Vertical structure of the outer accretion disk in persistent low-mass X-ray binaries

We have investigated the influence of X-ray irradiation on the vertical structure of the outer accretion disk in low-mass X-ray binaries by performing a self-consistent calculation of the vertical structure and X-ray radiation transfer in the disk. Penetrating deep into the disk, the field of scattered X-ray photons with energy E ≳ 10 keV exerts a significant influence on the vertical structure of the accretion disk at a distance R ≳ 10¹⁰ cm from the neutron star. At a distance R ∼ 10¹¹ cm, where the total surface density in the disk reaches Σ₀ ∼ 20 g cm⁻², X-ray heating affects all layers of an optically thick disk. The X-ray heating effect is enhanced significantly in the presence of an extended atmospheric layer with a temperature T _atm ≈ (2–3) × 10⁶ K above the accretion disk. We have derived simple analytic formulas for the disk heating by scattered X-ray photons using an approximate solution of the transfer equation by the Sobolev method. This approximation has a ≲10% accuracy in the range of X-ray photon energies E < 20 keV.     

Parameters of irradiated accretion disks from optical and X-ray observations of GS 1826-238

We show that the set of observational characteristics for low-mass X-ray binaries in the optical and X-ray bands can be explained in terms of the model of an optically thick accretion disk with an atmosphere irradiated by a central X-ray source. We show that this set of observational data can be successfully used to measure the orbital inclination of a binary, the geometric parameters of its accretion disk, and the reprocessing time of X-emission to optical one. For the burster GS 1826-238, a low-mass X-ray binary with a neutron star, we have estimated the binary inclination and the thickness of the disk atmosphere at the outer edge from the mean optical flux and the amplitude of periodic modulations in the optical light curve: i = 62.5° ± 5.5° and H _d/R _d = 0.145 ± 0.009. The optical response time of the binary to an X-ray burst disagrees with the geometric delay in the propagation of X-ray photons in the binary. We believe that this points to a finite X-ray reprocessing/reradiation time, 1.0 s ≲ τ _repr ≲ 2.2 s, in the hot atmosphere above the accretion disk.     

Spectral fit of Cygnus X-1 in high energy— a self-consistent study

We fit the spectra of Cyg X-1 using two component advective flows with Keplerian accretion disks on the equatorial plane surrounded by sub-Keplerian disks when standing shocks are present. The soft photons generated by the bremsstrahlung and synchrotron processes in the sub-Keplerian flow, as well as the multi-colour black body emission from the Keplerian disk are Comptonized by the thermal and non-thermal electrons. By varying Keplerian and sub-Keplerian rates we are able to reproduce the observed soft and hard states as far as X-ray region is concerned and ‘low γ-ray intensity’ and ‘high γ-ray intensity’ states as far as the soft γ-ray region is concerned. We also find two pivotal points where the spectra intersect as is observed in Cyg X-1.      

A submillimeter view of protoplanetary dust disks

We present some results from our submillimeter single-dish and aperture synthesis imaging surveys of protoplanetary disks using the JCMT, CSO, and Submillimeter Array (SMA) on Mauna Kea, Hawaii. Employing a simple disk model, we simultaneously fit the spectral energy distributions and spatially resolved submillimeter continuum emission from our SMA survey to constrain disk structure properties, including surface density profiles and sizes. The typical disk structure we infer is consistent with a fiducial accretion disk model with a viscosity parameter α≈0.01. Combined with a large, multiwavelength single-dish survey of similar disks, we show how these observations provide evidence for significant grain growth and rapid evolution in the outer regions of disks, perhaps due to an internal photoevaporation process. In addition, we discuss SMA observations of the disks in the Orion Trapezium (proplyds) in the context of disk evolution in a more extreme environment.     

Spectral Features of Photon Bubble Models of Ultraluminous X-ray Sources

The nature of Ultraluminous X-ray Sources – X-ray sources which exceed the Eddington luminosity for a ∼10 M _⊙ black hole – remains a mystery. One possible explanation is an inhomogeneous accretion disk around a solar mass black hole where photon transport through radiation-pressure dominated “photon bubbles” can lead to super-Eddington accretion. While previous studies of this model have focused primarily on its radiation-hydrodynamics aspects, here we explore some observational implications of such a model with a Monte Carlo–Fokker Planck radiation transport code.     

Evolutionary processes in protoplanetary accretion disks: the propagation of axisymmetric shock waves

In this paper we investigate both the global and the local hydrodynamics of axisymmetric accretion disks around young stellar objects under the simultaneous action of viscosity, self-gravity and pressure forces. For simplicity, we take for the global model a polytropic equation of state, make the infinitely thin disk approximation and characterize the surface density and temperature profiles in the disk as power laws in the radial distance r from the protostar. We solve the problem of the general density profile of a Keplerian disk showing that self-gravity could not be an important factor for the fast formation of the rocky cores of giant gaseous planets in our solar system. Under the hypothesis that the unperturbed rotation curve of the disk is nearly Keplerian throughout the radial extent, we can estimate with our polytropic model a lower limit for the resulting masses M_d(r) of stable disks up to 100 AU. These masses are in the range of the so-called minimum mass solar nebular (_d/M_s ≈ 0.01–0.02).By adopting a simplified viscosity model, where the height-integrated turbulent dynamical viscosity ν is a function of the surface density σ like η ∝ σ^Γ, we derive in the local shearing sheet model linearized evolution equations for small density perturbations describing both a diffusion process and the propagation of acoustic density waves. We solve a special initial value problem and calculate the appropriate Green's function. The analytical solutions so obtained describe in the case Γ < 0 the successive formation of quasi-stationary ring-shaped density structures in a disk with a definite mode of maximum instability, whereas in the case Γ > Γ_c the density wave equation describes the propagation of an “overstable” ring-shaped acoustic density wavelet to the outer ranges of the accretion disk. Whereas the group velocity of the wave packet is subsonic, the phase velocities of individual wave crests in the wave packet are supersonic. The mode of maximum instability, the growth rate and the number of growing waves in the wavelet are controlled by Γ and α. Our present knowledge concerning turbulent viscosity in protoplanetary disks is not sufficient to decide whether or not the case Γ > Γ_c is realized.The suggested structuring processes in the linear theory should initiate in the non-linear regime the formation of narrow ring-shaped density shock waves moving through the protoplanetary disk. These non-linear waves could produce extremely spatially and temporally heterogeneous temperature regions in the disk. We speculate that ring-shaped density waves, excited by inner boundary conditions and which have dominated the disk's evolution at early times, are responsible both for the fast growth of dust to planetesimals and at least for the rapid accretion of the rocky cores of giant gaseous planets in the protoplanetary accretion disk (shock wave trigger hypothesis). We derive provisional scaling rules for planetary systems regarding the spacing of orbits as a function of the mass ratio of the protoplanetary disk to the protostar. However, further analytical work and linear as well as nonlinear numerical simulations of density waves excited by inner boundary conditions are needed to consolidate the results and speculations of our linear wave mechanics in the future.     

The local stability of accretion disk models with considering the role of various viscosity and cooling mechanisms

The standard thin accretion disk model can explain the soft X-ray spectra of Galactic black hole systems and AGN successfully. However, there are still a few observational documents for Radiation pressure theory in X-ray novae in black hole binary systems and AGN. The luminosity in accretion onto black holes is corresponds to L>0.01L _E . According to standard thin disk model, when the accretion rate is over a small fraction of the Eddington rate, L>0.01L _E , the inner region of the disk is radiation-pressure-dominated and thermally unstable. However, observations of the high/soft state of black hole X-ray binaries with luminosity within (0.01L _E <L<0.5L _E ) show that the disk is quite stable. Thus, this contradiction shows the objection of this model and maybe it is essential to change the standard viscosity law or one of the other basic assumptions in order to get a stable disk models. In this paper, we revisit and recalculate the thermal instability with a different models of viscosity and cooling functions and show that the choosing of an arbitrary cooling and viscosity functions can affect on the stability of a general disk model and hence maybe answer to a this problem in accretion disk theory. We choose an arbitrary functions of surface density Σ and half thickness of disk H for cooling and viscosity. Also, we discuss a general disk with thermal conduction, radial force and advection. Then, we solve the equations numerically. We obtain a fourth degree dispersions relation and discuss solutions and instability modes. This analysis shows the great sensitivity of stability of disk to the form of viscosity, so there are various effective factors to stabilize the disk. For example the exist of advection and thermal conduction can effect to stability of disks also.     

Present Evidence for Intermediate Mass Black Holes in ULXs and Future Prospects

In a number of the most luminous ULXs (those with L_X ∼ 10⁴⁰ erg s⁻¹) in nearby galaxies, observations with XMM-Newton and Chandra are revealing evidence which suggests that these ULXs may harbor intermediate-mass black holes (IMBHs). The detection of accretion disk spectral components with temperatures 5–10 times lower than the temperatures observed in stellar-mass black hole binaries near to their Eddington limit may be particularly compelling evidence for IMBH primaries, since T ∝ M^−1/4 for disks around black holes. In some sources, X-ray timing diagnostics also hint at IMBHs. Evidence for IMBHs in a subset of the most luminous ULXs, a discussion of the robustness of this evidence and alternatives to the IMBH interpretation, and prospects for better determining the nature of these sources in the future, are presented in this work.     

Magnetic fields and quasi-periodic oscillations of black hole radiation

Various relations are found between the key parameters of black holes and active galactic nuclei. Some have a statistical property, others follow from the theoretical consideration of the evolution of these objects. In this paper we use a recently discovered empirical relation between the characteristic frequency of quasi-periodic oscillations of radiation ν _br of black holes, their masses and matter accretion rates to determine the magnetic field strength B _H at the black hole event horizon. Since the characteristic frequency can be determined from observations, the use of a new relation for the estimations of magnetic field B _H can yield more definite results, since we are decreasing the number of the unknown or poorly-determined parameters of objects (it especially concerns the accretion rate Ṁ). The typical values which we have found are B _H ≃ 10⁸G for the stellar mass black holes, and B _H ≃ 10⁴G for the supermassive black holes. Besides, we demonstrate that if the linear polarization of an object is caused by the radiation of a magnetized accretion disk, then the degree of observable polarization is p ∼ ν _br ^−1/2.     

The role of toroidal magnetic field in the stability of accretion disks with alpha viscosity and other viscosities

The standard thin accretion disk model predicts that the inner regions of alpha model disks, where radiation pressure is dominant, are thermally and viscously unstable. However, observations show that the bright X-ray binaries and AGN accretion disks, corresponding to radiation-pressure thin disks, are stable. In this paper, we reconsider the linear and local instability of accretion disks in the presence of a toroidal magnetic field. In the basic equations, we consider physical quantities such as advection, thermal conduction, arbitrary viscosity, and an arbitrary cooling function also. A fifth order diffusion equation is obtained and is solved numerically. The solutions are compared to non-magnetic cases. The results show that the toroidal magnetic field can make the thermal instability in radiation pressure-dominated slim disks disappear if ? _m ≥0.3. However, it causes a more thermal instability in radiation pressure alpha disks without advection. Also, we consider the thermal instability in accretion disks with other values of the viscosity and obtain a general criterion for thermal instability in the long-wavelength limit and in the presence of a toroidal magnetic field.     

Resonant Oscillations of Accretion Flow and Khz QPOS

High-frequency quasi-periodic variations (HF QPOs) in the X-ray light curves of black hole X-ray novae can be understood as oscillations of the accretion disk in a nonlinear 3:2 resonance. An m = 0 vertical oscillation near a black hole modulates the X-ray emission through gravitational lensing (light-bending) at the source. Certain oscillations of the accretion disk will also modulate the mass accretion rate, and in neutron-star systems this would lead to nearly periodic variations in brightness of the luminous boundary layer on the stellar surface – the amplitude of the neutron-star HF QPOs would be thus increased relative to the black hole systems. The “kHz QPOs” in black holes are in the hecto-Hz range.     

Structure and stability of rotating fluid disks around massive objects. I. Newtonian formulation

In this paper we have presented a very general class of solutions for rotating fluid disks around massive objects (neglecting the self gravitation of the disk) with density as a function of the radial coordinate only and pressure being nonzero. Having considered a number of cases with different density and velocity distributions, we have analysed the stability of such disks under both radial and axisymmetric perturbations. For a perfect gas disk with γ= 5/3 the disk is stable with frequency (MG/r³)^1/2 for purely radial pulsation with expanding and contracting boundary. In the case of axisymmetric perturbation the critical γ_c for neutral stability is found to be much less than 4/3 indicating that such disks are mostly stable under such perturbations. On leave of absence from Government College, Jagdalpur 494005.     

On the Relationship Between the Jets from X-Ray Binaries and Agn

The scale invariance model (Heinz, S. and Sunyaev, R.A.: 2003, MNRAS 343, L59) can be used to derive robust scaling relations between the radio luminosity from accreting black holes and the black hole mass and accretion rate. These relations agree well with the recently found “fundamental plane” of black hole activity (Merloni, A., Heinz, S. and Di Matteo, T.: 2003, MNRAS 345, 1057). This relation provides a new, powerful tool for the comparison of jets from black holes of different masses and accretion rates. The regression coefficients of this relation contain information about the nature of the X-ray emission mechanism driving the correlation. We argue that X-ray synchrotron emission from the base of the jets is unlikely to be the dominant contribution to the X-ray spectrum in most of the sources.     

Viscous-resistive ADAF with a general large-scale magnetic field

We have studied the structure of hot accretion flow bathed in a general large-scale magnetic field. We have considered magnetic parameters , where are the Alfvén sound speeds in three direction of cylindrical coordinate (r,φ,z). The dominant mechanism of energy dissipation is assumed to be the magnetic diffusivity due to turbulence and viscosity in the accretion flow. Also, we adopt a more realistic model for kinematic viscosity (ν=αc _s H), with both c _s and H as a function of magnetic field. As a result in our model, the kinematic viscosity and magnetic diffusivity (η=η ₀ c _s H) are not constant. In order to solve the integrated equations that govern the behavior of the accretion flow, a self-similar method is used. It is found that the existence of magnetic resistivity will increase the radial infall velocity as well as sound speed and vertical thickness of the disk. However the rotational velocity of the disk decreases by the increase of magnetic resistivity. Moreover, we study the effect of three components of global magnetic field on the structure of the disk. We found out that the radial velocity and sound speed are Sub-Keplerian for all values of magnetic field parameters, but the rotational velocity can be Super-Keplerian by the increase of toroidal magnetic field. Also, Our numerical results show that all components of magnetic field can be important and have a considerable effect on velocities and vertical thickness of the disk.     

Disk galaxy evolution: from the Milky Way to high-redshift disks

We develop a detailed model of the Milky Way (a `prototypical' disk galaxy) and extend it to other disks with the help of some simple scaling relations, obtained in the framework of Cold Dark Matter models. This phenomenological (`hybrid') approach to the study of disk galaxy evolution allows us to reproduce successfully a large number of observed properties of disk galaxies in the local Universe and up to redshift z ∼ 1. The important conclusion is that, on average, massive disks have formed the bulk of their stars earlier than their lower mass counterparts: the `star formation hierarchy' has been apparently opposite to the `dark matter assembly' hierarchy. It is not yet clear whether `feedback' (as used in semi-analytical models of galaxy evolution) can explain that discrepancy. This revised version was published online in August 2006 with corrections to the Cover Date.     

Low-Mass Young Stars in Cep OB2: Ages, Distribution and Accretion Disks

We are studying the young clusters Tr37 and NGC7160 in the Cep OB2 region as part of a program to understand the evolution of accretion disks at the ages of disk dissipation and planet formation. Here, we present the first identifications of low mass (spectral types K-M) members of the clusters and study the presence and characteristics of their accretion disks, finding evidences of disk evolution. Using optical photometry and spectroscopy, we have identified ～70 members in Tr37 and ～20 in NGC7160, confirming age estimates of 3 and 10 Myr respectively. Accretion rates are ～10^?8 M _⊙yr^?1 in Tr37. We have not found any accreting members in NGC7160, suggesting that disk accretion generally ends before the age of 10 Myr, which is consistent with the results from other populations.     

Observational evidence for massive black holes in the centers of active galaxies

Naturally occurring water vapor maser emission at 1.35 cm wavelength provides an accurate probe for the study of accretion disks around highly compact objects, thought to be black holes, in the centers of active galaxies. Because of the exceptionally fine angular resolution, 200 microarcseconds, obtainable with very long baseline interferometry, accompanied by high spectral resolution, <0.1 km s^-1, the dynamics and structures of these disks can be probed with exceptional clarity. The data on the galaxy NGC 4258 are discussed here in detail. The mass of the black hole binding the accretion disk is 3·9 × 10⁷ M_⊙. Although the accretion disk has a rotational period of about 800 years, the physical motions of the masers have been directly measured with VLBI over a period of a few years. These measurements also allow the distance from the earth to the black hole to be estimated to an accuracy of 4 per cent. The status of the search for other maser/black hole candidates is also discussed.     

Spread of matter over a neutron-star surface during disk accretion: Deceleration of rapid rotation

The accretion of hot slowly rotating gas onto a supermassive black hole is considered. The important case where the velocities of turbulent pulsations at the Bondi radius r _B are low, compared to the speed of sound c _s, is studied. Turbulence is probably responsible for the appearance of random average rotation. Although the angular momentum at r _B is low, it gives rise to the centrifugal barrier at a depth r _c = l ² /GM _BH ≪ r _B, that hinders supersonic accretion. The numerical solution of the problem of hot gas accretion with finite angular momentum is found taking into account electron thermal conductivity and bremsstrahlung energy losses of two temperature plasma for density and temperature near Bondi radius similar to observed in M87 galaxy. The saturation of the Spitzer thermal conductivity was also taken into account. The parameters of the saturated electron thermal conductivity were chosen similar to the parameters used in the numerical simulations of interaction of the strong laser beam radiation with plasma targets. These parameters are confirmed in the experiments. It is shown that joint action of electron thermal conductivity and free-free radiation leads to the effective cooling of accreting plasma and formation of the subsonic settling of accreting gas above the zone of a centrifugal barrier. A toroidal condensation and a hollow funnel that separates the torus from the black hole emerge near the barrier. The barrier divides the flow into two regions: (1) the settling zone with slow subKeplerian rotation and (2) the zone with rapid supersonic nearly Keplerian rotation. Existence of the centrifugal barrier leads to significant decrease of the accretion rate Ṁ in comparison with the critical Bondi solution for γ = 5/3 for the same values of density and temperature of the hot gas near Bondi radius. Shear instabilities in the torus and related friction cause the gas to spread slowly along spirals in the equatorial plane in two directions.As a result, outer (r > r _c) and inner (r < r _c) disks are formed. The gas enters the immediate neighborhood of the black hole or the zone of the internal ADAF flow along the accretion disk (r < r _c). Since the angular momentum is conserved, the outer disk removes outward an excess of angular momentum along with part of the matter falling into the torus. It is possible, that such outer Keplerian disk was observed by Hubble Space Telescope around the nucleus of the M87 galaxy in the optical emission lines. We discuss shortly the characteristic times during which the accretion of the gas with developed turbulence should lead to the changes in the orientation of the torus, accretion disk and, possibly, of the jet.