Masaaki Kusunose

Winds from hot accretion disks

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. 10 refs.

Relativistic thermal plasmas - time development of electron-positron pair concentration. [In active galactic nuclei]

The time evolution of a relativistic pair-equilibrium plasma confined in a uniformly or impulsively heated sphere is investigated analytically, starting from initial conditions appropriate for a plasma in an AGN. The formulations for pair processes, radiative transfer, photon production, Compton scattering, and the energy equations are explained, and numerical results are presented in extensive graphs and characterized in detail. It is found that the pair equilibrium is stable under constant heating, but that extreme changes in electron temperature, pair density, and luminosity occur for larger amounts of impulsive heating. 25 references.

Geometrically thin, hot accretion disks : topology of the thermal equilibrium curves

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.

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. 18 refs.

Pair-density transitions in accretion disk coronae

The thermal and e(+)e(-)-pair equilibrium structure of two-temperature disk coronae above a cool (about 10 exp 6 K) disk around a black hole of 10 solar masses are investigated. Soft photons are assumed to be amply supplied from the cool disk. Two-pair thermal equilibrium points are found for a given proton column density: the low state with very small pair density and the high state dominated by pairs. Both states are thermally unstable, while for perturbations in pair density the high state is unstable and the low state is stable. Two possible scenarios are discussed for the fate of a two-temperature corona. When the proton optical depth is relatively small (e.g., less than 1) and the temperature of input soft photons is low (e.g., less than 10 exp 6 K), the corona will undergo a limit cycle between the high state and the low state on a time scale of milliseconds. As a consequence of Compton scattering of the soft photons, the emergent spectrum in the high state is rather flat with a big Wien bump at about 100 keV, whereas it is composed of a power-law component in the low state. Some observational consequences are briefly discussed in connection with the high-low spectral transition in Cyg X-1.

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.

A Photo-hadronic Model of the Large-scale Jets of 3C 273 and PKS 1136–135

X-ray bright knots of kpc-scale jets of several radio loud quasars have been an actively discussed issue. Among various models to explain observations, synchrotron radiation from the electron population different from radio to IR emitting electrons is promising. However, the origin of this electron population has been debated. Recently, we proposed that this electron population is produced by proton-photon collisions (mainly, Bethe-Heitler process), and we applied this model to PKS 0637-752. We found that this model works if the proton power is by an order of magnitude larger than the Eddington power. In this paper we apply this model to the X-ray emission in the knots of 3C 273 and PKS 1136-135. The target photons for electron-positron pair production are supplied by synchrotron radiation at radio-IR by primary electrons and by the active galactic nucleus (AGN) core as well as cosmic microwave background (CMB) radiation. The effects of the AGN photons are included for the first time in the hadronic model. Though the observed X-ray flux is obtained with the contribution of the AGN photons, the required proton power turns out to be highly super-Eddington. However, we find that our model works for a nearly Eddington proton power, if the photon density of the AGN is enhanced. This can occur if the AGN photons are more beamed toward the X-ray knots than toward our line of sight and the AGN photon frequency is shifted by the Doppler effect.

A novel mechanism of the formation of electron-positron outflow from hot accretion disks

We propose a mechanism of the relativistic jet formation in active galactic nuclei and galactic black hole binaries. We consider the ejection of electron-positron pairs produced in the two-temperature accretion disks, by solving the pair momentum equation in the one-zone approximation, in which we assume that the electron-positron component can escape independently of the electron-proton one which forms a hydrostatic atmosphere. The results show that, in the inner region of the disks, when the mass accretion rate becomes larger than about a tenth of the Eddington rate, most of the viscously dissipated energy is converted into the thermal and kinetic energy of the ejected electron-positron pairs. The produced pairs are accelerated in the vertical direction by its own gas pressure rather than by the radiative force. Thus, this mechanism is successful in extracting accretion power to form powerful electron-positron jets as suggested by recent observations.