It was found that the advection-dominated accretion flow (ADAF)+thin disk model calculations can reproduce the observed spectral energy distributions (SED) of the two low luminosity AGN, provided they are accreting at ~0.01-0.03 Eddington rates and the thin disks are truncated to ADAFs at ~100 R_s (Schwarzschild radii) for M81 and NGC4579 (Quataert et al., 1999). However, the black hole masses adopted in their work are about one order of magnitude lower than recent measurements on these two sources. Adopting the well estimated black hole masses, our ADAF+thin disk model calculations can reproduce the observed SEDs of these two LLAGN, if the black hole is accreting at 2.5e-4 Eddington rates with the thin disk truncated at 120 R_s for M81 (3.3e-3 and R_tr = 80R_s are required for NGC4579). The observed widths of the thermal X-ray iron lines at 6.8 keV are consistent with the Doppler broadening by the Keplerian motion of the gases in the transition zones at ~100R_s. The observed thermal X-ray lines provide a useful diagnosis on the physical properties of the transition zones. We calculate the thermal X-ray line emission from the transition zone between the ADAF and the thin disk with standard software package Astrophysical Plasma Emission Code (APEC), and the physical implications on the models of the transition zones are discussed.
We review basic properties of advection-dominated accretion flows (ADAFs) and their applications to astrophysical systems ranging from Galactic binary systems to galactic nuclei. A new classification scheme for low-luminosity, X-ray bright galactic nuclei is highlighted. Some outstanding unresolved issues are discussed.
We derive all relevant equations needed for constructing a global general relativistic model of advectively cooled, very hot, optically thin accretion disks around black holes and present solutions which describe advection dominated flows in the gravitational field of a Kerr black hole.
We report a 3 sigma upper limit of 220 microJanskys on any 22-GHz continuum emission coincident with the central engine in NGC 4258. If NGC 4258 is powered by an advection-dominated accretion flow, this radio upper limit implies that the inner advection-dominated flow cannot extend significantly beyond about 100 Schwarzschild radii.
Accretion disk instabilities are briefly reviewed. Some details are given to the short-wavelength thermal instabilities and the convective instabilities. Time-dependent calculations of two-dimensional advection-dominated accretion flows are presented.
I review particle heating by MHD turbulence in collisionless plasmas appropriate to advection-dominated accretion flows. These considerations suggest that the preferential turbulent heating of protons assumed by theoretical models is only achieved for relatively subthermal magnetic fields.
We present a modified description of advection-dominated accretion flows (ADAF) with a geometrical viscosity. A simplified treatment of the dynamics of the flow in the inner relativistic part is suggested and results are compared with the galactic center source Sgr A*.
We obtained global transonic solutions representing optically thin advection-dominated accretion flow by solving the full set of differential equations describing such systems. We found that far from the sonic point self-similar solutions are an excellent approximation of the global flow structure if the accretion rate is well below the maximum value above which no optically thin solutions exist.
We calculate the radiation spectra from advection-dominated accretion flows (ADAFs), taking into account the effects of a global magnetic field. Calculation is based on the analytic model for magnetized ADAFs proposed by Kaburaki, where a large-scale magnetic field controls the accretion process. Adjusting a few parameters, we find that our model can well reproduce the observed spectrum of Sagittarius A$^{*}$. The result is discussed in comparison with those of well-known ADAF models, where the turbulent viscosity controls the accretion process.
Thermal instability is examined for advection-dominated one-temperature accretion disks. We consider axisymmetric perturbations with short wavelength in the radial direction. The viscosity is assumed to be sufficiently small for the vertical hydrostatic balance to hold in perturbed states. The type of viscosity is given either by the $α$-viscosity or by a diffusion-type stress tensor. Optically thick disks are found to be in general more unstable than optically thin ones. When the thermal diffusion is present, the optically thin disks become stable, but the optically thick disks are still unst…
This paper reviews our current understanding of low-luminosity AGNs (LLAGNs) in the context of the advection-dominated accretion flow. The best investigated source, the supermassive black hole in our galactic center, Sgr A*, is emphasized since the physics of accretion should be the same for various LLAGNs except for their different accretion rates. The important role of jets is discussed, but this is less well established.
Using simple arguments we show that the canonical thin keplerian accretion disks cannot smoothly match any plain advection dominated flow (ADAF) model. By 'plain' ADAF model we mean the ones with zero cooling. The existence of sonic points in exact solutions is critical and imposes constraints that cannot be surpassed adopting 'reasonable' physical conditions at the hypothetical match point. Only the occurrence of new critical physical phenomena may produce a transition. We propose that exact advection models are a class of solutions which don't necessarily involve the standard thin cool disks…
We provide complete and global solutions of transonic flows around black holes, in presence of advection, rotation, heating and cooling. We show that for any degree of advection, there may exist two critical viscosity parameters $α_{c1, c2}$ such that for $α<α_{c1}$ the flow may pass through the inner sonic point only. For $α_{c1} <α<α_{c2}$, the flow may have shock waves, and for $α>α_{c2}$, the flow may again pass through the inner sonic point, depending on flow parameters. No new topologies emerge other than what we found earlier while studying viscous isothermal transonic flows. These find…
Using only the assumption that all interactions between particles in an accretion flow are electromagnetically mediated, it is shown that the time to establish equipartition between ions and electrons is shorter than the characteristic accretion time. Consequently, two-temperature fits to the spectra of accreting objects are unphysical, and models in which significant thermal energy is carried across the event horizon are effectively ruled out.
We consider viscous rotating accretion flows in which most of the viscously dissipated energy is stored as entropy rather than being radiated. Such advection-dominated flows may occur when the optical depth is either very small or very large. We obtain a family of self-similar solutions where the temperature of the accreting gas is nearly virial and the flow is quasi-spherical. The gas rotates at much less than the Keplerian angular velocity; therefore, the central stars in such flows will cease to spin up long before they reach the break-up limit. Further, the Bernoulli parameter is positive,…
We consider the role of advection in the two-temperature accreting corona with an underlying optically thick disc. The properties of coronal solutions depend significantly on the description of advection. Local parameterization of advection by a constant coeficient $δ$ replacing the radial derivatives lead to complex topology of solutions, similar to some extent to other advection-dominated accretion flow solutions. One, radiatively cooled branch exists for low accertion rates. For higher accretion rates two solutions exist in a broad range of radii: one is radiatively cooled and the other one…
We show the results of two-dimensional, azimuthally symmetric simulations of advection dominated fluid flows around black holes. We use the SPH method, and the alpha-prescription for the viscosity, including all terms in the stress tensor. We have performed calculations with 0.001 < alpha < 0.2 and find strong circulation patterns in the flow, on spatial scales that depend on the magnitude of the viscosity.
We extend and reconcile recent work on turbulence and particle heating in advection-dominated accretion flows. For approximately equipartition magnetic fields, the turbulence primarily heats the electrons. For weaker magnetic fields, the protons are primarily heated. The division between electron and proton heating occurs between $β\sim 5$ and $β\sim 100$ (where $β$ is the ratio of the gas to the magnetic pressure), depending on unknown details of how Alfvén waves are converted into whistlers on scales of the proton Larmor radius. We also discuss the possibility that magnetic reconnection coul…
We present self-similar solutions for advection-dominated accretion flows with radial viscous force in the presence of outflows from the accretion flow or infall. The axisymmetric flow is treated in variables integrated over polar sections and the effects of infall and outflows on the accretion flow are parametrised for possible configurations compatible with the self-similar solution. We investigate the resulting accretion flows for three different viscosity laws and derive upper limits on the viscosity parameter alpha. In addition, we find a natural connection to non-rotating and spherical a…
We report here on a calculation of thermalization time-scale of the two temperature advection dominated accretion flow (ADAF) model. It is established that time required to equalize the electron and ion temperatures via electron-ion collisions in the ADAF with plausible physical parameters greatly exceeds age of the Universe, which corroborates validity one of the crucial assumptions of the ADAF model, namely the existence of a hot two temperature plasma. This work is motivated by the recent success (Mahadevan 1998a,b) of ADAF model in explaining the emitted spectrum of Sgr A*.
The global structure of optically thin hot accretion disks with radial advection included has been investigated. We solve the full energy conservation equation explicitly and construct the radial structure of the disk. It is found that advection is a real cooling process and that there are two solutions co-exist for a given mass accretion rate less than a critical limit. One is fully advection cooling dominated and the other is dominated by local radiative cooling. The advection dominated accretion disks are hotter than the local cooling dominated disks; they are most probably in the two-tempe…
The response of advection-dominated accretion disks to local disturbances is examined by one-dimensional numerical simulations. It is generally believed that advection-dominated disks are thermally stable. We, however, find that any disurbance added onto accretion flow at large radii does not decay so rapidly that it can move inward with roughly the free-fall velocity. Although disturbances continue to be present, the global disk structure will not be modified largely. This can account for persistent hard X-ray emission with substantial variations observed in active galactic nuclei and stellar…
The effects of radial advection and thermal diffusion were considered in investigating the linear stability of an optically thin, two-temperature accretion disc. If the disc has only very little advection, we proved that the thermal instability exists when the disc is geometrically thin. But it dispears in a geometrically slim disc if the thermal diffusion was considered. Moreover, if the disc is advection dominated, the thermal instability does not exist. In addition, we found that the instabilities of inertial-acoustic modes exist only in a geometrically thin disc or an advection-dominated d…
It has been found that a class of optically-thin two-temperature advection-dominated accretion solutions explains many observations of low-luminosity accreting black holes. Here it is shown that these models give a satisfactory description also of higher luminosity systems, provided the viscosity parameter $α$ is large. The models reproduce the spectra of black hole X-ray binaries in the Low State, and explain the transition from the Low State to the High State at a critical mass accretion rate. The models also show that the X-ray/$γ$-ray spectra of X-ray binaries and active galactic nuclei sh…
We have analyzed archival ASCA data on the soft X-ray transient source V404 Cyg in quiescence. We find that in the energy range 0.7 to 8.5 keV the spectrum is a hard power-law with a photon spectral index between 1.8 and 2.6 (90% confidence limits). We present a model of V404 Cyg in which the accretion flow has two components: (1) an outer thin disk with a small annular extent, and (2) a large interior region where the flow is advection-dominated. Nearly all the radiation in the infrared, optical, UV and X-ray bands is from the advection-dominated zone; the thin disk radiates primarily in the …
A model for advection-dominated accretion flows (ADAFs) in a global magnetic field is proposed. In contrast to the well known ADAF models in which the viscosity of a fluid determines both angular momentum transfer and energy dissipation in the flow, the magnetic field and the electric resistivity, respectively, control them in this model. A manageable set of analytic solutions for the flow and the magnetic field is obtained to vertically non-integrated basic equations. This set describes mathematically a fully advective accretion flow and, in physically plausible situations for most AGNs, it i…
General properties of advection-dominated accretion flows are discussed. Special emphasis is given to the optically thin branch of solutions, which has very high ion and electron temperatures and is thermally stable. This solution branch has been applied to a number of low-luminosity accreting black holes. The models have resolved some puzzles and have provided a straightforward explanation of the observed spectra. The success of the models confirms that the central objects in these low-luminosity sources are black holes. There is some indication that advection-dominated models may be relevant…
Using mean field MHD, we discuss the energetics of optically thin, two temperature, advection-dominated accretion flows (ADAFs). If the magnetic field is tangled and roughly isotropic, flux freezing is insufficient to maintain the field in equipartition with the gas. In this case, we expect a fraction of the energy generated by shear in the flow to be used to build up the magnetic field strength as the gas flows in; the remaining energy heats the particles. We argue that strictly equipartition magnetic fields are incompatible with a priori reasonable levels of particle heating; instead, the pl…
We calculate spectral models of advection-dominated accretion flows, taking into account the possibility that significant mass may be lost to a wind. We apply the models to the soft X-ray transient V404 Cyg in quiescence and the Galactic center source Sgr A*. We show that there are qualitative degeneracies between the mass loss rate in the wind and parameters characterizing the microphysics of the accretion flow; of particular importance is $δ$, the fraction of the turbulent energy which heats the electrons. For small $δ$, current observations of soft X-ray transients and Sgr A* suggest that a…
We use the standard Runge-Kutta method to solve the set of basic equations describing black hole accretion flows composed of two-temperature plasma. We do not invoke any extra energy transport mechanism such as thermal conduction and do not specify any ad hoc outer boundary condition for the advection-dominated accretion flow (ADAF) solution. We find that in the case of high viscosity and non-zero radiative cooling, the ADAF solution can have an asymptotic approach to the Shakura-Sunyaev disk (SSD) solution, and the SSD-ADAF transition radius is close to the central black hole. Our results fur…
We study the formation of the spectra in advection dominated accretion flows (ADAFs) around Kerr black holes. We use a Monte Carlo approach and fully general relativistic treatment to follow the paths of individual photons and model their scattering with mildly relativistic, thermal electrons of the two temperature plasma present in the flow. We are mostly interested in the dependence of the spectra on the black hole angular momentum, and we find that the influence of the black hole rotation rate on the flow structure has an impact on the resulting spectra. The flows around the fast rotating h…
In this Letter we investigate the properties of advection-dominated accretion flows (ADAFs) fed by the evaporation of a Shakura-Sunyaev accretion disk (SSD). In our picture the ADAF fills the central cavity evacuated by the SSD and extends beyond the transition radius into a coronal region. We find that, because of global angular momentum conservation, a significant fraction of the hot gas flows away from the black hole forming a transsonic wind, unless the injection rate depends only weakly on radius (if $r^2\dotσ\propto r^{-ξ}$, $ξ< 1/2$). The Bernoulli number of the inflowing gas is negativ…
A model of magnetic coupling (MC) of a rotating black hole (BH) with advection- dominated accretion flow (MCADAF) is proposed. It turns out that MCADAF providers a natural explanation for the transition radius between ADAF and SSD, and could be used to interpret the highest luminosity of GX 339-4 in hard-state. A very steep emissivity index can be produced in the innermost part of the MCADAF,which is consistent with the recent XMM-Newton observations of the nearby bright Seyfert 1 galaxy MCG-6-30-15 and with two X-ray binaries (XRBs): XTE J1655-500 and GX 339-4. In addition, we estimate the BH…
Recent papers discussing advection dominated accretion flows (ADAF) as a solution for astrophysical accretion problems should be treated with some caution because of their uncertain physical basis. The suggestions underlying ADAF involve ignoring the magnetic field reconnection in heating of the plasma flow, assuming electron heating due only to binary Coulomb collisions with ions. Here, we analyze the physical processes in optically thin accretion flows at low accretion rates including the influence of an equipartition turbulent magnetic field. For these conditions there is continuous destruc…
We write down and solve equations describing steady state, optically thin, advection-dominated accretion onto a Kerr black hole. The mean flow, described by the relativistic fluid equations, is axisymmetric and vertically averaged. The effect of turbulence in the flow is represented by a viscous shear stress. Our treatment differs in several important ways from earlier work: we use a causal prescription for the shear stress, we do not assume the relativistic enthalpy is unity (this is important for rapidly rotating holes), and we use a relativistic equation of state. We present several represe…
We study the formation of the spectra in advection dominated accretion flows (ADAFs) around Kerr black holes. We use a Monte Carlo approach and fully general relativistic treatment to follow the paths of individual photons and model their scattering with mildly relativistic, thermal electrons of the two temperature plasma present in the flow. We study the influence of the accretion rate, black hole mass, black hole angular momentum, and the strength of the small scale magnetic field present in the plasma on the resulting spectra. The impact of the black hole angular momentum on the spectra is …
The local stability of accretion disks with advection is studied together with the considerations of radial viscous force and thermal diffusion. For a geometrically thin, radiative cooling dominated disk, the thermal diffusion has nearly no effects on the thermal and viscous modes. The including of thermal diffusion, however, tends to stabilize the acoustic modes which, if without advection, are unstable if the disk is optically thick, radiation pressure dominated or optically thin, and are stable if the disk is optically thick, gas pressure dominated. The including of very little advection ha…
We obtain global solutions of radiatively inefficiently accretion flows around black holes. Whether and where convection develops in a flow are self-consistently determined with the mixing-length theory. The solutions can be divided into three types according to the strength of normal viscosity. Type I solution corresponds to large viscosity parameter $α\ga 0.1$, which is purely advection-dominated and with no convection, and has been extensively studied in the literature. Type II solution is for moderate $α\sim 0.01$, which has a three-zone structure. The inner zone is advection-dominated, th…
The defining characteristic of a black hole is that it possesses an event horizon through which matter and energy can fall in but from which nothing escapes. Soft X-ray transients (SXTs), a class of X-ray binaries, appear to confirm this fundamental property of black holes. SXTs that are thought to contain accreting black holes display a large variation of luminosity between their bright and faint states, while SXTs with accreting neutron stars have a smaller variation. This difference is predicted if the former stars have horizons and the latter have normal surfaces.
Viscous rotating accretion flows around black holes become advection-dominated when the accretion rate $\dot M$ is sufficiently low. Most of the accretion energy in such flows is stored within the gas and advected radially inward. The temperature is therefore very high, and much of the radiation comes out in hard X-rays and $γ$-rays. We have constructed an advection-dominated accretion flow model for the Galactic Center source Sagittarius A$^*$. The model consists of a $7\times10^5M_\odot$ black hole accreting at $\dot M=1.2\times10^{-5}α\, M_\odot{\rm yr^{-1}}$, where $α$ is the usual viscosi…
We present a model for the high-low state transition of the X-Ray Nova GS 1124-68 (Nova Muscae 1991) observed by Ginga. The model consists of an advection-dominated accretion flow (ADAF) near the central black hole surrounded by a thin accretion disk. During the rise phase of the outburst, as the mass accretion rate increases, the transition radius between the thin disk and the ADAF moves closer to the center, until the thin disk extends all the way in to the last stable orbit. The transition radius increases again during decline. We reproduce the basic features of the spectra taken during and…
This article reviews the physics of advection-dominated accretion flows (ADAFs) and describes applications to several black hole X-ray binaries and galactic nuclei. The possibility of using ADAFs to explore the event horizons of black holes is highlighted.
The predictability, or lack thereof, of the solar cycle is governed by numerous separate physical processes that act in unison in the interior of the Sun. Magnetic flux transport and the finite time delay it introduces, specifically in the so-called Babcock-Leighton models of the solar cycle with spatially segregated source regions for the alpha and omega effects, play a crucial rule in this predictability. Through dynamo simulations with such a model, we study the physical basis of solar cycle predictions by examining two contrasting regimes, one dominated by diffusive magnetic flux transport…
We report the results of a broadband (0.1-100 keV) X-ray observation of the nearby elliptical galaxy NGC1052, performed with the BeppoSAX observatory. We confirm the presence of a bright (2-10 keV luminosity ~ 4 x 10^{42} erg/s) and strongly absorbed (NH ~ 2 x 10^{23} atoms/cm/cm) X-ray source. The flatness of the X-ray spectrum (photon index ~1.4), the estimated low accretion rate (m-dot \~ 10^{-4}) and the radio-to-X-ray spectral energy distribution suggest that this observation may represent the first direct measurement above 10 keV of an accretion-dominated flow in an elliptical galaxy.
The global structure of optically thin advection dominated accretion flows which are composed of two-temperature plasma around black holes is calculated. We adopt the full set of basic equations including the advective energy transport in the energy equation for the electrons. The spectra emitted by the optically thin accretion flows are also investigated. The radiation mechanisms which are taken into accout are bremsstrahlung, synchrotron emission, and Comptonization. The calculation of the spectra and that of the structure of the accretion flows are made to be completely consistent by calcul…
We present analytical scaling laws for self-similar advection dominated flows. The spectra from these systems range from 10$^{8}$ - 10$^{20}$ Hz, and are determined by considering cooling of electrons through synchrotron, bremsstrahlung, and Compton processes. We show that the spectra can be quite accurately reproduced without detailed numerical calculations, and that there is a strong testable correlation between the radio and X-ray fluxes from these systems. We describe how different regions of the spectrum scale with the mass of the accreting black hole, $M$, the accretion rate of the gas, …
We investigate advection dominated, transsonic accretion flows in the vicinity of a Kerr black hole. We take into account all relativistic effects in the dynamics of the flow and in the propagation of light. We assume the matter to be weakly magnetized and cool via the thermal synchrotron and Bremsstrahlung radiation. We include also the effects of Comptonization. We calculate the spectra of radiation as seen by observers located at different positions relative to the equatorial plane of the disk. The radiation emitted by the accreting matter is anisotropic and observers near the equatorial pl…
There is increasing evidence for the presence of an optically thin advection-dominated accretion flow (ADAF) in low luminosity active galactic nuclei and radio-loud quasars. The present paper is devoted to explore the fate of a blob ejected from an ADAF, and to discuss its observational consequences. It is inevitable for the ejected blob to drastically expand into its surroundings. Consequently, it is expected that a group of relativistic electrons should be accelerated, which may lead to nonthermal flares, since a strong shock will be formed by the interaction between the blob and its surroun…
Advection-dominated, high-temperature, quasi-spherical accretion flow onto a compact object, recently considered by a number of authors, assume that the dissipation of turbulent energy of the flow heats the ions and that the dissipated energy is advected inward. It is suggested that the efficiency of conversion of accretion energy to radiation can be very much smaller than unity. However, it is likely that the flows have an equipartition magnetic field with the result that dissipation of magnetic energy at a rate comparable to that for the turbulence must occur by Ohmic heating. We argue that …
We investigate the form of the momentum distribution function for protons and electrons in an advection-dominated accretion flow (ADAF). We show that for all accretion rates, Coulomb collisions are too inefficient to thermalize the protons. The proton distribution function is therefore determined by the viscous heating mechanism, which is unknown. The electrons, however, can exchange energy quite efficiently through Coulomb collisions and the emission and absorption of synchrotron photons. We find that for accretion rates greater than \sim 10^{-3} of the Eddington accretion rate, the electrons…
We demonstrate that at relatively low mass accretion rates, black hole candidate (BHC) X-ray binaries (XRBs) should enter `jet-dominated' states, in which the majority of the liberated accretion power is in the form of a (radiatively inefficient) jet and not dissipated as X-rays in the accretion flow. This result follows from the empirically established non-linear relation between radio and X-ray power from low/hard state BHC XRBs, which we assume also to hold for neutron star (NS) XRBs. Conservative estimates of the jet power indicate that all BHC XRBs in `quiescence' should be in this jet-do…
A large-scale closed magnetic field can transfer angular momentum and energy between a black hole (BH) and its surrounding accretion flow. We investigate the effects of this magnetic coupling (MC) process on the dynamics of a hot accretion flow (e.g., an advection dominated accretion flow, hereafter ADAF). The energy and angular momentum fluxes transported by the magnetic field are derived by an equivalent circuit approach. For a rapidly rotating BH, it is found that the radial velocity and the electron temperature of the accretion flow decrease, whereas the ion temperature and the surface den…
We calculate the flux and spectrum of γ-rays emitted by a two-temperature advection-dominated accretion flow (ADAF) around a black hole. The γ-rays are from the decay of neutral pions produced through proton-proton collisions. We discuss both thermal and power-law distributions of proton energies and show that the γ-ray spectra in the two cases are very different. We apply the calculations to the γ-ray source, 2EG J1746-2852, detected by EGRET from the direction of the Galactic Center. We show that the flux and spectrum of this source are consistent with emission from an ADAF around the superm…
In a previous paper we have written down equations describing steady-state, optically thin, advection-dominated accretion onto a Kerr black hole (Gammie and Popham 1997, hereafter Paper I). In this paper we survey the numerical solutions to these equations. We find that the temperature and density of the gas in the inner part of the accretion flow depend strongly on the black hole spin parameter $a$. The rate of angular momentum accretion is also shown to depend on $a$; for $a$ greater than an equilibrium spin parameter $a_{eq}$ the black hole is de-spun by the accretion flow. We also investig…
We describe new optically thin solutions for rotating accretion flows around black holes and neutron stars. These solutions are advection dominated, so that most of the viscously dissipated energy is advected radially with the flow. We model the accreting gas as a two temperature plasma and include cooling by bremsstrahlung, synchrotron, and Comptonization. We obtain electron temperatures $T_e\sim 10^{8.5}-10^{10}$K. The new solutions are present only for mass accretion rates $\dot M$ less than a critical rate $\dot M_{crit}$ which we calculate as a function of radius $R$ and viscosity paramet…
Relativistic outflows (jets) of matter are commonly observed from systems containing black holes. The strongest outflows occur in the radio-loud systems, in which the accretion disk is likely to have an advection-dominated structure. In these systems, it is clear that the binding energy of the accreting gas is emitted primarily in the form of particles rather than radiation. However, no comprehensive model for the disk structure and the associated outflows has yet been produced. In particular, none of the existing models establishes a direct physical connection between the presence of the outf…
It has recently been demonstrated that the Shakura-Sunyaev prescription for the kinematic viscosity in an advection-dominated accretion disk yields physically reasonable solutions for the structure of the inflow close to the event horizon. In particular, no violations of relativistic causality occur at the horizon. This is somewhat surprising considering the diffusive nature of the angular momentum transport in the Shakura-Sunyaev scenario, and it is therefore natural to ask whether one can also obtain acceptable solutions for the disk structure based on the various alternative models for the …
In this Letter, we suggest that the relativistic protons powering the outflows emanating from radio-loud systems containing black holes are accelerated at standing, centrifugally-supported shocks in hot, advection-dominated accretion disks. Such disks are ideal sites for first-order Fermi acceleration at shocks because the gas is tenuous, and consequently the mean free path for particle-particle collisions generally exceeds the thickness of the disk. The accelerated particles are therefore able to avoid thermalization, and as a result a small fraction of them achieve very high energies and esc…
When we model black hole accretion sources such as active galactic nuclei and black hole X-ray binaries as advection-dominated accretion flows (ADAFs), it is neccesary to use the global solution to the equations rather than the simpler self-similar solution, since the latter is inaccurate in the region near the black hole where most of the radiation is emitted. However, technically, it is a difficult task to calculate the global solution because of the transonic nature of the flow, which makes it a two-point boundary value problem. In this paper we propose a simplified approach for calculating…
Recently, Narayan & Raymond (1999) proposed that the thermal emission lines from the hot plasma in advection dominated accretion flows (ADAFs) are potentially observable with the next generation of X-ray observatories, with which the physical properties of some X-ray sources can be probed. In ADAFs, the temperature of the ion is so high that the thermal broadening of the line is important. We calculate the profiles of thermal line emission from ADAFs, in which both the thermal and Doppler broadening have been considered. It is found that the double-peaked profiles are present for high inclinat…
We present the basic equations and sample solutions for the steady-state global transonic solutions of the advection-dominated accretion flows (ADAFs) with a causal viscosity prescription. The procedures for the stable numerical calculations and all explicit formula for obtaining the solutions of ADAFs are also presented. We solve the transonic solutions of ADAFs by using the explicit numerical integrations, as in the Runge-Kutta method. In this calculation method, we first solve the physical values at the sonic radius where L'Hopital's rule is used. Then, we numerically solve the coupled diff…
