Structure Of Accretion Disk Research Articles

Fast transition between high-soft and low-soft states in GRS 1915 + 105: Evidence for a critically viscous accretion flow

We present the results of a detailed analysis of RXTE observations of classω (Klein-Woltet al. 2002) which show an unusual state transition between high-soft and low-soft states in the Galactic microquasar GRS 1915 + 105. Out of about 600 pointed RXTE observations, the source was found to exhibit such state transition only on 16 occasions. An examination of the RXTE/ASM data in conjunction with the pointed observations reveals that these events appeared as a series of quasi-regular dips in two stretches of long duration (about 20 days during each occasion) when hard X-ray and radio flux were very low. The X-ray light curve and colour-colour diagram of the source during these observations are found to be different from any reported so far. The duration of these dips is found to be of the order of a few tens of seconds with a repetition time of a few hundred seconds. The transition between these dips and non-dips which differ in intensity by a factor of ∼ 3.5, is observed to be very fast (∼ a few seconds). It is observed that the low-frequency narrow QPOs are absent in the power density spectrum (PDS) of the dip and non-dip regions of classω and the PDS is a power law in the 0.1–10 Hz frequency range. There is a remarkable similarity in the spectral and timing properties of the source during the dip and non-dip regions in this set of observations. These properties of the source are distinctly different from those seen in the observations of other classes. This indicates that the basic accretion disk structure during both dip and non-dip regions of classω is similar, but differ only in intensity. To explain these observations, we invoke a model in which the viscosity is very close to critical viscosity and the shock wave is weak or absent.

Structures of magnetized thin accretion disks

Structures of magnetized thin accretion disks

The Optical Light Curves of XTE J2123−058. III. The Mass of the Binary Components and the Structure of the Quiescent Accretion Disk

We present optical photometry of XTE J2123-058 taken in 1999 and 2000, during its quiescent state. The dominant feature of our R-band light curve is the ellipsoidal modulation of the secondary star; however, in order to fit this satisfactorily, we require additional components that comprise an X-ray-heated Roche lobe-filling secondary star and an accretion disk bulge, i.e., where the gas stream impacts the accretion disk. The observed dip near phase 0.8 is interpreted as the eclipse of inner parts of the accretion disk by the bulge. This scenario is highly plausible given the high binary inclination. Our fits allow us to constrain the size of the quiescent accretion disk to lie in the range (0.26-0.58)RL1 (68% confidence). Using the distance of 9.6 kpc and the X-ray flux inferred from the heated hemisphere of the companion, we obtain an unabsorbed X-ray luminosity of 1.2 × 1033 ergs s-1 for XTE J2123-058 in quiescence. From the observed quiescent optical/IR colors, we find that the power-law index (-1.4) for the spectral distribution of the accretion disk compares well with other quiescent X-ray transients. We also reanalyze the optical light curves of the soft X-ray transient XTE J2123-058 taken during its outburst and decay in 1998. We use a robust method to fit the data using a refined X-ray binary model. The model computes the light arising from a Roche lobe-filling star and flared accretion disk irradiated by X-rays and calculates the effects of shadowing and mutual star/disk eclipses. We obtain relatively accurate values for the binary inclination and mass ratio, which when combined with spectroscopic results obtained in Paper II, gives a neutron star mass in the range 1.04-1.56 M☉ (68% confidence).

Structures of magnetized thin accretion disks

We investigate the magnetohydrodynamic (MHD) process in thin accretion disks. The relevant momentum as well as magnetic reduction equations in the thin disk approximation are included. On the basis of these equations, we examine numerically the stationary structures, including distributions of the surface mass density, temperature and flow velocities of a disk around a young stellar object (YSO). The numerical results are as follows: (i) There should be an upper limit to the magnitude of magnetic field, such an upper limit corresponds to the equipartition field. For relevant magnitude of magnetic field of the disk's interior the disk remains approximately Keplerian. (ii) The distribution of effective temperature T(r) is a smoothly decreasing function of radius with power index corresponding to the observed radiation flux density, provided that the magnetic field is suitably chosen.

The magnetic geometry of magnetic-dominated thin accretion disks

The presence of an imposed external magnetic field drastically influences the structure of thin accretion disks. The magnetic field energy is assumed to be in balance with the thermal energy of the accretion flow, and a diusion approximation simulates the (vertical) energy transport. Our main result is that inside the corotation radius the resulting radial inclination i of the magnetic field lines from the rotation axis easily exceeds the critical value 30 (required to launch cold jets) even for mag- netic Prandtl numbers of order unity. The self-consistent consideration of both magnetic field and accretion flow demonstrates only a weak dependence of the inclination angle on the magnetic Prandtl number. The surface values of the toroidal magnetic fields necessary to induce considerably high values for the radial inclination are much smaller than expected. As the innermost part of the disk produces the largest B tor , the largest radial inclination can also be expected there. The idea is therefore supported that the cold jets are basically launched in the central disk area.

IP Pegasi: Investigation of the accretion disk structure

IP Pegasi: Investigation of the accretion disk structure

On the effects of the stellar magnetic field on the structure of T Tauri accretion discs

The structure of accretion discs around magnetic T Tauri stars is numerically calculated using a particle hydrodynamical code, in which magnetic interaction is included in the framework of King's diamagnetic blob accretion model. Setting up the calculation so as to simulate the density structure of a quasi-steady disc in the equatorial plane of a T Tauri star, we find that the central star's magnetic field typically produces a central hole in the disc and spreads out the surface density distribution. We argue that this result suggests a promising mechanism for explaining the unusual flatness (IR excess) of T Tauri accretion disc spectra.

Radiation Hydrodynamics of 2-D Accretion Disks

To examine the structure and dynamics of thick accretion disks, we use a two-dimensional viscous hydrodynamic code coupled with radiation transport. The -model and the full viscous stress-tensor description for the kinematic viscosity are used. The radiation transport is treated in the gray, flux-limited diffusion approximation. The finite difference methods used are based on an explicit-implicit method. We apply the numerical code to the Super-Eddington black-hole model for SS 433.@The result for a very small viscosity parameter a reproduces well the characteristic features of SS 433, such as the relativistic jets with 0.26c, the small collimation degree of the jets, the mass-outflow rate of , and the formation of the X-ray iron emission lines.

Multiwavelength Monitoring of the Narrow‐Line Seyfert 1 Galaxy Arakelian 564. II. Ultraviolet Continuum and Emission‐Line Variability

We present results of an intensive 2 month campaign of approximately daily spectrophotometric monitoring of the narrow-line Seyfert 1 galaxy (NLS1) Ark 564 with the Hubble Space Telescope. The fractional variability amplitude of the continuum variations between 1365 and 3000 is D6%, about a Ae factor of 3 less than that found in typical Seyfert 1 galaxies over a similar period of time. However, large-amplitude, short timescale —aring behavior is evident, with trough-to-peak —ux changes of about 18% in approximately 3 days. We present evidence for wavelength-dependent continuum time delays, with the variations at 3000 lagging behind those at 1365 by about 1 day. These delays may be Ae Ae interpreted as evidence for a strati—ed continuum reprocessing region, possibly an accretion disk structure. The Ly aj 1216 emission line exhibits —ux variations of about 1% amplitude. These variations lag those at 1365 by days, and combining this with the line width yields a virial black hole mass limit Ae [3 of . We caution that the low-amplitude Ly aj 1216 variations may indicate the bulk of [ 8 ] 106 M _ the emission region is at larger radii. This scenario aUects the veracity of our black hole mass upper limit in an uncertain manner owing to the unknown nature of the gas velocity —eld. Our mass estimate is thus unreliable; however, it is consistent with the independent estimate M D 1 ] 107 of Pounds et al. M _ based on a —uctuation power spectrum analysis of X-ray variability in Ark 564. The black hole mass and 5100 luminosity of Ark 564 are consistent with the hypothesis that, relative to Seyfert 1 galaxies, Ae NLS1s have lower black hole masses and higher accretion rates. Other strong emission lines (e.g., C IV j1549 and He II j1640) are constrained to vary with amplitudes of less than 5%. This low level of emission-line variability is diUerent from most Seyfert 1 galaxies, which characteristically display variations of D10% on similar timescales.

Mirror eclipses in the cataclysmic variable IP Peg

We present time resolved K-band infrared spectra of the dwarf nova (DN) IP Peg in early quiescence. The Brackett-gamma and HeI lines in our data show hitherto unseen behaviour, which we term a mirror eclipse, and interpret as an eclipse of the secondary star by an optically thin accretion disc. Mirror eclipses are a direct probe of the structure and physical conditions of accretion discs. For example, on assuming the relevant level populations to be in LTE, we constrain the temperature and density of the optically thin material causing the mirror eclipse to be 10,000 < T < 20,000 K and \rho ~10^-11 g/cm^3 respectively. In order to match our data we find that at least the outermost 20% of the disc (in radius) must be entirely optically thin. Implications for time-dependant disc models are examined.

Non‐LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei. IV. Effects of Compton Scattering and Metal Opacities

We extend our models of the vertical structure and emergent radiation field of accretion disks around supermassive black holes described in previous papers of this series. Our models now include both a self-consistent treatment of Compton scattering and the effects of continuum opacities of the most important metal species (C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni). With these new effects incorporated, we compute the predicted spectrum from black holes accreting at nearly the Eddington luminosity (L/LEdd ≈ 0.3) and central masses of 106, 107, and 108 M☉. We also consider two values of the Shakura-Sunyaev α parameter, 0.1 and 0.01, but in contrast to our previous papers, we consider a kinematic viscosity that is independent of depth. Although it has little effect when M > 108 M☉, Comptonization grows in importance as the central mass decreases and the central temperature rises. It generally produces an increase in temperature with height in the uppermost layers of hot atmospheres. Compared to models with coherent electron scattering, Comptonized models have enhanced extreme ultraviolet/soft X-ray emission, but they also have a more sharply declining spectrum at very high frequencies. Comptonization also smears the hydrogen and the He II Lyman edges. The effects of metals on the overall spectral energy distribution are smaller than the effects of Comptonization for these parameters. Compared to pure hydrogen-helium models, models with metal-continuum opacities have reduced flux in the high-frequency tail, except at the highest frequencies, where the flux is very low. Metal photoionization edges are not present in the overall disk-integrated model spectra. The viscosity parameter α has a more dramatic effect on the emergent spectrum than do metal-continuum opacities. As α increases (and therefore the disk column density decreases), the flux at both the high- and low-frequency extremes of the spectrum increases, while the flux near the peak decreases. Multitemperature blackbodies are a very poor approximation to accretion disk spectra in the soft X-ray region, and such crude modeling may greatly overestimate the accretion luminosity required to explain observed soft X-ray excesses in active galactic nuclei. In addition to our new grid of models, we also present a simple analytic prescription for the vertical temperature structure of the disk in the presence of Comptonization, and show under what conditions a hot outer layer (a corona) is formed.

Evidence for accretion disc reprocessing in QSO 0957+561

We use archival g- and r-band photometry of the gravitational lens system QSO 0957+561A, B to estimate the intrinsic variability of the quasar during 1996 February–June. The light curves span 234 d with temporal resolutions of about 2.5 d. Both light curves display a single large-amplitude event, of ∼0.1 mag (max-to-min) in about 100 d, followed by small-amplitude variations of ∼0.02 mag on time-scales of tens of days. We find the r-band variations lag those at g by 3.4−1.4+1.5d for the large-amplitude event. This lag is greater than zero at no less than 98 per cent confidence. The delayed coupling of the rest-frame UV intrinsic variations strongly suggests the existence of a stratified reprocessing region extending ∼light-days from the putative central black hole source. The observed lag is consistent with that expected from a reverberation within an irradiated accretion disc structure. However, any definitive statement requires further detailed theoretical modelling and high-quality, signal-to-noise ratio of about 100, optical/IR simultaneous monitoring with about 3-d resolution for approximately 6 months.

Timing in the Time Domain: Cygnus X-1

Quantities characterizing temporal property, e.g. power density, coherence, and time lag, can be defined and calculated directly in the time domain without using the Fourier transformation. Spectral hardness, variability duration, and correlation between different characteristic quantities on different time scale can be studied in the time domain as well. The temporal analysis technique in the time domain is a powerful tool, particularly in studying rapid variability on short time scales (or in high frequencies). Results of studying variabilities of X-rays from Cyg X-1 with the analysis technique in the time domain and RXTE data reveal valuable clues to understanding production and propagation processes of X-rays and structure of accretion disk in the black hole system.

The structure of matter flows in semi-detached binaries after the termination of mass transfer

We present the results of three-dimensional gas-dynamical simulations of matter flows in semi-detached binaries after termination of the mass transfer between the components of the system. The structure of the residual accretion disk is studied. When the mass transfer has ended, the quasi-elliptical disk becomes circular and its structure changes: tidal interactions result in the formation of a second arm in the spiral shock wave. In addition, a condensation (blob) moving through the disk with variable velocity is formed. The blob is maintained by interactions with the arms of the spiral shock and exists essentially over the entire lifetime of the disk. We also show that, for a viscosity corresponding to α∼0.01 (typical for observed accretion disks), the lifetime of the residual disk is about 50 orbital periods.

The structure of protosolar accretion disk

The structure of protosolar accretion disk during the stage of viscous evolution is calculated. The standard, steady-state model of accretion disk is adopted to describe the turbulent viscosity in the disk. Radial energy transport is neglected, and the vertical structure is treated as an one-dimensional problem. It is assumed that the disk performs Keplerian differential rotation and is in hydrostatic and thermal equilibrium. The disk is heated by viscous dissipation, and energy is transported outward by convection and radiation. The results show that the temperature-sensitive opacity is an important factor for the structure of the disk. The protosolar accretion disk is a cold and thin disk. The boundary of instability of heat convection in the disk moves from outer to inner regions and from upper to lower layers. Formation of planets should begin in the region where convection terminates.

Launching of Jets and The Vertical Structure of Accretion Disk

The launching of magnetohydrodynamic outflows from accretion disks is considered. We formulate a model for the local vertical structure of a thin disk threaded by a poloidal magnetic field of dipolar symmetry. The model consists of an optically thick disk matched to an isothermal atmosphere. The disk is supposed to be turbulent and possesses an effective viscosity and an effective magnetic diffusivity. In the atmosphere, if the magnetic field lines are inclined sufficiently to the vertical, a magnetocentrifugal outflow is driven and passes through a slow magnetosonic point close to the surface. We determine how the rate of mass loss varies with the strength and inclination of the magnetic field. In particular, we find that for disks in which the mean poloidal field is sufficiently strong to stabilize the disk against the magnetorotational instability, the mass-loss rate decreases extremely rapidly with increasing field strength and is maximal at an inclination angle of 40°-50°. For turbulent disks with weaker mean fields, the mass-loss rate increases monotonically with increasing strength and inclination of the field, but the solution branch terminates before achieving excessive mass-loss rates. Our results suggest that efficient jet launching occurs for a limited range of field strengths and a limited range of inclination angles in excess of 30°. In addition, we determine the direction and rate of radial migration of the poloidal magnetic flux and discuss whether configurations suitable for jet launching can be maintained against dissipation.

Spectra of Maser Radiation from a Turbulent, Circumnuclear Accretion Disk. III. Circular Polarization

Calculations are performed for the circular polarization of maser radiation from a turbulent, Keplerian disk intended to represent the subparsec disk at the nucleus of the galaxy NGC 4258. The polarization in the calculations is a result of the Zeeman effect when the Zeeman splitting is much less than the spectral line breadth. Plausible configurations for turbulent magnetic and velocity fields in the disk are created by statistical methods. This turbulence, the Keplerian velocity gradients, and the blending of the three hyperfine components to form the 616-523 masing transition of water are key ingredients in determining the appearance of the calculated polarized spectra. These spectra are quite different from the polarized spectra that would be expected for a two-level transition where there is no hyperfine structure. The effect of the hyperfine structure on the polarization is most striking in the calculations for the maser emission that represents the central (or systemic) features of NGC 4258. Information about magnetic fields is inferred from observations for polarized maser radiation and bears on the structure of accretion disks.

Thermal stability of self-gravitating, optically thin accretion disks

In the dynamics of accretion disks, the presence of collective eects associated with the self-gravity of the disk is expected to aect not only the momentum transport, but also the relevant energy balance equations, which could dier substantially from the non-self-gravitating case. Here we follow the model that, when the disk is suciently cold, the stirring due to Jeans-related instabilities acts as a source of eective heating. The corresponding reformulation of the energy equations allows us to: (i) demonstrate how self-regulation can be established, so that the stability parameter Q is maintained close to a threshold value, with weak dependence on radius; (ii) rediscuss the opacity properties in the self-gravitating regime. In particular, we show that, if cooling is dominated by bremsstrahlung, an optically thin stationary accretion solution is thermally stable, even in the non-advective case, provided the disk is self-gravitating. The details of the cooling function have little eect on the structure of such accretion disk, which is in any case induced by self-gravity to self-regulate. This condition of self-gravitating accretion is expected to be appropriate for the outer regions of many disks of astrophysical interest. With the reformulation of the energy equations described in this paper, we have also secured: (iii )a starting point for the study of the emission properties of self-gravitating accretion disks; (iv) a tool to analyze the structure of the transition region, where the disk becomes self-gravitating.

On the Structure of Advective Accretion Disks at High Luminosity

Global solutions of optically thick advective accretion disks around black holes are constructed. The solutions are obtained by solving numerically a set of ordinary differential equations corresponding to a steady axisymmetric geometrically thin disk. We pay special attention to consistently satisfy the regularity conditions at singular points of the equations. For this reason we analytically expand a solution at the singular point, and use coefficients of the expansion in our iterative numerical procedure. We obtain consistent transonic solutions in a wide range of values of the viscosity parameter alpha and mass acretion rate. We compare two different form of viscosity: one takes the shear stress to be proportional to the pressure, while the other uses the angular velocity gradient-dependent stress. We find that there are two singular points in solutions corresponding to the pressure-proportional shear stress. The inner singular point locates close to the last stable orbit around black hole. This point changes its type from a saddle to node depending on values of alpha and accretion rate. The outer singular point locates at larger radius and is always of a saddle-type. We argue that, contrary to the previous investigations, a nodal-type inner singular point does not introduce multiple solutions. Only one integral curve, which corresponds to the unique global solution, passes simultaneously the inner and outer singular points independently of the type of inner singular point. Solutions with the angular velocity gradient-dependent shear stress have one singular point which is always of a saddle-type and corresponds to the unique global solution. The structure of accretion disks corresponding to both viscosities are similar.

Relation between blue/ultraviolet continuum shape and the ratio of radio-to-optical emission for B3-VLA quasars

The low-frequency radio luminosity is believed to be an indicator of jet power, while the optical/ultraviolet (UV) emission is probably from accretion discs in the nuclei of steep-spectrum radio quasars. We present a correlation between the ratio of radio-to-optical luminosities and the continuum spectral index in blue/UV bands, which might indicate that the continuum shape in blue/UV bands is related to the ratio of jet to accretion power. The results may imply that the spectra and structure of accretion discs are probably affected by the interactions between jets and discs.