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Xingming Chen
Physics , 1995,
Abstract: 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-temperature regime and effects such as electron-positron pair production and annihilation may need to be considered to study the microphysics of the hot plasma. However, the global disk structure will not be much affected by the local radiative process.
Highly Viscous Accretion Disks with Advection  [PDF]
I. V. Artemova,G. S. Bisnovatyi-Kogan,G. Bjoernsson,I. D. Novikov
Physics , 1997,
Abstract: We consider the effects of advection and radial gradients of pressure and radial drift velocity on the structure of optically thick accretion disks. We concentrate our efforts on highly viscous disks, $\alpha=1.0$, with large accretion rates. Contrary to disk models neglecting advection, we find that continuous solutions extending from the outer disk regions to the inner edge exist for all accretion rates we have considered. We show that the sonic point moves outward with increasing accretion rate, and that in the innermost disk region advection acts as a heating process that may even dominate over dissipative heating. Despite the importance of advection on it's structure, the disk remains geometrically thin.
The Local Stability of Accretion Disks with Advection  [PDF]
Xue-Bing Wu,Qi-Bin Li
Physics , 1995, DOI: 10.1086/177825
Abstract: 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 has significant effects on two acoustic modes. Independent on the optical depth, the instability of the outward propagating mode (O-mode) is enhanced and that of the inward propagating mode (I-mode) is damped if the disk is gas pressure dominated, while the instability of O-mode is damped and that of I-mode is enhanced if the disk is radiation pressure dominated. For a geometrically slim, advection-dominated disk, both the thermal and viscous modes, as well as I-mode, are always stable if the disk is optically thin. The including of thermal diffusion tends to make these modes more stable. However, the O-mode can become unstable when $q/m$ is very large ($q$ is the ratio of advective to viscous dissipated energy and $m$ the Mach number), even if the thermal diffusion is considered. On the other hand, if the advection-dominated disk is optically thick, we found there is no self-consistent acoustic modes in our local analyses. The thermal diffusion has no effect on the stable viscous mode but has a significant contribution to enhance the thermal instability.
Advection-Dominated Accretion Disks: Geometrically Slim or Thick?  [PDF]
Wei-Min Gu,Li Xue,Tong Liu,Ju-Fu Lu
Physics , 2009, DOI: 10.1093/pasj/61.6.1313
Abstract: We revisit the vertical structure of black hole accretion disks in spherical coordinates. By comparing the advective cooling with the viscous heating, we show that advection-dominated disks are geometrically thick, i.e., with the half-opening angle > 2\pi/5, rather than slim as supposed previously in the literature.
Coronae above accretion disks around black holes: The effect of Compton cooling  [PDF]
E. Meyer-Hofmeister,B. F. Liu,F. Meyer
Physics , 2012, DOI: 10.1051/0004-6361/201219245
Abstract: The geometry of the accretion flow around stellar mass and supermassive black holes depends on the accretion rate. Broad iron emission lines originating from the irradiation of cool matter can indicate that there is an inner disk below a hot coronal flow.These emission lines have been detected in X-ray binaries. Observations with the Chandra X-ray Observatory, XMM Newton and Suzaku have confirmed the presence of these emission lines also in a large fraction of Seyfert-1 active galactic nuclei (AGN). We investigate the accretion flow geometry for which broad iron emission lines can arise in hard and soft spectral state. We study an ADAF-type coronal flow, where the ions are viscously heated and electrons receive their heat only by collisions from the ions and are Compton cooled by photons from an underlying cool disk. For a strong mass flow in the disk and the resulting strong Compton cooling only a very weak coronal flow is possible. This limitation allows the formation of ADAF-type coronae above weak inner disks in the hard state, but almost rules them out in the soft state. The observed hard X-ray luminosity in the soft state, of up to 10% or more of the total flux, indicates that there is a heating process that directly accelerates the electrons. This might point to the action of magnetic flares of disk magnetic fields reaching into the corona. Such flares have also been proposed by observations of the spectra of X-ray black hole binaries without a thermal cut-off around 200 keV.
Cold Accretion Disks and Lineless Quasars  [PDF]
Ari Laor,Shane W. Davis
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.19310.x
Abstract: The optical-UV continuum of quasars is broadly consistent with the emission from a geometrically thin optically thick accretion disk (AD). The AD produces the ionizing continuum which powers the broad and narrow emission lines. The maximum AD effective temperature is given by Teff=fmax(Mdot/M^2)^1/4, where M is the black hole mass, Mdot the accretion rate, and fmax is set by the black hole spin a_*. For a low enough value of Mdot/M^2 the AD may become too cold to produce ionizing photons. Such an object will form a lineless quasar. This occurs for a local blackbody (BB) AD with a luminosity Lopt=10^46 erg/s for M>3.6E9 Msun, when a_*=0, and for M>1.4E10 Msun, when a_*=0.998. Using the AD based Mdot, derived from M and Lopt, and the reverberation based M, derived from Lopt and the Hbeta FWHM, v, gives Teff \propto Lopt^-0.13v^-1.45. Thus, Teff is mostly set by v. Quasars with a local BB AD become lineless for v> 8,000 km/s, when a_*=0, and for v> 16,000 km/s, when a_*=0.998. Higher values of v are required if the AD is hotter than a local BB. The AD becoming non-ionizing may explain why line emitting quasars with v>10,000 km/s are rare. Weak low ionization lines may still be present if the X-ray continuum is luminous enough, and such objects may form a population of weak emission line quasars (WLQ). If correct, such WLQ should show a steeply falling SED at lambda<1000A. Such an SED was observed by Hryniewicz et al. in SDSS J094533.99+100950.1, a WLQ observed down to 570A, which is well modeled by a rather cold AD SED. UV spectroscopy of z~1-2 quasars is required to eliminate potential intervening Lyman limit absorption by the intergalactic medium (IGM), and to explore if the SEDs of lineless quasars and some additional WLQ are also well fit by a cold AD SED.
The existence of warm and optically thick dissipative coronae above accretion disks  [PDF]
A. Rozanska,J. Malzac,R. Belmont,B. Czerny,P. -O. Petrucci
Physics , 2015, DOI: 10.1051/0004-6361/201526288
Abstract: In the past years, several observations of AGN and X-ray binaries have suggested the existence of a warm T around 0.5-1 keV and optically thick, \tau ~ 10-20, corona covering the inner parts of the accretion disk. These properties are directly derived from spectral fitting in UV to soft-X-rays using Comptonization models. However, whether such a medium can be both in radiative and hydrostatic equilibrium with an accretion disk is still uncertain. We investigate the properties of such warm, optically thick coronae and put constraints on their existence. We solve the radiative transfer equation for grey atmosphere analytically in a pure scattering medium, including local dissipation as an additional heating term in the warm corona. The temperature profile of the warm corona is calculated assuming it is cooled by Compton scattering, with the underlying dissipative disk providing photons to the corona. Our analytic calculations show that a dissipative thick, (\tau_{cor} ~ 10-12) corona on the top of a standard accretion disk can reach temperatures of the order of 0.5-1 keV in its upper layers provided that the disk is passive. But, in absence of strong magnetic fields, the requirement of a Compton cooled corona in hydrostatic equilibrium in the vertical direction sets an upper limit on the Thomson optical depth \tau_{cor} < 5 . We show this value cannot be exceeded independently of the accretion disk parameters. However, magnetic pressure can extend this result to larger optical depths. Namely, a dissipative corona might have an optical depth up to ~ 20 when the magnetic pressure is 100 times higher that the gas pressure. The observation of warm coronae with Thomson depth larger than ~ 5 puts tights constraints on the physics of the accretion disk/corona systems and requires either strong magnetic fields or vertical outflows to stabilize the system.
Coronae as Consequence of Large Scale Magnetic Fields in Turbulent Accretion Disks  [PDF]
Eric G. Blackman,Martin E. Pessah
Physics , 2009, DOI: 10.1088/0004-637X/704/2/L113
Abstract: Non-thermal X-ray emission in compact accretion engines can be interpreted to result from magnetic dissipation in an optically thin magnetized corona above an optically thick accretion disk. If coronal magnetic field originates in the disk and the disk is turbulent, then only magnetic structures large enough for their turbulent shredding time to exceed their buoyant rise time survive the journey to the corona. We use this concept and a physical model to constrain the minimum fraction of magnetic energy above the critical scale for buoyancy as a function of the observed coronal to bolometric emission. Our results suggest that a significant fraction of the magnetic energy in accretion disks resides in large scale fields, which in turn provides circumstantial evidence for significant non-local transport phenomena and the need for large scale magnetic field generation. For the example of Seyfert AGN, for which of order 30 per cent of the bolometric flux is in the X-ray band, we find that more than 20 per cent of the magnetic energy must be of large enough scale to rise and dissipate in the corona.
Models for X-ray Emission from Radio Quiet AGNs  [PDF]
Francesco Haardt
Physics , 1996,
Abstract: The current status of understanding of the X-ray emission from Seyfert galaxies involves thermal Comptonization of soft photons by mildrelativistic electrons and positrons. I review observational and theoretical arguments supporting such a view, discussing current models proposed for the structure of the innermost part of the accretion flow: extended coronae, small scale flaring blobs, advection dominated accretion disks and small scale cold cloudlets.
Advection-dominated Inflow/Outflows from Evaporating Accretion Disks  [PDF]
R. Turolla,C. P. Dullemond
Physics , 2000, DOI: 10.1086/312527
Abstract: 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 $r^2\dot\sigma\propto r^{-\xi}$, $\xi< 1/2$). The Bernoulli number of the inflowing gas is negative if the transition radius is $\lesssim 100$ Schwarzschild radii, so matter falling into the hole is gravitationally bound. The ratio of inflowing to outflowing mass is $\approx 1/2$, so in these solutions the accretion rate is of the same order as in standard ADAFs and much larger than in advection-dominated inflow/outflow models (ADIOS). The possible relevance of evaporation-fed solutions to accretion flows in black hole X-ray binaries is briefly discussed.
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