Slim Accretion Disks Research Articles

The Observational Appearance of Slim Accretion Disks

We reexamine the hypothesis that the optical/UV/soft X-ray continuum of Active Galactic Nuclei is thermal emission from an accretion disk. Previous studies have shown that fitting the spectra with the standard, optically thick and geometrically thin accretion disk models often led to luminosities which contradict the basic assumptions adopted in the standard model. There is no known reason why the accretion rates in AGN should not be larger than the thin disk limit. In fact, more general, slim accretion disk models are self-consistent even for moderately super-Eddington luminosities. We calculate here spectra from a set of thin and slim, optically thick accretion disks. We discuss the differences between the thin and slim disk models, stressing the implications of these differences for the interpretation of the observed properties of AGN. We found that the spectra can be fitted not only by models with a high mass and a low accretion rate (as in the case of thin disk fitting) but also by models with a low mass and a high accretion rate. In the first case fitting the observed spectra in various redshift categories gives black hole masses around 10^9 solar masses for a wide range of redshifts, and for accretion rates ranging from 0.4 to 8 solar masses/year. In the second case the accretion rate is around 10^2 solar masses/year for all AGN and the mass ranges from 3*10^6 to 10^8 solar masses. Unlike the disks with a low accretion rate, the spectra of the high-accretion-rate disks extend into the soft X-rays. A comparison with observations shows that such disks could produce the soft X-ray excesses claimed in some AGNs. We show also that the sequence of our models with fixed mass and different accretion rates can explain the time evolution of the observed spectra in Fairall 9.

The stability of slim accretion discs - the radial dependence

The stability of slim accretion discs - the radial dependence

Variability and Slim Accretion Disks

Many compact galactic and extra-galactic sources have a luminosity in the slim accretion disk regime, i.e. L/LE ∼ 1, where LE is the Eddington luminosity. The correspondingly large accretion rates diminish the relevance of variability interpretations based on the thin disk model. This paper explores the possible connection between variability in AGN and local instabilities in slim disks, and points out some relevant areas of future research.

Numerical Simulations of Pulsationally Unstable Accretion Disks around Supermassive Black Holes

AbstractThe innermost region of slim accretion disks with standard α viscosity is unstable against axisymmetric radial inertial acoustic perturbations under certain conditions. Numerical simulations are performed in order to demonstrate behaviors of such unstable disks. It is shown that oscillations with the period of ~ 10−3(MBH/M⊙) s can be excited near the inner edge of the disks, where MBH is the mass of the central object. This kind of unstable disks is a possible origin of the periodic X-ray time variabilities with period of ~ 104s observed in a Seyfert galaxy NGC 6814.

Stability of slim accretion discs – Effects of central mass and viscosity

Slim accretion discs have a total luminosity of the order |$L/L_{E}=\dot{m}\sim 1$|⁠, where LE is the Eddington luminosity and |$\dot{m}=\dot{M}/\dot{M}_{c}$|⁠, where Me is a critical accretion rate, related to the Eddington one. The local stability properties of such discs are examined, in the three-dimensional parameter space spanned by the (α, μ, m) axes, where α and μ are two viscosity parameters, and m = M/M⊙ the central mass. We suggest that various types of observed quasi-periodic behaviour may be connected with slim disc instabilities. If this turns out to be correct, the so-called normal and horizontal branch oscillations could be due to unstable thermal and acoustic modes, respectively. It is subsequently shown that some of the observed short-term (quasi-periodic) variability in active galactic nuclei may also originate from short-wavelength acoustic modes in the innermost region of the disc. Consequently, observational characteristics, in connection with stability theory, may yield estimates of basic accretion parameters. In the case of the Seyfert galaxy NGC6814, this process seems to favour |$(\alpha ,\mu ,m,\dot{m})\simeq (0.5,0,{10}^{6},{10}^{-2})$|⁠. We finally conclude that this line of work may provide additional evidence for both accretion discs and black holes, in various compact sources.

Slim accretion disks

view Abstract Citations (1364) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Slim Accretion Disks Abramowicz, M. A. ; Czerny, B. ; Lasota, J. P. ; Szuszkiewicz, E. Abstract A new branch of equilibrium solutions for stationary accretion disks around black holes is found. These solutions correspond to moderately super-Eddington accretion rates. The existence of the new branch is a consequence of an additional cooling due to general relativistic Roche lobe overflow and horizontal advection of heat. On an accretion rate versus surface density plane the new branch forms, together with the two 'standard' branches (corresponding to the Shakura-Sunyaev accretion disk models) a characteristically S-shaped curve. This could imply a limit cycle-type behavior for black hole accretion flows with accretion rates close ot the Eddington one. Publication: The Astrophysical Journal Pub Date: September 1988 DOI: 10.1086/166683 Bibcode: 1988ApJ...332..646A Keywords: Accretion Disks; Astronomical Models; Black Holes (Astronomy); Stellar Mass Accretion; Computational Astrophysics; Eddington Approximation; Navier-Stokes Equation; Roche Limit; Astrophysics; BLACK HOLES; STARS: ACCRETION full text sources ADS | data products SIMBAD (1)

Slim accretion disks around black holes

We have constructed black hole accretion disk models at moderately high accretion rates, taking correctly into account dissipation and heat transport in the transonic part of the flow. Our models differ from those of Shakura and Sunyaev in many important respects. The surface density at a given radius relates to the accretion rate through an S-shaped function.