Time-Dependent Stochastic Particle Acceleration in Astrophysical Plasmas: Exact Solutions Including Momentum-Dependent Escape

Stochastic acceleration of charged particles due to interactions with magnetohydrodynamic (MHD) plasma waves is the dominant process leading to the formation of the high-energy electron and ion distributions in a variety of astrophysical systems. Collisions with the waves influence both the energization and the spatial transport of the particles, and therefore it is important to treat these two aspects of the problem in a self-consistent manner. We solve the representative Fokker-Planck equation to obtain a new, closed-form solution for the time-dependent Green's function describing the acceleration and escape of relativistic ions interacting with Alfven or fast-mode waves characterized by momentum diffusion coefficient $D(p)\propto p^q$ and mean particle escape timescale $t_esc(p) \propto p^{q-2}$, where $p$ is the particle momentum and $q$ is the power-law index of the MHD wave spectrum. In particular, we obtain solutions for the momentum distribution of the ions in the plasma and also for the momentum distribution of the escaping particles, which may form an energetic outflow. The general features of the solutions are illustrated via examples based on either a Kolmogorov or Kraichnan wave spectrum. The new expressions complement the results obtained by Park and Petrosian, who presented exact solutions for the hard-sphere scattering case ($q=2$) in addition to other scenarios in which the escape timescale has a power-law dependence on the momentum. Our results have direct relevance for models of high-energy radiation and cosmic-ray production in astrophysical environments such as $\gamma$-ray bursts, active galaxies, and magnetized coronae around black holes.Comment: Accepted for publication in Ap

Remodeling After Myocardial Infarction in Humans Is Not Associated With Interstitial Fibrosis of Noninfarcted Myocardium

AbstractObjectives. This study was specifically designed to evaluate whether noninfarcted hypertrophic myocardium in patients with end-stage heart failure after myocardial infarction (MI) is associated with an increase in interstitial fibrous tissue.Background. Postinfarction remodeling consists of complex alterations that involve both infarcted and noninfarcted myocardium. The question arises whether ventricular dysfunction is due to physical events, such as inadequate myocardial hypertrophy to compensate for increased tangential wall stress, or is caused by the development of progressive interstitial fibrosis in noninfarcted myocardium.Methods. Fifteen hearts were obtained as cardiac explants (n = 13) or at autopsy (n = 2) from patients with end-stage coronary artery disease. Sixteen normal hearts served as reference hearts. Samples were taken from the left ventricular (LV) wall that contained the infarcted area, the border area and noninfarcted myocardium remote from scar areas. Collagen was quantified biochemically and microdensitophotometrically. Collagen type I and III ratios were analyzed by using the cyanogen bromide method and immunohistochemical staining, followed by microdensitophotometric quantification.Results. In noninfarcted myocardium remote from the scar areas, total collagen levels and collagen type I/III ratios did not differ statistically from those in reference hearts. These observations contrasted with high total collagen content and high collagen type I/III ratios in scar and border areas.Conclusions. Remodeling of LV myocardium after MI in patients with end-stage heart failure is not necessarily associated with interstitial fibrosis in noninfarcted hypertrophic myocardium remote from scar areas. This finding raises questions regarding therapeutic interventions designed to prevent or retard the development of interstitial fibrosis.(J Am Coll Cardiol 1997;30:76–82

A two-way photonic interface for linking Sr+ transition at 422 nm to the telecommunications C-band

We report a single-stage bi-directional interface capable of linking Sr+ trapped ion qubits in a long-distance quantum network. Our interface converts photons between the Sr+ emission wavelength at 422 nm and the telecoms C-band to enable low-loss transmission over optical fiber. We have achieved both up- and down-conversion at the single photon level with efficiencies of 9.4% and 1.1% respectively. Furthermore we demonstrate noise levels that are low enough to allow for genuine quantum operation in the future.Comment: 5 pages, 4 figure

Formation of Relativistic Outflows in Shearing Black Hole Accretion Coronae

We examine the possibility that the relativistic jets observed in many active galactic nuclei may be powered by the Fermi acceleration of protons in a tenuous corona above a two-temperature accretion disk. In this picture the acceleration arises as a consequence of the shearing motion of the magnetic field in the corona, which is anchored in the underlying Keplerian disk. The protons in the corona have a power-law distribution because the density there is too low for proton-proton collisions to thermalize the energy supplied via Fermi acceleration. The same shear acceleration mechanism also operates in the disk itself, however, there the density is high enough for thermalization to occur and consequently the disk protons have a Maxwellian distribution. Particle acceleration in the corona leads to the development of a pressure-driven wind that passes through a critical point and subsequently transforms into a relativistic jet at large distances from the black hole. We combine the critical conditions for the wind with the structure equations for the disk and the corona to obtain a coupled disk/corona/wind model. Using the coupled model we compute the asymptotic Lorentz factor $\Gamma_\infty$ of the jet as a function of the cylindrical starting radius at the base of the outflow, in the corona. Our results suggest that \Gamma_\infty \lapprox 10, which is consistent with observations of superluminal motion in blazars. We show that collisions between the jet and broad-line emission clouds can produce high-energy radiation with a luminosity sufficient to power the $\gamma$-rays observed from blazars. Subject headings: radiation mechanisms: non-thermal, accretion, accretion disks, acceleration of particles, gamma rays: theoryComment: 50 pages, 13 figures, accepted by ApJ, 199

Spatially resolved simulation of a radio frequency driven micro atmospheric pressure plasma jet and its effluent

Radio frequency driven plasma jets are frequently employed as efficient plasma sources for surface modification and other processes at atmospheric pressure. The radio-frequency driven micro atmospheric pressure plasma jet ($\mu$APPJ) is a particular variant of that concept whose geometry allows direct optical access. In this work, the characteristics of the $\mu$APPJ operated with a helium-oxygen mixture and its interaction with a helium environment are studied by numerical simulation. The density and temperature of the electrons, as well as the concentration of all reactive species are studied both in the jet itself and in its effluent. It is found that the effluent is essentially free of charge carriers but contains a substantial amount of activated oxygen (O, O$_3$ and O$_2(^1\Delta)$). The simulation results are verified by comparison with experimental data

Relative entropy, Haar measures and relativistic canonical velocity distributions

The thermodynamic maximum principle for the Boltzmann-Gibbs-Shannon (BGS) entropy is reconsidered by combining elements from group and measure theory. Our analysis starts by noting that the BGS entropy is a special case of relative entropy. The latter characterizes probability distributions with respect to a pre-specified reference measure. To identify the canonical BGS entropy with a relative entropy is appealing for two reasons: (i) the maximum entropy principle assumes a coordinate invariant form; (ii) thermodynamic equilibrium distributions, which are obtained as solutions of the maximum entropy problem, may be characterized in terms of the transformation properties of the underlying reference measure (e.g., invariance under group transformations). As examples, we analyze two frequently considered candidates for the one-particle equilibrium velocity distribution of an ideal gas of relativistic particles. It becomes evident that the standard J\"uttner distribution is related to the (additive) translation group on momentum space. Alternatively, imposing Lorentz invariance of the reference measure leads to a so-called modified J\"uttner function, which differs from the standard J\"uttner distribution by a prefactor, proportional to the inverse particle energy.Comment: 15 pages: extended version, references adde