spectra are calculated, assuming pure absorption, for sample models at various inclination angles and observing times. We show that these models have mass fluxes and flow speeds which are comparable to those which have been inferred from observations of Seyfert 1 warm absorbers, and that they can produce rich absorption line spectra.
We report on time-dependent axisymmetric simulations of an X-ray excited flow from a parsec-scale, rotating, cold torus around an active galactic nucleus. Our simulations account for radiative heating and cooling and radiation pressure force. The simulations follow the development of a broad bi-conical outflow induced mainly by X-ray heating. We compute synthetic spectra predicted by our simulations. The wind characteristics and the spectra support the hypothesis that a rotationally supported torus can serve as the source of a wind which is responsible for the warm absorber gas observed in the…
The purpose of this paper is to explore how accretion discs manifest the phenomenon of transient growth on a global scale. We investigate analytically the time response of a thin accretion disc to particular axisymmetric perturbations. To facilitate an analytical treatment we replace the energy equation with a general polytropic assumption. The asymptotic expansion of Kluźniak & Kita (2000), which extended the method of Regev (1983) to a full steady polytropic disc (with $n=3/2$), is further developed and implemented for both the steady (for any polytropic index) and time-dependent problems. T…
We examine how perturbed shear flows evolve in two-dimensional, incompressible, inviscid hydrodynamical fluids, with the ultimate goal of understanding the dynamics of accretion disks. To linear order, vorticity waves are swung around by the background shear, and their velocities are amplified transiently before decaying. It has been speculated that sufficiently amplified modes might couple nonlinearly, leading to turbulence. Here we show how nonlinear coupling occurs in two dimensions. This coupling is remarkably simple because it only lasts for a short time interval, when one of the coupled …
Aims : We re-examine the conditions required to steadily deviate an accretion flow from a circumstellar disc into a magnetospheric funnel flow onto a slow rotating young forming star. Methods : New analytical constraints on the formation of accretion funnels flows due to the presence of a dipolar stellar magnetic field disrupting the disc are derived. The Versatile Advection Code is used to confirm these constraints numerically. Axisymmetric MHD simulations are performed, where a stellar dipole field enters the resistive accretion disc, whose structure is self-consistently computed. Results : …
We present a systematic numerical study of two-dimensional axisymmetric accretion flows around black holes. The flows have no radiative cooling and are treated in the framework of the hydrodynamical approximation. The models calculated in this study cover the large range of the relevant parameter space. There are four types of flows, determined by the values of the viscosity parameter $α$ and the adiabatic index $γ$: convective flows, large-scale circulations, pure inflows and bipolar outflows. Thermal conduction introduces significant changes to the solutions, but does not create a new flow t…
The linear marginal instability of an axisymmetric MHD Taylor-Couette flow of infinite vertical extension is considered. The dependence of the flow stability on magnetic Prandtl number, Pm, and gap-width between rotating cylinders is investigated. There is an unstability for flows with resting outer cylinder even without a magnetic field. Nevertheless, there are solutions with smaller critical Reynolds numbers for certain (weak) magnetic fields. These solutions, however, exist for not too small Pm or large enough gap. For hydrodynamically stable flow, we found that a magnetic field amplitude a…
We report on the first phase of our study of slightly rotating accretion flows onto black holes. We consider inviscid accretion flows with a spherically symmetric density distribution at the outer boundary, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum. We study accretion flows by means of numerical 2D, axisymmetric, hydrodynamical simulations. Our main result is that the properties of the accretion flow do not depend as much on the outer boundary conditions (i.e., the amount as well as distribution of the angular momentum) as on the geo…
My lecture is devoted to the analytical results available for a large class of axisymmetric stationary flows in the vicinity of compact astrophysical objects. First, the most general case is formulated corresponding to the axisymmetric stationary MHD flow in the Kerr metric. Then, I discuss the hydrodynamical version of the Grad-Shafranov equation. Although not so well-known as the full MHD one, it allows us to clarify the nontrivial structure of the Grad-Shafranov approach as well as to discuss the simplest version of the 3+1-split language -- the most convenient one for the description of id…
We report on the fourth phase of our study of slightly rotating accretion flows onto black holes. The main new element of this study is that we used fully three dimensional (3-D) numerical simulations. We consider hydrodynamics of inviscid accretion flows. We assume a spherically symmetric density distribution at the outer boundary, but brake the flow symmetry by introducing a small, latitude-dependent angular momentum. We also consider cases where angular momentum at large radii is latitude- and azimuth-dependent. For the latitude-dependent angular momentum, 3-D simulations confirm axisymmetr…
The instability of non-homoentropic axisymmetric flow of perfect fluid with respect to non-axisymmetric infinitesimal perturbations was investigated by numerical integration of hydrodynamical differential equations in two-dimensional approximation. The non-trivial influence of entropy gradient on unstable sound and surface gravity waves was revealed. In particular, both decrease and growth of entropy against the direction of effective gravitational acceleration $g_{eff}$ give rise to growing surface gravity modes which are stable with the same parameters in the case of homoentropic flow. At th…
The properties of axisymmetric accretion flows of cold adiabatic gas with zero total energy in the vicinity of a Newtonian point mass are characterized by a single dimensionless parameter, the thickness of incoming flow. In the limit of thin accretion flows with vanishing thickness, we show that the governing equations become self-similar, involving no free parameters. We study numerically thin accretion flows with finite thickness as well as those with vanishing thickness. Mass elements of the incoming flow enter the computational regime as thin rings. In the case with finite thickness, after…
Explosive nucleosynthesis under the axisymmetric explosion in Type II supernova has been performed by means of two dimensional hydrodynamical calculations. We have compared the results with the observations of SN 1987A. Our chief findings are as follows: (1) $^{44}Ti$ is synthesized so much as to explain the tail of the bolometric light curve of SN 1987A. We think this is because the alpha-rich freezeout takes place more actively under the axisymmetric explosion. (2) $^{57}Ni$ and $^{58}Ni$ tend to be overproduced compared with the observations. However, this tendency relies strongly on the pr…
We present numerical simulations of cold, axisymmetric, magnetically driven relativistic outflows. The outflows are initially sub-Alfvénic and Poynting flux-dominated, with total--to--rest-mass energy flux ratio up to $μ\sim 620$. To study the magnetic acceleration of jets we simulate flows confined within a funnel with rigid wall of prescribed shape, which we take to be $z\propto r^a$ (in cylindrical coordinates, with $a$ ranging from 1 to 2). This allows us to eliminate the numerical dissipative effects induced by a free boundary with an ambient medium. We find that in all cases they converg…
We present results from axisymmetric, time-dependent hydrodynamical (HD) and magnetohydrodynamical (MHD) simulations of a gaseous envelope collapsing onto a black hole (BH). We consider gas with so small angular momentum that after an initial transient, the flow in the HD case, accretes directly onto a BH without forming a rotationally support torus. However, in the MHD case even with a very weak initial magnetic field, the flow settles into a configuration with four components: (i) an equatorial inflow, (ii) a bipolar outflow, (iii) polar funnel outflow, and (iv) polar funnel inflow. We focus…
Highly supercritical accretion discs are probable sources of dense optically thick axisymmetric winds. We introduce a new approach based on diffusion approximation radiative transfer in a funnel geometry and obtain an analytical solution for the energy density distribution inside the wind assuming that all the mass, momentum and energy are injected well inside the spherization radius. This allows to derive the spectrum of emergent emission for various inclination angles. We show that self-irradiation effects play an important role altering the temperature of the outcoming radiation by about 20…
We have studied non-axisymmetric standing accretion shock instability, or SASI, by 3D hydrodynamical simulations. This is an extention of our previous study on axisymmetric SASI. We have prepared a spherically symmetric and steady accretion flow through a standing shock wave onto a proto-neutron star, taking into account a realistic equation of state and neutrino heating and cooling. This unperturbed model is supposed to represent approximately the typical post-bounce phase of core-collapse supernovae. We then have added a small perturbation (~1%) to the radial velocity and computed the ensuin…
Many systems of interest in general relativistic astrophysics, including neutron stars, accreting compact objects in X-ray binaries and active galactic nuclei, core collapse, and collapsars, are assumed to be approximately spherically symmetric or axisymmetric. In Newtonian or fixed-background relativistic approximations it is common practice to use spherical polar coordinates for computational grids; however, these coordinates have singularities and are difficult to use in fully relativistic models. We present, in this series of papers, a numerical technique which is able to use effectively s…
We present the results of axisymmetric, time-dependent magnetohydrodynamic simulations of accretion flows around black holes. The calculations begin from a rotationally supported thick torus which contains a weak poloidal field. Accretion is produced by growth and saturation of the magnetorotational instability (MRI) provided the wavelength of the fastest growing mode is less than the thickness of the torus. Using a computational grid that spans more than two decades in radius, we compare the time-averaged properties of the flow to previous hydrodynamical simulations. The net mass accretion ra…
We report on the second phase of our study of slightly rotating accretion flows onto black holes. We consider magnetohydrodynamical (MHD) accretion flows with a spherically symmetric density distribution at the outer boundary, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum and a weak radial magnetic field. We study accretion flows by means of numerical 2D, axisymmetric, MHD simulations with and without resistive heating. Our main result is that the properties of the accretion flow depend mostly on an equatorial accretion torus which is ma…
We present global solutions that describe advection-dominated accretion flows around black holes. The solutions are obtained by numerically solving a set of coupled ordinary differential equations corresponding to a steady axisymmetric height-integrated flow. The solutions satisfy consistent boundary conditions at both ends. We obtain well-behaved transonic solutions for a wide range of values of the viscosity parameter $α$, from 0.001 to 0.3. We do not find any need for shocks in our solutions, and disagree with previous claims that viscous accretion flows with low values of $α$ must have sho…
Pulsar bow-shock nebulae are a class of pulsar wind nebulae (PWNe) that form when the pulsar wind is confined by the ram pressure of the ambient medium, and are usually associated with old pulsars, that have already emerged from the progenitor Supernova Remnant (SNR). Until a few years ago these nebulae were mainly observed as \halpha sources; recently, also non-thermal emission has been detected. This is the signature of accelerated particles gyrating in a magnetic field. In the same way as \halpha radiation is a tool for studying the layer of shocked Interstellar Medium (ISM), the non-therma…
Two-dimensional accretion flows near black holes have been investigated by time-dependent hydrodynamical calculations. We assume that the flow is axisymmetric and that radiative losses of internal energy are negligible, so that the disc is geometrically thick and hot. Accretion occurs due to the overflow of the effective potential barrier near the black hole, similar to the case of the Roche lobe overflowing star in a binary system. We make no pre-assumptions on the properties of the flow, instead our models evolve self-consistently from an initially non-accreting state. The viscosity is due t…
X-Ray observations of groups and clusters of galaxies show that the Intra-Cluster Medium (ICM) in their cores is hotter than expected from cosmological numerical simulations of cluster formation which include star formation, radiative cooling and SN feedback. We investigate the effect of the injection of supersonic AGN jets into the ICM using axisymmetric hydrodynamical numerical simulations. A simple model for the ICM, describing the radial properties of gas and the gravitational potential in cosmological N-Body+SPH simulations of one cluster and three groups of galaxies at redshift z=0, is o…
