Hot Accretion Disk Research Articles

Hot accretion disks with pairs: Effects of magnetic field and thermal cyclocsynchrotron radiation

We show the effects of thermal cyclosynchrotron radiation and magnetic viscosity on the structure of hot, two-temperature accretion disks. Magnetic field, B, is assumed to be randomly oriented and the ratio of magnetic pressure to either gas pressure, alpha = P(sub mag)/P(sub gas), or the sum of the gas and radiation pressures, alpha = (P(sub mag)/P(sub gas) + P(sub rad)), is fixed. We find those effects do not change the qualitative properties of the disks, i.e., there are still two critical accretion rates related to production of e(sup +/-) pairs, (M dot)((sup U)(sub cr)) and (M dot)((sup L)(sub cr)), that affect the number of local and global disk solutions, as recently found by Bjoernsson and Svensson for the case with B = 0. However, a critical value of the alpha-viscosity parameter above which those critical accretion rates disappear becomes smaller than alpha(sub cr) = 1 found in the case of B = 0, for P(sub mag) = alpha(P(sub gas) + P(sub rad)). If P(sub mag) = alpha P(sub gas), on the other hand, alpha(sub cr) is still about unity. Moreover, when Comptonized cyclosynchrotron radiation dominates Comptonized bremsstrahlung, radiation from the disk obeys a power law with the energy spectral index of approximately 0.5, in a qualitative agreement with X-ray observations of active galactic nuclei (AGNS) and Galactic black hole candidates. We also extend the hot disk solutions for P(sub mag) = alpha(P(sub gas) + P(sub rad)) to the effectively optically thick region, where they merge with the standard cold disk solutions. We find that the mapping method by Bjoernsson and Svensson gives a good approximation to the disk structure in the hot region and show where it breaks in the transition region. Finally, we find a region in the disk parameter space with no solutions due to the inability of Coulomb heating to supply enough energy to electrons.

Hot Accretion Disks with Magnetic Field and Thermal Cyclo-Synchrotron Radiation

We study the structure of hot, two-temperature accretion disks around black holes, including the effects of thermal cyclo-synchrotron radiation and magnetic viscosity. This work is an extension of previous work by Björnsson & Svensson (1991a, b, 1992) and Kusunose & Mineshige (1992), which did not include those effects. Magnetic field, B, is assumed to be randomly oriented and determined by prescribing the ratio α = Pmag/Pgas or α = Pmag/(Pgas + Prad), where Pmag, Pgas, and Prad are the pressures of magnetic field, gas, and radiation, respectively. We find those effects do not change the qualitative properties of the disks, i.e., there are two critical accretion rates related to production of e± pairs, ṀcrU and ṀcrL that affect the number of local and global disk solutions, as recently found for the case with B = 0 (Björnsson & Svensson 1991a, b, 1992). However, a critical value of the α-viscosity parameter above which those critical accretion rates disappear becomes smaller than αcr = 1 found in the case of B = 0, for Pmag = α(Pgas + Prad). If Pmag = αPgas, on the other hand, αcr is still about unity. Moreover, when Comptonized cyclo-synchrotron radiation dominates Comptonized bremsstrahlung, radiation from the disk obeys a power law with the energy spectral index of ∼ 0.5, in a qualitative agreement with X-ray observations of AGNs and Galactic black hole candidates. The spectral index is weakly dependent on the mass accretion rate.

Relativistic Particle Transport in Hot Accretion Disks

AbstractAccretion disks around rapidly rotating black holes provide one of the few plausible models for the production of intense radiation in AGNs above energies of several hundred MeV. The rapid rotation of the hole increases the binding energy per nucleon in the last stable orbit relative to the Schwarzschild case, and naturally leads to ion temperatures in the range 1012 -1013 K for sub-Eddington accretion rates. The protons in the hot inner region of a steady, two-temperature disk form a reservoir of energy that is sufficient to power the observed EGRET outbursts if the black hole mass is 1010M⊙ . Moreover, the accretion timescale for the inner region is comparable to the observed transient timescale of ~1 week. Hence EGRET outbursts may be driven by instabilities in hot, two-temperature disks around supermassive black holes. In this paper we discuss turbulent (stochastic) acceleration in hot disks as a possible source of GeV particles and radiation. We constrain the model by assuming the turbulence is powered by a collective instability that drains energy from the hot protons. We also provide some ideas concerning new, high-energy Penrose processes that produce GeV emission be directly tapping the rotational energy of Kerr black holes.Subject headings: acceleration of particles — accretion, accretion disks — galaxies: nuclei — quasars: individual (3C 279) — radiation mechanisms: nonthermal

Hot pair-dominated accretion disks

view Abstract Citations (38) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hot Pair-dominated Accretion Disks Bjornsson, Gunnlaugur ; Svensson, Roland Abstract The structure and properties of geometrically thin steady hot two-temperature accretion disks, where Comptonized bremsstrahlung is the dominant emission mechanism, are analyzed using a new method based on the fact that the disk solutions depend on two parameters only. The method allows all possible local disk solutions to be displayed as a surface in a single figure. Local disk solutions are found for all accretion rates, viscosity parameters, and radii. As an example, disk solutions are presented for two particular values of the viscosity parameter, 0.1 and 2.0. The existence of global disk solutions is also discussed. Publication: The Astrophysical Journal Pub Date: August 1992 DOI: 10.1086/171601 Bibcode: 1992ApJ...394..500B Keywords: Accretion Disks; Electron-Positron Pairs; Pair Production; Astronomical Models; Bremsstrahlung; Computational Astrophysics; High Temperature; Astrophysics; ACCRETION; ACCRETION DISKS; BLACK HOLE PHYSICS; RADIATION MECHANISMS: BREMSSTRAHLUNG full text sources ADS | data products SIMBAD (1)

Geometrically thin, hot accretion disks - Topology of the thermal equilibrium curves

view Abstract Citations (17) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Geometrically Thin, Hot Accretion Disks: Topology of the Thermal Equilibrium Curves Kusunose, Masaaki ; Mineshige, Shin Abstract All the possible thermal equilibrium states of geometrically thin alpha-disks around stellar-mass black holes are presented. A (vertically) one-zone disk model is employed and it is assumed that a main energy source is viscous heating of protons and that cooling is due to bremsstrahlung and Compton scattering. There exist various branches of the thermal equilibrium solution, depending on whether disks are effectively optically thick or thin, radiation pressure-dominated or gas pressure-dominated, composed of one-temperature plasmas or of two-temperature plasmas, and with high concentration of e(+)e(-) pairs or without pairs. The thermal equilibrium curves at high temperatures (greater than or approximately equal to 10 exp 8 K) are substantially modified by the presence of e(+)e(-) pairs. The thermal stability of these branches are examined. Publication: The Astrophysical Journal Pub Date: June 1992 DOI: 10.1086/171466 Bibcode: 1992ApJ...392..653K Keywords: Accretion Disks; Black Holes (Astronomy); Bremsstrahlung; Compton Effect; Stellar Mass; Black Body Radiation; Radiation Pressure; Radiative Transfer; Thermodynamic Equilibrium; Astrophysics; ACCRETION; ACCRETION DISKS; BLACK HOLE PHYSICS; RADIATION MECHANISMS: THERMAL full text sources ADS | data products SIMBAD (1)

Vortices on accretion disks

EVERY rotating cosmic fluid that can be observed sufficiently closely displays either vortices or magnetic flux tubes on its surface; examples are tornadoes in the Earth's atmosphere1, the Great Red Spot and other vortices in Jupiter's atmosphere, and sunspots. We suggest here that hot accretion disks also produce coherent objects, and that these vortices and magnetic flux tubes will cause significant dissipation and other observable physical effects. They will facilitate the escape of collimated radiation from deep within hot disks, producing spectral changes and time variability in the radiation from the disk. In the case of active galactic nuclei, modification of X-ray spectra due to the presence of vortices on accretion disks permits us to explain several observational puzzles, including short-term variability and the low degree of linear polarization.

The visual wavelength continua of quasars - A reexamination

In recent years the preferred model of origin of the ultraviolet photionizing continuum of QSOs is that of the hot thermal accretion disk. The model spectra of these disks are very broad and extend through the visual spectral range. However, observations indicate a power-law description of the visual continuum with negative spectral index typically, and it is not clear that accretion disk spectra can satisfy this constraint as well as extend to the low-energy X-ray regime.

A recipe for making hot accretion disks

view Abstract Citations (21) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Recipe for Making Hot Accretion Disks Bjornsson, Gunnlaugur ; Svensson, Roland Abstract A powerful new method to determine the structure of effectively optically thin accretion disks is described. The method reduces the set of equations needed to be numerically solved to the microphysical equations only and reduces the dimension of the parameter space needed to be explored from three to two. It is shown why proton optical depth and compactness are natural parameters in studying hot plasma clouds (HPCs), and the structure equations of geometrically thin alpha disk are studied and the accretion disk parameters are related to the HPC parameters. As an example, the method is applied to an effectively optically thin bremsstrahlung disk. It is shown how a full disk solution is constructed from the generic solution profile. Publication: The Astrophysical Journal Pub Date: April 1991 DOI: 10.1086/186004 Bibcode: 1991ApJ...371L..69B Keywords: Accretion Disks; Black Holes (Astronomy); High Temperature Plasmas; Optical Thickness; Plasma Clouds; Bremsstrahlung; X Ray Sources; Astrophysics; ACCRETION; BLACK HOLES; X-RAYS: GENERAL; X-RAYS: SOURCES full text sources ADS |

Two-temperature accretion disks with winds in a fluid approximation

The polytropic relation between pressure and density of wind gases is used to obtain solutions for optically thin hot accretion disks with winds. It is assumed that the cooling mechanisms in the disks are bremsstrahlung and Compton scattering. This extends the previous work of Takahara et al. (1989) which did not analyze winds within a self-consistent disk model. It is shown that the disks have winds for accretion rates higher than about 10 percent of the Eddington rate, depending on the viscosity parameter of the disks and the polytropic exponent of wind gases. The poloidal velocity of winds at the foot-point is shown to be as high as 10 percent of light speed.

Time-dependent X-ray emission from unstable accretion disks around black holes

The spectral evolution of accretion disks in X-ray binaries containing black holes is studied, based on the disk instability model. The thermal transition of the outer portions of the disk controls the mass flow rate into the inner portions of the disk, thus modulating the soft X-ray flux which is thought to arise from the inner disk. Calculated soft X-ray spectra are consistent with the observations of the X-ray transient A0620 - 00 and especially ASM 2000 + 25, the soft X-ray spectra of which are well fitted by blackbody radiation with a fixed inner edge of the disk, Rin, and with monotonically decreasing temperature at Rin with time. Since the gas pressure is always dominant over the radiation pressure during the decay in these models, a two-temperature region is difficult to create. Instead, it is suggested that hard X-rays are generated in a hot (kT greater than 10 keV) accretion disk corona above the cool (kT less than 1 keV) disk.

Instabilities and time evolution of hot accretion disks with electron-positron pairs

view Abstract Citations (20) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Instabilities and Time Evolution of Hot Accretion Disks with Electron-Positron Pairs White, Timothy R. ; Lightman, Alan P. Abstract Since the 1970s, hot, optically thin, thermal accretion disks have been proposed in some models of galactic and extragalactic X-ray sources. Recent investigations of such disks, including electron-positron pairs, have found that in disks cooled by internally produced photons, there exists a critical accretion rate above which equilibrium is not possible in an annular region around r = 10GM/c^2^, where M is the mass of the central object. (The critical accretion rate corresponds to a luminosity of a few percent of the Eddington limit.) Below the critical rate, there are two self-consistent disk structures at each radius, one with a low density of pairs and one with a high density of pairs. We consider the stability and time-dependent behavior of such disks. At accretion rates below the critical value, the high pair density equilibrium is unstable to pair runaway. Such disks, when perturbed, produce pairs rampantly, then cool, and collapse toward cool, optically thick disks without pairs on a couple of hydrodynamical time scales (~ 10 hr at r = 10GM/c^2^ for M = 10^8^ M_sun_). This behavior should be accompanied by the reduction or elimination of all hard X-ray emission and an increase in UV emission. Optically thin disks subject to accretion rates above the critical rate would behave in a similar way. Because the inner regions of the optically thick disks are known to be secularly unstable, such a disk may fluctuate between optically thick and optically thin states, with an accompanying change in the spectrum. The absence of such fluctuations would suggest that the dominant X-ray and gamma-ray emission of these objects is not produced by a thermal plasma, unless the plasma is cooled by an external source of soft photons. When the spectral and luminosity fluctuations are present, their time scale provides an estimate of the mass of the central black hole. Publication: The Astrophysical Journal Pub Date: April 1990 DOI: 10.1086/168553 Bibcode: 1990ApJ...352..495W Keywords: Accretion Disks; Electron-Positron Pairs; Galactic Nuclei; Galactic Radiation; Optical Thickness; X Ray Sources; Compton Effect; Digital Simulation; Electron Energy; Elementary Particles; Astrophysics; ELEMENTARY PARTICLES; GALAXIES: NUCLEI; RADIATION MECHANISMS full text sources ADS |

Winds from hot accretion disks

view Abstract Citations (29) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Winds from Hot Accretion Disks Takahara, Fumio ; Rosner, Robert ; Kusunose, Masaaki Abstract Hydrodynamical winds from hot accretion disks around compact objects are investigated using a simplified model. The flow pattern above the disk is solved assuming that specific angular momentum is conserved along a stream line and that the radial component of the gravitational force balances the centrifugal force everywhere. An isothermal or polytropic relation is also assumed. It is shown that there exists a critical solution for which the flow is subsonic at the disk plane and becomes supersonic beyond the critical point. The mass-loss rate is calculated for the critical solution, and it turns out to be comparable to the accretion rate if the temperature is near the virial temperature. Thus wind loss will significantly affect the properties of hot two-temperature accretion disks. Publication: The Astrophysical Journal Pub Date: November 1989 DOI: 10.1086/167990 Bibcode: 1989ApJ...346..122T Keywords: Accretion Disks; Active Galactic Nuclei; Black Holes (Astronomy); Computational Astrophysics; Hydrodynamics; Astronomical Models; Gravitational Effects; High Temperature; Temperature Distribution; X Ray Sources; Astrophysics; ACCRETION; BLACK HOLES; GALAXIES: NUCLEI; HYDRODYNAMICS full text sources ADS | data products SIMBAD (1)

Hot accretion disks with electron-positron pairs

view Abstract Citations (75) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hot Accretion Disks with Electron-Positron Pairs White, Timothy R. ; Lightman, Alan P. Abstract The hot thermal accretion disks of the 1970s are studied and consideration is given to the effects of electron-positron pairs, which were originally neglected. It is found that disks cooled by internally produced photons have a critical accretion rate above which equilibrium is not possible in a radial annulus centered around r = 10 GM/c-squared, where M is the mass of the central object. This confirms and extends previous work by Kusunose and Takahara. Above the critical rate, pairs are created more rapidly than they can be destroyed. Below the critical rate, there are two solutions to the disk structure, one with a high pair density and one with a low pair density. Depending on the strength of the viscosity, the critical accretion rate corresponds to a critical luminosity of about 3-10 percent of the Eddington limit. Publication: The Astrophysical Journal Pub Date: May 1989 DOI: 10.1086/167455 Bibcode: 1989ApJ...340.1024W Keywords: Accretion Disks; Bremsstrahlung; Compton Effect; Electron-Positron Pairs; Nuclear Astrophysics; Luminosity; Pair Production; X Ray Sources; Astrophysics; ELEMENTARY PARTICLES; STARS: ACCRETION full text sources ADS |

The 1987 outburst of the recurrent nova U Sco

Spectral observations obtained soon after the 1987 brightening of U Sco support a thermonuclear runaway model for outbursts of this object. Spectra later in the decline are, however, more characteristic of a hot accretion disc. These observations are reconciled in a model where the low-mass high-velocity shell ejected from the surface of the white dwarf collides with the accretion disc causing it to brighten.

Photoionization models with accretion discs

AbstractThe diagnositic possibilities for identifying the ionizing source in symbiotic systems are explored. As possible sources we consider hot blackbodies and accretion discs. It turns out that main sequence accretors and hot blackbodies may have the same appearance in both, emission line and continuum flux distribution. However, UV continuum indices of models containing an accretion disc around a white dwarf are confined to a very small region, separated from main sequence accretors and blackbodies. Furthermore, if symbiotic systems containing a white dwarf accretor exist, they might be recognizable by strong emission in Fe X λ6374.

The 1987 Outburst of the Recurrent Nova U SCO

AbstractSpectral observations obtained soon after the 1987 brightening of U Sco support a thermonuclear runaway model for outbursts of this object. Spectra later in the decline are, however, more characteristic of a hot accretion disc. These observations are reconciled in a model where the low-mass high-velocity shell ejected from the surface of the white dwarf collides with the accretion disc causing it to brighten.

Sub-arc second 2 centimeter continuum and SiO spectral line observations of R Aquarii

VLA observations with a limiting spatial resolution of about 0.15 arcsec were attempted along with complementary observations of SiO emission using the three-element Hat Creek interferometer to determine an accurate position of the long-period variable (LPV) of the R Aquarii binary system. The observations indicate that the LPV wind is distorted by the photoionization of the hot campanion's accretion disk, that precession of the accretion disk may occur, and that the observed v = 1, J = 2-1 transition emanates from a circumbinary layer far removed from the position of the LPV itself.

Spectral variability of Cygnus X-2 - Structure in the circumsource material

Cygnus X-2 was observed during 13 separate pointings in the course of 1 yr by the Einstein monitor proportional counter. During one of these pointings the objective grating spectrometer was also used. It is found that the data from these observations are well fitted by single-component thermal bremsstrahlung models with temperatures ranging from 4-12 keV; however, two-component models, where one component is a blackbody, are not excluded. The X-ray light curve appears to be correlated with the 9.843 day optical period, possibly implying a partial eclipse of the X-ray source. During the high states there are irregular dips of up to 30 percent in intensity that last for 300-700 s. The high spectra resolution grating data were taken during one of the high states when these dips occurred, and it is possible to distinguish variable variable emission and absorption features due to Fe, O, and Ne. The picture of Cygnus X-2 is consistent with an intrinsic thermal source surrounded by a hot accretion disk corona. This model can be used to explain the high and low X-ray states, the variability within the states, and the intensity dips which occur in the high states.

Quasi-periodic oscillations from accretion disk coronas

Recent observations of galactic bulge X-ray sources have revealed in some cases the existence of a hot accretion disk corona1,2 and in others quasi-periodic oscillations (QPOs) in the X-ray flux3–5. We suggest here that these properties are directly related and that the oscillations are produced in the innermost parts of the corona. The observed variations in the centroid frequency of the QPOs with source intensity3–6 are explained and further variations with the photon spectrum predicted. Unlike some alternative models7,8, we do not require that the central object is a rapidly spinning weakly magnetized neutron star.

The high-energy spectrum of hot accretion disks

A hot, two-temperature accretion disk can be a strong ..gamma..-ray and relativistic particle source. This occurs when the accretion rate is high enough: M/M> or approx. =3 x 10/sup -9/..cap alpha.. yr/sup -1/ for a canonical Kerr black hole: due to the high ion temperature in the inner disk. We present detailed photon and particle spectra for specific disk models. The predicted ..gamma..-ray flux is as high as 10% of the bolometric luminosity in sub-Eddington models. Most of the ..gamma..-radiation is continuous, due to the ..pi../sup 0/ decay, emitted around 100 MeV but degraded to a few MeV in optically thick models. Spectral lines, due to position annihilation or to excited nuclei, provide only a small amount of the primary ..gamma..-ray luminosity. The energy flux in approx.35 MeV pairs is comparable to the ..gamma..-ray luminosity; Penrose effects provide a smaller number of approx.1 GeV pairs.