Two-temperature coronal flow above a thin disk

We extended the disk corona model (Meyer & Meyer-Hofmeister 1994; Meyer, Liu, & Meyer-Hofmeister 2000a) to the inner region of galactic nuclei by including different temperatures in ions and electrons as well as Compton cooling. We found that the mass evaporation rate and hence the fraction of accretion energy released in the corona depend strongly on the rate of incoming mass flow from outer edge of the disk, a larger rate leading to more Compton cooling, less efficient evaporation and a weaker corona. We also found a strong dependence on the viscosity, higher viscosity leading to an enhanced mass flow in the corona and therefore more evaporation of gas from the disk below. If we take accretion rates in units of the Eddington rate our results become independent on the mass of the central black hole. The model predicts weaker contributions to the hard X-rays for objects with higher accretion rate like narrow-line Seyfert 1 galaxies (NLS1s), in agreement with observations. For luminous active galactic nuclei (AGN) strong Compton cooling in the innermost corona is so efficient that a large amount of additional heating is required to maintain the corona above the thin disk.Comment: 17 pages, 6 figures. ApJ accepte

Properties of Intercalated 2H-NbSe2, 4Hb-TaS2 and 1T-TaS2

The layered compounds 2H-NbSe, 24Hb-TaS, 2and 1T-TaS2 have been intercalated with organic molecules; and the resulting crystal structure, heat capacity, conductivity, and superconductivity have been studied. The coordination in the disulfide layers was found to be unchanged in the product phase. Resistance minima appear and the superconducting transition temperature is reduced in the NbSe2 complex. Conversely, superconductivity is induced in the 4Hb-TaS2 complex. Corresponding evidence of a large change of the density of states, negative for 2H-NbSe2 and positive for 4Hb-TaS2, was also observed upon intercalation. The transport properties of all the intercalation complexes show a pronounced dependence upon the coordination of the transition metal

Evolution of Supermassive Black Hole Binary and Acceleration of Jet Precession in Galactic Nuclei

Supermassive black hole binary (SMBHB) is expected with the hierarchical galaxy formation model. Currently, physics processes dominating the evolution of a SMBHB are unclear. An interesting question is whether we could observationally determine the evolution of SMBHB and give constraints on the physical processes. Jet precession have been observed in many AGNs and generally attributed to disk precession. In this paper we calculate the time variation of jet precession and conclude that jet precession is accelerated in SMBHB systems but decelerated in others. The acceleration of jet precession $dP_{\rm pr} / dt$ is related to jet precession timescale $P_{\rm pr}$ and SMBHB evolution timescale $\tau_{\rm a}$, ${dP_{\rm pr} \over dt} \simeq - \Lambda {P_{\rm pr} \over \tau_{\rm a}}$. Our calculations based on the models for jet precession and SMBHB evolution show that $dP_{\rm pr} / dt$ can be as high as about $- 1.0$ with a typical value -0.2 and can be easily detected. We discussed the differential jet precession for NGC1275 observed in the literature. If the observed rapid acceleration of jet precession is true, the jet precession is due to the orbital motion of an unbound SMBHB with mass ratio $q\approx 0.76$. When jets precessed from the ancient bubbles to the currently active jets, the separation of SMBHB decrease from about $1.46 {\rm Kpc}$ to $0.80 {\rm Kpc}$ with an averaged decreasing velocity $da/dt \simeq - 1.54 \times 10^6 {\rm cm/s}$ and evolution timescale $\tau_{\rm a} \approx 7.5\times 10^7 {\rm yr}$. However, if we assume a steady jet precession for many cycles, the observations implies a hard SMBHB with mass ratio $q\approx 0.21$ and separation $a\approx 0.29 {\rm pc}$.Comment: 29 pages, no figure, Accepted for publication in Ap

Acceleration of weakly collisional solar-type winds

One of the basic properties of the solar wind, that is the high speed of the fast wind, is still not satisfactorily explained. This is mainly due to the theoretical difficulty of treating weakly collisional plasmas. The fluid approach implies that the medium is collision dominated and that the particle velocity distributions are close to Maxwellians. However the electron velocity distributions observed in the solar wind depart significantly from Maxwellians. Recent kinetic collisionless models (called exospheric) using velocity distributions with a suprathermal tail have been able to reproduce the high speeds of the fast solar wind. In this letter we present new developments of these models by generalizing them over a large range of corona conditions. We also present new results obtained by numerical simulations that include collisions. Both approaches calculate the heat flux self-consistently without any assumption on the energy transport. We show that both approaches - the exospheric and the collisional one - yield a similar variation of the wind speed with the basic parameters of the problem; both produce a fast wind speed if the coronal electron distribution has a suprathermal tail. This suggests that exospheric models contain the necessary ingredients for the powering of a transonic stellar wind, including the fast solar one.Comment: Accepted for publication in The Astrophysical Journal Letters (accepted: 13 May 2005

Heavy Meson Production in NN Collisions with Polarized Beam and Target -- A new facility for COSY

The study of near--threshold meson production in pp and pd collisions involving polarized beams and polarized targets offers the rare opportunity to gain insight into short--range features of the nucleon--nucleon interaction. The Cooler Synchrotron COSY at FZ--J\"ulich is a unique environment to perform such studies. Measurements of polarization observables require a cylindrically symmetrical detector, capable to measure the momenta and the directions of outgoing charged hadrons. The wide energy range of COSY leads to momenta of outgoing protons to be detected in a single meson production reaction between 300 and 2500 MeV/c. Scattering angles of protons to be covered extend to about $45^{\circ}$ in the laboratory system. An azimuthal angular coverage of the device around 98% seems technically achievable. The required magnetic spectrometer could consist of a superconducting toroid, providing fields around 3 T.Comment: 6 pages, 1 figure, submitted to Czechoslovak Journal of Physic

Emergence of foams from the breakdown of the phase field crystal model

The phase field crystal (PFC) model captures the elastic and topological properties of crystals with a single scalar field at small undercooling. At large undercooling, new foam-like behavior emerges. We characterize this foam phase of the PFC equation and propose a modified PFC equation that may be used for the simulation of foam dynamics. This minimal model reproduces von Neumann's rule for two-dimensional dry foams, and Lifshitz-Slyozov coarsening for wet foams. We also measure the coordination number distribution and find that its second moment is larger than previously-reported experimental and theoretical studies of soap froths, a finding that we attribute to the wetness of the foam increasing with time.Comment: 4 pages, 4 figure

On the turbulent α\alpha-disks and the intermittent activity in AGN

We consider effects of the MHD turbulence on the viscosity during the evolution of the thermal-viscous ionization instability in the standard $\alpha$-accretion disks. We consider the possibility that the accretion onto a supermassive black hole proceeds through an outer standard accretion disk and inner, radiatively inefficient and advection dominated flow. In this scenario we follow the time evolution of the accretion disk in which the viscosity parameter $\alpha$ is constant throughout the whole instability cycle, as implied by the strength of MHD turbulence. We conclude that the hydrogen ionization instability is a promising mechanism to explain the intermittent activity in AGN.Comment: 13 pages, 9 figures; ApJ accepte

A Local One-Zone Model of MHD Turbulence in Dwarf Nova Disks

The evolution of the magnetorotational instability (MRI) during the transition from outburst to quiescence in a dwarf nova disk is investigated using three-dimensional MHD simulations. The shearing box approximation is adopted for the analysis, so that the efficiency of angular momentum transport is studied in a small local patch of the disk: this is usually referred as to a one-zone model. To take account of the low ionization fraction of the disk, the induction equation includes both ohmic dissipation and the Hall effect. We induce a transition from outburst to quiescence by an instantaneous decrease of the temperature. The evolution of the MRI during the transition is found to be very sensitive to the temperature of the quiescent disk. As long as the temperature is higher than a critical value of about 2000 K, MHD turbulence and angular momentum transport is sustained by the MRI. However, MHD turbulence dies away within an orbital time if the temperature falls below this critical value. In this case, the stress drops off by more than 2 orders of magnitude, and is dominated by the Reynolds stress associated with the remnant motions from the outburst. The critical temperature depends slightly on the distance from the central star and the local density of the disk.Comment: 20 pages, 2 tables, 6 figures, accepted for publication in Ap