Convergence of Monte Carlo Simulations to Equilibrium

We give two direct, elementary proofs that a Monte Carlo simulation converges to equilibrium provided that appropriate conditions are satisfied. The first proof requires detailed balance while the second is quite general.Comment: 4 pages. v2: published versio

Formation and dynamics of self-sustained neutron haloes in disk accreting sources

It has been recognized long ago that the presence of hot plasma in the inner accretion disks around black holes could lead to the neutron production via dissociation of helium nuclei. We show that, for a broad range of accretion parameters, neutrons effectively decouple from protons and pile up in the inner disk leading to the formation of self-sustained halo. This means that new neutrons in the halo are supplied mainly by the splitting of helium nuclei in their collisions with existing neutrons. Once formed, such a halo can exist even if the proton temperature is much lower than the energy threshold of helium dissociation. We show that neutron haloes can be the natural source of relativistic electrons and positrons, providing characteristic comptonization spectra and hard spectral tails observed in many black hole candidates, and also giving rise to relativistic outflows. Deuterium gamma-ray line at 2.2 MeV resulting from neutron capture is also expected at a level detectable by future INTEGRAL mission. Furthermore, the presence of a neutron halo strongly affects the dynamics of accretion and leads to the rich variety of transient dynamical regimes.Comment: 10 pages, submitted to Astronomy and Astrophysic

A test of local Lorentz invariance with Compton scattering asymmetry

We report on a measurement of the constancy and anisotropy of the speed of light relative to the electrons in photon-electron scattering. We used the Compton scattering asymmetry measured by the new Compton polarimeter in Hall~C at Jefferson Lab to test for deviations from unity of the vacuum refractive index ($n$). For photon energies in the range of 9 - 46 MeV, we obtain a new limit of $1-n < 1.4 \times 10^{-8}$. In addition, the absence of sidereal variation over the six month period of the measurement constrains any anisotropies in the speed of light. These constitute the first study of Lorentz invariance using Compton asymmetry. Within the minimal standard model extension framework, our result yield limits on the photon and electron coefficients $\tilde{\kappa}_{0^+}^{YZ}, c_{TX}, \tilde{\kappa}_{0^+}^{ZX}$, and $c_{TY}$. Although, these limits are several orders of magnitude larger than the current best limits, they demonstrate the feasibility of using Compton asymmetry for tests of Lorentz invariance. Future parity violating electron scattering experiments at Jefferson Lab will use higher energy electrons enabling better constraints.Comment: 7 pages, 5 figure

Hot One-Temperature Accretion Flows Around Black Holes

We describe hot, optically-thin solutions for one-temperature accretion disks around black holes. We include cooling by synchrotron, bremsstrahlung, and Comptonization. Our solutions are thermally and viscously stable, with gas temperatures on the order of $T \sim 10^9-10^{10.7}$K. The thermal stability is a direct result of the inclusion of synchrotron cooling. The new solution branch is related to the advection-dominated solution for a two-temperature gas described by Narayan \& Yi (1995b). It is present only for mass accretion rates less than some critical $\dot{M}_{crit}$ which depends on the radius $R$ and viscosity parameter $\alpha$. The solutions are advection-dominated for extremely low values of $\dot{M}$. However, for a range of intermediate accretion rates, the new solutions are both hot ($T \sim 10^{10}$K) and cooling-dominated. Because of this new feature, one-temperature solutions are significantly more luminous than the corresponding two temperature solutions. The radial profile of the new solutions is unusual. The inner parts of the flow are cooling-dominated and have a disk-like geometry, while the outer parts are fully advection-dominated and nearly quasi-spherical.Comment: 24 pages tex file and 7 postscript figures all included in one compressed tar file. Accepted for publication in ApJ. In case of problems, write to [email protected]

Magnetically Arrested Disk: An Energetically Efficient Accretion Flow

We consider an accretion flow model originally proposed by Bisnovatyi-Kogan & Ruzmaikin (1974), which has been confirmed in recent 3D MHD simulations. In the model, the accreting gas drags in a strong poloidal magnetic field to the center such that the accumulated field disrupts the axisymmetric accretion flow at a relatively large radius. Inside the disruption radius, the gas accretes as discrete blobs or streams with a velocity much less than the free-fall velocity. Almost the entire rest mass energy of the gas is released as heat, radiation and mechanical/magnetic energy. Even for a non-rotating black hole, the efficiency of converting mass to energy is of order 50% or higher. The model is thus a practical analog of an idealized engine proposed by Geroch and Bekenstein.Comment: 4 pages, 2 figure, new refs added, in print in PAS