Studies of the halo gas in the Milky Way and in nearby spiral galaxies show the presence of gas complexes that cannot be reconciled with an internal (galactic fountain) origin and are direct evidence of gas accretion. Estimating gas accretion rates from these features consistently gives values, which are one order of magnitude lower than what is needed to feed the star formation. I show that this problem can be overcome if most of the accretion is in fact "hidden" as it mixes with the galactic fountain material coming from the disk. This model not only provides an explanation for the missing gas accretion but also reproduces the peculiar kinematics of the halo gas in particular the vertical rotation gradient. In this view this gradient becomes indirect evidence for gas accretion.
In this paper, we analyse in detail the mass-accretion histories and structural properties of dark haloes in high-resolution N-body simulations. Modeling the density distribution in individual haloes with the NFW profile, we find, for all main progenitors of a given halo, there is a tight correlation between its inner scale radius $r_s$ and the mass within it, $M_s$, which is the basic reason why halo structural properties are closely related to their mass-accretion histories. This correlation can be used to predict accurately the structural properties of a dark halo at any time from its mass-…
We examine here the inner region accretion flows onto black holes. A variety of models are presented. We also discuss viscosity mechanisms under a variety of circumstances, for standard accretion disks onto galactic black holes and supermassive black holes and hot accretion disks. Relevant work is presented here on unified aspects of disk accretion onto supermassive black holes and the possible coupling of thick disks to beams in the inner regions. We also explore other accretion flow scenarios. We conclude that a variety of scenarios yield high temperatures in the inner flows and that viscosi…
N-body simulations predict that CDM halo-assembly occurs in two phases: 1) a fast accretion phase with a rapidly deepening potential well; and 2) a slow accretion phase characterised by a gentle addition of mass to the outer halo with little change in the inner potential well. We demonstrate, using one-dimensional simulations, that this two-phase accretion leads to CDM halos of the NFW form and provides physical insight into the properties of the mass accretion history that influence the final profile. Assuming that the velocities of CDM particles are effectively isotropised by fluctuations in…
It has recently been shown that relaxed spherically symmetric dark matter halos develop from the inside out, by permanently adapting their inner structure to the boundary conditions imposed by the current accretion rate. Such a growth allows one to infer the typical density profiles of halos. Here we follow the same approach to infer the typical spherically averaged profiles of the main structural and kinematic properties of triaxial, anisotropic, rotating halos. Specifically, we derive their density, spatial velocity dispersion, phase-space density, anisotropy and specific angular momentum pr…
Results are presented from a time-dependent, numerical investigation of super-Eddington spherical accretion onto black holes with different initial conditions. We have studied the stability of stationary solutions, the non-linear evolution of shocked models and the time-dependent accretion from an expanding medium.
We present a theoretical model for driving jets by accretion onto Kerr black holes and try to give an answer to the following question: How much energy could be extracted from a rotating black hole and its accretion disk in order to power relativistic jets in Active Galactic Nuclei?
The merger and accretion probabilities of dark matter halos have so far only been calculated for an infinitesimal time interval. This means that a Monte-Carlo simulation with very small time steps is necessary to find the merger history of a parent halo. In this paper we use the random walk formalism to find the merger and accretion probabilities of halos for a finite time interval. Specifically, we find the number density of halos at an early redshift that will become part of a halo with a specified final mass at a later redshift, given that they underwent $n$ major mergers, $n=0,1,2,...$ . W…
By using a properly generalized accretion formalism it is argued that the accretion of phantom energy onto a wormhole does not make the size of the wormhole throat to comovingly scale with the scale factor of the universe, but instead induces an increase of that size so big that the wormhole can engulf the universe itself before it reaches the big rip singularity, at least relative to an asymptotic observer.
It is shown that, contrary to the widespread opinion, particle collisions considerably increase accretion rate from the cosmological background onto 5D primordial black holes formed during the high-energy phase of the Randall-Sundrum Type II braneworld scenario. Increase of accretion rate leads to much tighter constraints on initial primordial black hole mass fraction imposed by the critical density limit and measurements of high-energy diffuse photon background and antiproton excess.
This paper explores the mapping between the observable properties of a stellar halo in phase- and abundance-space and the parent galaxy's accretion history in terms of the characteristic epoch of accretion and mass and orbits of progenitor objects. The study utilizes a suite of eleven stellar halo models constructed within the context of a standard LCDM cosmology. The results demonstrate that coordinate-space studies are sensitive to the recent (0-8 Gyears ago) merger histories of galaxies (this timescale corresponds to the last few to tens of percent of mass accretion for a Milky-Way-type gal…
Accretion onto supermassive black holes produces both the dramatic phenomena associated with active galactic nuclei and the underwhelming displays seen in the Galactic Center and most other nearby galaxies. I review selected aspects of the current theoretical understanding of black hole accretion, emphasizing the role of magnetohydrodynamic turbulence and gravitational instabilities in driving the actual accretion and the importance of the efficacy of cooling in determining the structure and observational appearance of the accretion flow. Ongoing investigations into the dynamics of the plungin…
We examine the radiatively driven mass accretion onto galactic nuclei due to the intensive radiation from circumnuclear starbursts. The radiation from a starburst not only contracts an inner gas disk by the radiation flux force, but also extracts angular momenta due to the relativistic radiation drag, thereby inducing an avalanche of the surface layer of disk. The present radiatively driven mass accretion may provide a physical mechanism which enables mass accretion from 100 pc scales down to pc, and it can eventually link to an advection-dominated viscous accretion onto a massive black hole. …
High resolution cosmological N-body simulations show that the density profiles of dark matter halos in hierarchical cosmogonies are universal, with low mass halos typically denser than more massive ones. This mass-density correlation is interpreted as reflecting the earlier formation of less massive objects. We investigate this hypothesis in the light of formation times defined as the epoch at which halos experience their last major merger. We find that the characteristic density and the scale radius of halos are essentially proportional, respectively, to the critical density of the universe a…
The role of radiation in general relativistic magnetohydrodynamic (GRMHD) accretion simulations is discussed through axisymmetric simulations of the evolution of an initial torus seeded with a weak magnetic field. The paper compares and contrasts the rate of accretion onto a Kerr black hole and mass flux out out of the initial torus at large radii in the GRMHD code of De Villiers and Hawley and a newly developed radiative GRMHD code. This rGRMHD code currently operates in the diffusion approximation, restricting the study of radiative effects to the bound portion of the accretion disk/jet syst…
Four quiescent elliptical galaxies were imaged with the NRAO VLA at 8.5 GHz. Within the context of canonical advection-dominated accretion flows (ADAFs), these VLA images plus published black hole masses constrain the accretion rates to be $ <1.6\times10^{-4}$, $ <3.6\times10^{-4}$, $\le7.8\times10^{-4}$, and $\le7.4\times10^{-4}$ of the Eddington rates. These ADAF accretion rates derived at 8.5 GHz have important implications for the levels of soft and hard X-rays expected from these quiescent galaxies.
We investigate the accretion flows onto the supermassive binary black holes (SMBBHs) from the circumbinary disk with the equal mass, eccentric binary on the subparsec scale, using Smoothed Particle Hydrodynamics (SPH) code. We find that the material can be supplied from circumbinary disk, which leads to the formation of two accretion disks around the SMBBHs. The mass accretion rates significantly modulate with the binary orbital motion. These could provide the observable diagnosis of the existence of the supermassive binary black holes (e.g. OJ287) on the subparsec scale in merged galactic nuc…
We investigate Hoyle-Lyttleton accretion for the case where the central source is a luminous accretion disk. %In classical Hoyle-Lyttleton accretion onto a ``spherical'' source, accretion takes place in an axially symmetric manner around a so-called accretion axis. The accretion rate of the classical Hoyle-Lyttleton accretion onto a non-luminous object and $Γ$ the luminosity of the central object normalized by the Eddington luminosity. %If the central object is a compact star with a luminous accretion disk, the radiation field becomes ``non-spherical''. %Although the gravitional field remains …
In this work a general formalism for the accretion of dark energy onto astronomical objects, black holes and wormholes, is considered. It is shown that in models with four dimensions or more, any singularity with a divergence in the Hubble parameter may be avoided by a big trip, if it is assumed that there is no coupling between the bulk and this accreting object. If this is not the case in more than four dimensions, the evolution of the cosmological object depends on the particular model.
In this paper, we initiate a new study of steady funnel-flow accretion onto strongly magnetized neutron stars, including a full treatment of shock generation. As a first step, we adopt a simplified model considering the flow within Newtonian theory and neglecting radiative pressure and cooling. The flow is taken to start from an accretion disc and then to follow magnetic field lines, forming a transonic funnel flow onto the magnetic poles. A standing shock occurs at a certain point in the flow and beyond this material accretes subsonically onto the star with high pressure and density. We calcu…
The detection and characterization of accretion processes in the disks surrounding young stars may be directly relevant to studies of planet formation. Especially the study of systems with very low accretion rates (<< 10^{-10} M_sun yr^{-1}) is important, since at those rates radial mixing becomes inefficient and disk material will have to be dissipated into larger bodies at its present location. In these proceedings, we compare the different methods of tracing accretion onto Herbig Ae/Be stars and conclude that high-resolution infrared spectroscopy is currently the only reliable method that o…
I review our current theoretical understanding of the different possible rotating modes of accretion onto a black hole. I discuss both thick adiabatic flows and radiatively efficient thin disc solutions. I present a new self-consistent unified analytical description of two-phase thin disc--corona systems in which magneto-rotational instability is responsible for angular momentum transport in the disc and for the corona generation. Finally, I briefly discuss the role of magnetic fields in bridging the gap between accretion discs theory and jet production mechanisms
The dynamics of accretion onto a schwarzschild black hole is studied using Paczynski-Wiita pseudo newtonian potential. Steady state solution of the flow equations is obtained using thin disc approximation, including the effect of bremsstrahlung cooling in the energy equation. The topology of transonic solutions are got and the conditions for shock formation (Rankine-Hugoniot conditions) are checked. Shock and no-shock regions in the parameter space of accretion rate and specific angular momentum are identified. We motivate the time-dependent numerical simulation of the flow, as a candidate for…
We consider hot accretion onto a rapidly spinning neutron star (or any other compact object with a surface). A radiative hot settling flow has been discovered at low accretion rates in the early work by Medvedev & Narayan (2001) and analytical solution has been presented. It was shown later that this flow can match external medium smoothly, thus enforcing its physical feasibility. Here we complete the study of the global structure of such hot accretion by presenting the analytical solution for the boundary later, which forms between the bulk of the flow and the stellar surface. We confirm our …
The role that minor mergers have played in the formation and structure of the Milky Way is still an open question, about which there is much debate. We use numerical simulations to explore the evolution of debris from a tidally disrupted satellite, with the aim of developing a method that can be used to identify and quantify signatures of accretion in a survey of halo stars. For a Milky Way with a spherical halo, we find that debris from minor mergers can remain aligned along great circles throughout the lifetime of the Galaxy. We exploit this result to develop the method of Great Circle Cell …
Although disk accretion onto compact objects - white dwarfs, neutron stars, and black holes - is central to much of high energy astrophysics, the mechanisms which enable this process have remained observationally elusive. Accretion disks must transfer angular momentum for matter to travel radially inward onto the compact object. Internal viscosity from magnetic processes and disk winds can in principle both transfer angular momentum, but hitherto we lacked evidence that either occurs. Here we report that an X-ray-absorbing wind discovered in an observation of the stellar-mass black hole binary…
We present new results on the angular momentum evolution of dark matter halos. Halos, from N-body simulations, are classified according to their mass growth histories into two categories: the accretion category contains halos whose mass has varied continuously and smoothly, while the merger category contains halos which have undergone sudden and significant mass variations (greater than 1/3 of their initial mass per event). We observe for the growth of angular momentum that it continues well into the nonlinear regime, that it is correlated with mass growth and that it occurs in two phases: a f…
When a black hole accretes slowly, the radiative cooling of the infalling gas is weak and the accretion disk does not form. A hot collisionless quasi-spherical halo is formed instead. The properties of such halos are discussed. The rate of accretion, the radiative efficiency, and the temperature and density near the hole are evaluated.
Aims: We investigate the cosmological growth of dark halos and follow the consequences of coeval growth for the accretion history of associated supermassive black holes. Methods: The Press-Schechter approximation is used to obtain an analytic expression for the mean rate of growth of dark matter halos. Dark halo accretion rates are compared with numerical work and the consequences for understanding AGN evolution are described. Results: The mean accretion rate onto dark matter halos is shown to have a simple analytic form that agrees with previous numerical work and that may easily be calculate…
It is now accepted that accretion onto classical T Tauri stars is controlled by the stellar magnetosphere, yet to date most accretion models have assumed that their magnetic fields are dipolar. By considering a simple steady state accretion model with both dipolar and complex magnetic fields we find a correlation between mass accretion rate and stellar mass of the form $\dot{M} \propto M_{\ast}^α$, with our results consistent within observed scatter. For any particular stellar mass there can be several orders of magnitude difference in the mass accretion rate, with accretion filling factors of…
High-resolution N-body simulations of hierarchical clustering in a wide variety of cosmogonies show that the density profiles of dark matter halos are universal, with low mass halos being denser than their more massive counterparts. This mass-density correlation is interpreted as reflecting the earlier typical formation time of less massive objects. We investigate this hypothesis in the light of formation times defined as the epoch at which halos experience their last major merger. Such halo formation times are calculated by means of a modification of the extended Press & Schechter formalism w…
The rate of turbulent spherical accretion onto a compact object might be much smaller than the Bondi rate. It is suggested that the rate of accretion onto Sgr A-star is much smaller than the Bondi rate.
Fast and slow magnetosonic shock formation is presented for stationary and axisymmetric magnetohydrodynamical (MHD) accretion flows onto a black hole. The shocked black hole accretion solution must pass through magnetosonic points at some locations outside and inside the shock location. We analyze critical conditions at the magnetosonic points and the shock conditions. Then, we show the restrictions on the flow parameters for strong shocks. We also show that a very hot shocked plasma is obtained for a very high-energy inflow with small number density. Such a MHD shock can appear very close to …
We performed two-dimensional radiation hydrodynamic simulations of supercritical accretion flows around neutron stars (NSs). In contrast with the accretion flows onto black holes (BHs), we find that the shell-shaped high-density regions form around the NSs, since the radiation force is enhanced in the innermost regions. The enhanced radiation force drives strong outflows above and below the disk. The mass-accretion rate onto the NS exceeds the critical rate, $L_{\rm E}/c^2$, with $L_{\rm E}$ being the Eddington luminosity. However it is about $20-30%$ of that onto the BH, under the condition t…
Stationary, spherical accretion onto an unmagnetized neutron star is here reconsidered on the wake of the seminal paper by Zel'dovich \& Shakura (1969). It is found that new ``hot'' solutions may exist for a wide range of luminosities. These solutions are characterized by a high temperature, $10^{9}÷10^{11}$ K, and arise from a stationary equilibrium model where the dominant radiative mechanisms are multiple Compton scattering and bremsstrahlung emission. For low luminosities, $\mincir 10^{-2} \ L_{E}$, only the ``cold'' (à la Zel'dovich and Shakura) solution is present.
Numerical simulations of MHD accretion flows in the vicinity of a supermasssive black hole provide important insights to the problem of why and how systems -- such as the Galactic Center -- are underluminous and variable. To access applicability of such flows to real objects, we examine the dynamical MHD studies with computations of the time dependent radiation spectra predicted by the simulations. We apply Monte Carlo methods to calculate spectra predicted by the time-dependent model of an axisymmetic MHD flow accreting onto a black hole presented by Proga and Begelman. Our calculations show …
(abridged) We use a one-dimensional hydrodynamical code to investigate the effects of preheating on gas accretion and cooling in cold dark matter halos. In the absence of radiative cooling, preheating reduces the amount of gas that can be accreted into a halo, and the accreted gas fraction is determined by the ratio of the initial specific entropy of the gas to the virial entropy of the halo. In the presence of radiative cooling, preheating affects the gas fraction that can cool in two different ways. For small halos with masses <10^12Msun, preheating suppresses gas accretion, but most of the …
We present evidence for cosmological gas accretion onto spiral galaxies in the local universe. The accretion is seen through its effects on the dynamics of the extra-planar neutral gas. The accretion rates that we estimate for two nearby spiral galaxies are of the order of their star formation rates. Our model shows that most of the extra-planar gas is produced by supernova feedback (galactic fountain) and only 10-20 % comes from accretion. The accreting material must have low specific angular momentum about the disc's spin axis, although the magnitude of the specific angular-momentum vector c…
The influence of the magnetic field on process of the accretion onto White Dwarfs in Cataclysmic Variables (CVs) is discussed. Except for the Polars or AM Her objects, the strength of magnetic field can not be measured directly in CVs by modern techniques. But there is growing evidence that most of the types of Cataclysmic Variables classified on the basis of their observational characteristics are behaving in one or the other way under the influence of the magnetic field of the accreting White Dwarf, among other things. Here, we discuss the bulk of CVs that are traditionally considered as non…
We explore the effects of small scale structure on the formation and equilibrium of dark matter halos in a universe dominated by vacuum energy. We present the results of a suite of four N-body simulations, two with a LCDM initial power spectrum and two with WDM-like spectra that suppress the early formation of small structures. All simulations are run into to far future when the universe is 64Gyr/h old, long enough for halos to essentially reach dynamical equilibrium. We quantify the importance of hierarchical merging on the halo mass accretion history, the substructure population, and the equ…
We present dynamically consistent solutions for hot accretion onto unmagnetized, rotating white dwarfs (WDs) in five quiescent dwarf novae. The measured WD rotation rates (and other system parameters) in RX And, SS Cyg, U Gem, VW Hyi and WZ Sge imply spindown of the WD by an extended hot flow emitting most of its X-rays in the vicinity of the stellar surface. In general, energy advection is absent and the flow is stable to convection and hydrodynamical outflows. In rapidly rotating systems, the X-ray luminosity provides only an upper limit on the quiescent accretion rate because of substantial…
The present day chemical and dynamical properties of the Milky Way bear the imprint of the Galaxy's formation and evolutionary history. One of the most enduring and critical debates surrounding Galactic evolution is that regarding the competition between ``satellite accretion'' and ``monolithic collapse''; the apparent strong correlation between orbital eccentricity and metallicity of halo stars was originally used as supporting evidence for the latter. While modern-day unbiased samples no longer support the claims for a significant correlation, recent evidence has been presented by Chiba & Be…
Results are presented from a time--dependent, numerical investigation of spherical accretion onto black holes, within the framework of relativistic radiation hydrodynamics. We have studied the stability of self--consistent, stationary solutions of black hole accretion with respect to thermal and radiative perturbations and also the non--linear evolution of unstable, high temperature models, heated by the hard radiation produced by the accretion flow itself in the inner region near to the horizon. In some cases, a hydrodynamic shock forms at around $10^3$--$10^4$ Schwarzschild radii, where Comp…
We study accretion onto the neutron star in Be/X-ray binaries, using a 3D SPH code and the data imported from a high resolution simulation by Okazaki et al.\ (2002) for a coplanar system with a short period ($P_{\rm orb}=24.3 \rm d$) and moderate eccentricity $(\rm{e=0.34})$. We find that a time-dependent accretion disk is formed around the neutron star in Be/X-ray binaries. The disk shrinks after the periastron passage of the Be star and restores its radius afterwards. Our simulations show that the truncated Be disk model for Be/X-ray binaries is consistent with the observed X-ray behavior.
The topologies are obtained for accretion flows around black holes as the parameters, accretion rate and specific angular momentum are varied. Paczy{ń}ski-Wiita pseudo-Newtonian potential is used to mimic the gravitational field of a Schwarzschild black hole. Both stellar mass($14M_{\odot}$) and supermassive($10^8M_{\odot}$) black holes are considered. The effects of bremsstrahlung and synchrotron cooling processes on the flow dynamics are studied. Rankine-Hugoniot conditions are checked for possible shock formation of solution branches of the topology. Stability of the branches and the bounda…
Quasi-spherical supersonic (Bondi-type) accretion to a star with a dipole magnetic field is investigated using resistive magnetohydrodynamic simulations. A systematic study is made of accretion to a non-rotating star, while sample results for a rotating star are also presented. A new stationary subsonic accretion flow is found with a steady rate of accretion to the magnetized star smaller than the Bondi accretion rate. Dependences of the accretion rate and the flow pattern on the magnetic momentum of the star and the magnetic diffusivity are presented. For slow star's rotation the accretion fl…
We present a detailed model for the formation of massive objects at the centers of galaxies. The effects of supernovae heating and the conditions of gas loss are revisited. The escape time of the gas is compared with the cooling time, which provides an additional condition not previously considered. Its consequences for the allowed mass range of the halo is calculated. It is shown that sufficient gas is retained to form massive dark objects and quasars even for moderately massive halos but a decline is expected at low redshifts. Subsequently, a gaseous disk forms with a radial extent of a ~kpc…
We describe general-relativistically a spherically symmetric stationary fluid accretion onto a black hole. Relativistic effects enhance mass accretion, in comparison to the Bondi model predictions, in the case when backreaction is neglected. That enhancement depends on the adiabatic index and the asymptotic gas temperature and it can magnify accretion by one order in the ultrarelativistic regime.
The stationary, spherically symmetric accretion of dark energy onto a Schwarzschild black hole is considered in terms of relativistic hydrodynamics. The approximation of an ideal fluid is used to model the dark energy. General expressions are derived for the accretion rate of an ideal fluid with an arbitrary equation of state p=p(ρ) onto a black hole. The black hole mass was found to decrease for the accretion of phantom energy. The accretion process is studied in detail for two dark energy models that admit an analytical solution: a model with a linear equation of state, p=α(ρ-ρ_0), and a Cha…
We construct a simple accretion model of a rotating pressureless gas sphere onto a Schwarzschild black hole. We show how to build analytic solutions in terms of Jacobi elliptic functions. This construction represents a general relativistic generalisation of the Newtonian accretion model first proposed by Ulrich (1976). In exactly the same form as it occurs for the Newtonian case, the flow naturally predicts the existence of an equatorial rotating accretion disc about the hole. However, the radius of the disc increases monotonically without limit as the flow reaches its maximum allowed angular …
The equations governing general relativistic, spherically symmetric, hydrodynamic accretion of polytropic fluid onto black holes are solved in Schwarzschild metric to investigate some of the transonic properties of the flow. Only stationary solutions are discussed. For such accretion, it has been shown that real physical sonic points may form {\it even for flow with $γ< {4/3}$ or $γ> {5/3}$}. Behaviour of some flow variables in the close vicinity of the event horizon are studied as a function of specific energy and polytropic index of the flow.
We present a new geometrical approach to the study of accretion flows onto rotating (Kerr) black holes. Instead of Boyer-Lindquist coordinates, the standard choice in all existing numerical simulations in the literature, we employ the simplest example of a horizon adapted coordinate system, the Kerr-Schild coordinates. This choice eliminates boundary ambiguities and unphysical divergent behavior at the event horizon. Computations of Bondi-Hoyle accretion onto extreme Kerr black holes, performed here for the first time, demonstrate the key advantages of this procedure. We argue it offers the be…
Cosmic strings with small-scale structure have a coarse-grained mass per unit length $μ$ which is larger than the string tension. This leads to an effective Newtonian gravitational line source and to a characteristic translational velocity which is smaller than for strings without small-scale structure. Here, the accretion of hot dark matter onto such strings is studied by means of the Zel'dovich approximation. We find that clustering is greatly enhanced by the Newtonian line source. In the limit of vanishing translational velocity, the first nonlinear filaments form at a redshift of greater t…
