951 research outputs found
Diffusive shock acceleration in extragalactic jets
We calculate the temporal evolution of distributions of relativistic
electrons subject to synchrotron and adiabatic processes and Fermi-like
acceleration in shocks. The shocks result from Kelvin-Helmholtz instabilities
in the jet. Shock formation and particle acceleration are treated in a
self-consistent way by means of a numerical hydrocode. We show that in our
model the number of relativistic particles is conserved during the evolution,
with no need of further injections of supra-thermal particles after the initial
one. From our calculations, we derive predictions for values and trends of
quantities like the spectral index and the cutoff frequency that can be
compared with observations.Comment: 12 pages containing 7 postscript figures; uses A&A macros. Accepted
for publication in Astronomy and Astrophysic
Neutrino emission via the plasma process in a magnetized plasma
Neutrino emission via the plasma process using the vertex formalism for QED
in a strongly magnetized plasma is considered. A new vertex function is
introduced to include the axial vector part of the weak interaction. Our
results are compared with previous calculations, and the effect of the axial
vector coupling on neutrino emission is discussed. The contribution from the
axial vector coupling can be of the same order as or greater than the vector
vector coupling under certain plasma conditions.Comment: 20 pages, 3 figure
Particle Acceleration in Turbulence and Weakly Stochastic Reconnection
Fast particles are accelerated in astrophysical environments by a variety of
processes. Acceleration in reconnection sites has attracted the attention of
researchers recently. In this letter we analyze the energy distribution
evolution of test particles injected in three dimensional (3D)
magnetohydrodynamic (MHD) simulations of different magnetic reconnection
configurations. When considering a single Sweet-Parker topology, the particles
accelerate predominantly through a first-order Fermi process, as predicted in
previous work (de Gouveia Dal Pino & Lazarian, 2005) and demonstrated
numerically in Kowal, de Gouveia Dal Pino & Lazarian (2011). When turbulence is
included within the current sheet, the acceleration rate, which depends on the
reconnection rate, is highly enhanced. This is because reconnection in the
presence of turbulence becomes fast and independent of resistivity (Lazarian &
Vishniac, 1999; Kowal et al., 2009) and allows the formation of a thick volume
filled with multiple simultaneously reconnecting magnetic fluxes. Charged
particles trapped within this volume suffer several head-on scatterings with
the contracting magnetic fluctuations, which significantly increase the
acceleration rate and results in a first-order Fermi process. For comparison,
we also tested acceleration in MHD turbulence, where particles suffer
collisions with approaching and receding magnetic irregularities, resulting in
a reduced acceleration rate. We argue that the dominant acceleration mechanism
approaches a second order Fermi process in this case.Comment: 6 pages, 1 figur
Periodicity and the determinant bundle
The infinite matrix `Schwartz' group is a classifying group for
odd K-theory and carries Chern classes in each odd dimension, generating the
cohomology. These classes are closely related to the Fredholm determinant on
We show that while the higher (even, Schwartz) loop groups of
again classifying for odd K-theory, do \emph{not} carry
multiplicative determinants generating the first Chern class, `dressed'
extensions, corresponding to a star product, do carry such functions. We use
these to discuss Bott periodicity for the determinant bundle and the eta
invariant. In so doing we relate two distinct extensions of the eta invariant,
to self-adjoint elliptic operators and to elliptic invertible suspended
families and show that the corresponding invariant is a determinant in
this sense
Elliptic operators on manifolds with singularities and K-homology
It is well known that elliptic operators on a smooth compact manifold are
classified by K-homology. We prove that a similar classification is also valid
for manifolds with simplest singularities: isolated conical points and fibered
boundary. The main ingredients of the proof of these results are: an analog of
the Atiyah-Singer difference construction in the noncommutative case and an
analog of Poincare isomorphism in K-theory for our singular manifolds.
As applications we give a formula in topological terms for the obstruction to
Fredholm problems on manifolds with singularities and a formula for K-groups of
algebras of pseudodifferential operators.Comment: revised version; 25 pages; section with applications expande
Circular Polarization Induced by Scintillation in a Magnetized Medium
A new theory is presented for the development of circular polarization as
radio waves propagate through the turbulent, birefringent interstellar medium.
The fourth order moments of the wavefield are calculated and it is shown that
unpolarized incident radiation develops a nonzero variance in circular
polarization. A magnetized turbulent medium causes the Stokes parameters to
scintillate in a non-identical manner. A specific model for this effect is
developed for the case of density fluctuations in a uniform magnetic field.Comment: 16 pages, 1 figure, Phys. Rev. E, accepte
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