One-temperature accretion flows with advection and nonthermal e pair production

By means of the self-similar relations of the hot flow with advection and nonthermal e^± pair production, we examine the radial structure of such a flow. Some typical features have been confirmed, and several new results obtained: there exists a critical radius r_cr; the local cooling rate is inversely proportional to the square of the mass of the central object; the pair density in the accretion flow with advection is higher than in the SLE disk.     

Hot accretion discs with thermal Comptonization and advection in luminous black hole sources

We solve for the structure of a hot accretion disc with unsaturated thermal Comptonization of soft photons and with advection, generalizing the classical model of Shapiro et al. The upper limit on the accretion rate due to advection constrains the luminosity to ≲ 0.15 y^3/5 α^7/5 of the Eddington limit, where y and α are the Compton and viscosity parameters, respectively. The characteristic electron temperature and Thomson optical depth of the inner flow at accretion rates within an order of magnitude of that upper limit are ∼ 10⁹ K and ∼ 1, respectively. The resulting spectra are then in close agreement with the X-ray and soft γ-ray spectra from black hole binaries in the hard state and Seyferts. At low accretion rates, bremsstrahlung becomes the dominant radiative process.     

黑洞吸积理论的新进展(Ⅰ):ADAF吸积理论

黑洞的吸积是天体物理学中最重要的基础理论之一。近年来该理论取得了引人瞩目的重大进展,具体表现在两个方面。其一是根据黑洞吸积必定跨声速这一特性,提出在一定条件下吸积流中会出现激波,这可称为含激波的吸积理论;其二是基于对一种局域致冷机制-贮导（advection）致冷的作用的重新认识而建立的,称为ADAF理论。在吸积盘的光学厚度很小或很大两种情况下,粘滞产生的大部分热量没有像在标准薄盘模型中那样辐射出去,而是贮存在流体中随流体的径向运动进入黑洞。与标准薄盘模型相比,贮导吸积盘具有高得多的温度和大得多的径向速度,但角动量小于开普勒角动量,吸积致能的效率要低得多。     

Luminous hot accretion discs

We find a new two-temperature hot branch of equilibrium solutions for stationary accretion discs around black holes. In units of Eddington accretion rate defined as 10 L _Edd c ², the accretion rates to which these solutions correspond are within the range ̇ ₁≲ ̇ ≲1, where ̇ ₁ is the critical rate of advection-dominated accretion flow (ADAF). In these solutions, the energy loss rate of the ions by Coulomb energy transfer between the ions and electrons is larger than the viscously heating rate and it is the advective heating together with the viscous dissipation that balances the Coulomb cooling of ions. When ̇ ₁≲ ̇ ≲ ̇ ₂, where ̇ ₂∼5 ̇ ₁<1, the accretion flow remains hot throughout the disc. When ̇ ₂≲ ̇ ≲1, Coulomb interaction will cool the inner region of the disc within a certain radius ( r _tr∼several tens of Schwarzschild radii or larger depending on the accretion rate and the outer boundary condition) and the disc will collapse on to the equatorial plane and form an optically thick cold annulus. Compared with ADAF, these hot solutions are much more luminous because of the high accretion rate and efficiency; therefore, we call them luminous hot accretion discs.     

The effects of thermal conduction on the ADAF with a toroidal magnetic field

The observation of the hot gas surrounding Sgr A * and a few other nearby galactic nuclei imply that electron and proton mean free paths are comparable to the gas capture radius. So, the hot accretion flows are likely to proceed under week-collision conditions. Hence, thermal conduction has been suggested as a possible mechanism by which the sufficient extra heating is provided in hot advection-dominated accretion flow (ADAF) accretion discs. We consider the effects of thermal conduction in the presence of a toroidal magnetic field in an ADAF around a compact object. For a steady-state structure of such accretion flows, a set of self-similar solutions are presented. We find two types of solutions which represent high and slow accretion rate. They have different behaviours with saturated thermal conduction parameter, φ.     

Radiation spectrum of a magnetized supercritical accretion disc with thermal conduction

we examine the effect of thermal conduction on the observational properties of a super critical hot magnetized flow. We obtained self-similar solution of a magnetized disc when the thermal conduction plays an important role. Follow of our first paper (Ghasemnezhad et al. in Astrophys. J. 750, 2012 (hereafter GKA12)) we have extended our solution on the observational appearance of the disc to show how physical condition such as thermal conduction, viscosity, and advection will change the observed luminosity of the disc, Continuous spectra and surface temperature of such discs was plotted. We apply the present model to black-hole X-ray binary LMC X-3 and narrow-line seyfert 1 galaxies, which are supposed to be under critical accretion rate. Our results show clearly that the surface temperature is strongly depends on the thermal conduction, the magnetic field and advection parameter. However we see that thermal conduction acts to oppose the temperature gradient as we expect and observed luminosity of the disc will reduce when thermal conduction is high. We have shown that in this model the spectra of critical accretion flows strongly depends on the inclination angle.     

Super-Eddington accretion discs around Kerr black holes

We calculate the structure of the accretion disc around a rapidly rotating black hole with a super-Eddington accretion rate. The luminosity and height of the disc are reduced by the advection effect. In the case of large viscosity parameter, α>0.03, the accretion flow deviates strongly from thermodynamic equilibrium and overheats in the central region. With increasing accretion rate, the flow temperature steeply increases, reaches maximum, and then falls off. The maximum is achieved in the advection-dominated regime of accretion. The maximum temperature in the disc around a massive black hole of M =10⁸ M⊙ with α=0.3 is of order 3×10⁸ K. The discs with large accretion rates can emit X-rays in quasars as well as in galactic black hole candidates.     

Component thermalization time-scale estimate for the advection dominated accretion flow around Sgr 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 the accretion time, 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 of the ADAF model [Nature 394 (1998) 651; MNRAS 304 (1999) 501] in explaining the emitted spectrum of Sgr A*, and it is complementary to the similar analysis of Mahadevan and Quataert [ApJ 490 (1997) 605].     

Hot accretion flow with ordered magnetic field, outflow, and saturated conduction

The importance of thermal conduction on hot accretion flow is confirmed by observations of hot gas that surrounds Sgr A^? and a few other nearby galactic nuclei. On the other hand, the existence of outflow in accretion flows is confirmed by observations and magnetohydrodynamic (MHD) simulations. In this research, we study the influence of both thermal conduction and outflow on hot accretion flows with ordered magnetic field. Since the inner regions of hot accretion flows are, in many cases, collisionless with an electron mean free path due to Coulomb collision larger than the radius, we use a saturated form of thermal conduction, as is appropriate for weakly collisional systems. We also consider the influence of outflow on accretion flow as a sink for mass, and the radial and the angular momentum, and energy taken away from or deposited into the inflow by outflow. The magnetic field is assumed to have a toroidal component and a vertical component as well as a stochastic component. We use a radially self-similar method to solve the integrated equations that govern the behavior of such accretion flows. The solutions show that with an ordered magnetic field, both the surface density and the sound speed decrease, while the radial and angular velocities increase. We found that a hot accretion flow with thermal conduction rotates more quickly and accretes more slowly than that without thermal conduction. Moreover, thermal conduction reduces the influences of the ordered magnetic field on the angular velocities and the sound speed. The study of this model with the magnitude of outflow parameters implies that the gas temperature decreases due to mass, angular momentum, and energy loss. This property of outflow decreases for high thermal conduction.     

A unified description of the global structure of accretion disks

The global structure of accretion disks is investigated in a unified scheme. We use a general radiative cooling formula applicable to both optically thick and thin regimes and we include radial advection cooling. Within the -viscosity models, we found distinct families of global solutions. If the accretion rate is low, there are three non-intersecting solutions, corresponding to optically thick and thin, local cooling and optically thin advection cooling. If the accretion rate is high, and the viscosity coefficient is large, the two local cooling solutions coincide at radii R₁ and R₂ and exist independently below R₁ and above R₂ while the advection solution is stable at all radii. If the accretion is high and the viscosity is low, the two optically thin solutions will cross each other while the optically thick solution exists at all radii.     

Shakura—Sunyaev盘向径移主导吸积流的转变

对Shakura-Sunyaev盘向径移主导吸积流转变的机制作了更详细的研究，即考虑了有韧致辐射，同步辐射和康普顿化的双温等离子体吸积流，结果进一步表明这种转变在相当广泛的物理条件下都可能实现。     

The magnetic prandtl number on structure of hot and cold accretion flows

The aim of this work is to investigate the effect of the presence of a magnetic Prandtl number on the structure of an accretion flow with a bipolar outflow by focusing on the density structure. Two cold and hot classes are considered for accretion flows. According to the self-similar assumptions in the radial direction and boundary conditions as well, we solve the MHD equations along the θ-direction to obtain the density structure. In addition, we consider the results in two gas-pressure-dominated and radiation-pressure-dominated regions. The obtained results show that the existence of a magnetic prandtl number may lead to bump formation in hot accretion flows, which may have consequences for planet formation. Furthermore, some discontinuations in the density structure are seen at some regions resulting in the production of a gap in the case of cold accretion flows. The results of this work may be useful in the consideration of the Rossby wave instability in both classes of accretion flows.     

The effect of wind on magnetized advection dominated accretion flows

We study the effects of winds on advection dominated accretion flows in the presence of a global magnetic field under a self-similar treatment. The disk gas is assumed to be isothermal. For a steady state structure of such accretion flows a set of self similar solutions are presented. We consider the wind in a general magnetic field with three components (r,φ,z) in advection-dominated accretion flows. The mass-accretion rate $\dot{M}$ decreases with radius r as $\dot{M}\propto r^{s+1/2}$ , where s is an arbitrary constant. We will see, by increasing the wind parameter s, radial and rotational velocities increase.     

X-ray emissions from two-temperature accretion flows within a dipole magnetic funnel

We investigate the hydrodynamics of accretion channelled by a dipolar magnetic field (funnel flows). We consider situations in which the electrons and ions in the flow cannot maintain thermal equilibrium [two-temperature (2T) effects] due to strong radiative loss, and determine the effects on the keV X-ray properties of the systems. We apply this model to investigate the accretion shocks of white dwarfs in magnetic cataclysmic variables (mCVs). We have found that the incorporation of 2T effects could harden the keV X-rays. Also, the dipolar model yields harder X-ray spectra than the standard planar model if white dwarf is sufficiently massive  (≳1 M_⊙)  . When fitting observed keV X-ray spectra of mCVs, the inclusion of 2T hydrodynamics and a dipolar accretion geometry lowers estimates for white dwarf masses when compared with masses inferred from models excluding these effects. We find mass reductions ≲9 per cent in the most massive cases.     

The stability of spherically symmetric accretion flows

The stability of spherically symmetric accretion flows is reconsidered. An analytical proof on stability for isothermal critical flow is given and numerical stability analyses are carried out for critical accretion flows in the range of 1<<5/3. Moreover, numerical results for subsonic flows are given.     

Episodic accretion in magnetically layered protoplanetary discs

We study protoplanetary disc evolution assuming that angular momentum transport is driven by gravitational instability at large radii, and magnetohydrodynamic (MHD) turbulence in the hot inner regions. At radii of the order of 1 au such discs develop a magnetically layered structure, with accretion occurring in an ionized surface layer overlying quiescent gas that is too cool to sustain MHD turbulence. We show that layered discs are subject to a limit cycle instability, in which accretion on to the protostar occurs in ∼10⁴-yr bursts with ̇ ∼10⁻⁵ M_⊙ yr⁻¹, separated by quiescent intervals lasting ∼10⁵ yr where ̇ ≈10⁻⁸ M_⊙ yr⁻¹. Such bursts could lead to repeated episodes of strong mass outflow in young stellar objects. The transition to this episodic mode of accretion occurs at an early epoch ( t ≪1 Myr), and the model therefore predicts that many young pre-main-sequence stars should have low rates of accretion through the inner disc. At ages of a few Myr, the discs are up to an order of magnitude more massive than the minimum-mass solar nebula, with most of the mass locked up in the quiescent layer of the disc at r ∼1 au. The predicted rate of low-mass planetary migration is reduced at the outer edge of the layered disc, which could lead to an enhanced probability of giant planet formation at radii of 1–3 au.     

Hydrodynamical non-radiative accretion flows in two dimensions

Two-dimensional (axially symmetric) numerical hydrodynamical calculations of accretion flows that cannot cool through emission of radiation are presented. The calculations begin from an equilibrium configuration consisting of a thick torus with constant specific angular momentum. Accretion is induced by the addition of a small anomalous azimuthal shear stress which is characterized by a function ν . We study the flows generated as the amplitude and form of ν are varied. A spherical polar grid which spans more than two orders of magnitude in radius is used to resolve the flow over a wide range of spatial scales. We find that convection in the inner regions produces significant outward mass motions that carry away both the energy liberated by and a large fraction of the mass participating in the accretion flow. Although the instantaneous structure of the flow is complex and dominated by convective eddies, long-time averages of the dynamical variables show remarkable correspondence to certain steady-state solutions. The two-dimensional structure of the time-averaged flow is marginally stable to the Høiland criterion, indicating that convection is efficient. Near the equatorial plane, the radial profiles of the time-averaged variables are power laws with an index that depends on the radial scaling of the shear stress. A stress in which ν ∝ r ^1/2 recovers the widely studied self-similar solution corresponding to an ' α -disc'. We find that, regardless of the adiabatic index of the gas, or the form or magnitude of the shear stress, the mass inflow rate is a strongly increasing function of radius, and is everywhere nearly exactly balanced by mass outflow. The net mass accretion rate through the disc is only a fraction of the rate at which mass is supplied to the inflow at large radii, and is given by the local, viscous accretion rate associated with the flow properties near the central object.