3,049 research outputs found
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 and mean particle escape timescale , where
is the particle momentum and 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 () 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 -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 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 -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
(APPJ) is a particular variant of that concept whose geometry allows
direct optical access. In this work, the characteristics of the 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 and O). 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
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