2,385 research outputs found
Kinetic approaches to particle acceleration at cosmic ray modified shocks
Kinetic approaches provide an effective description of the process of
particle acceleration at shock fronts and allow to take into account the
dynamical reaction of the accelerated particles as well as the amplification of
the turbulent magnetic field as due to streaming instability. The latter does
in turn affect the maximum achievable momentum and thereby the acceleration
process itself, in a chain of causality which is typical of non-linear systems.
Here we provide a technical description of two of these kinetic approaches and
show that they basically lead to the same conclusions. In particular we discuss
the effects of shock modification on the spectral shape of the accelerated
particles, on the maximum momentum, on the thermodynamic properties of the
background fluid and on the escaping and advected fluxes of accelerated
particles.Comment: 22 pages, 7 figures, accepted for publication in MNRA
The Littlewood-Gowers problem
We show that if A is a subset of Z/pZ (p a prime) of density bounded away
from 0 and 1 then the A(Z/pZ)-norm (that is the l^1-norm of the Fourier
transform) of the characterstic function of A is bounded below by an absolute
constant times (log p)^{1/2 - \epsilon} as p tends to infinity. This improves
on the exponent 1/3 in recent work of Green and Konyagin.Comment: 31 pp. Corrected typos. Updated references
Nonthermal Emission from a Supernova Remnant in a Molecular Cloud
In evolved supernova remnants (SNRs) interacting with molecular clouds, such
as IC 443, W44, and 3C391, a highly inhomogeneous structure consisting of a
forward shock of moderate Mach number, a cooling layer, a dense radiative shell
and an interior region filled with hot tenuous plasma is expected. We present a
kinetic model of nonthermal electron injection, acceleration and propagation in
that environment and find that these SNRs are efficient electron accelerators
and sources of hard X- and gamma-ray emission. The energy spectrum of the
nonthermal electrons is shaped by the joint action of first and second order
Fermi acceleration in a turbulent plasma with substantial Coulomb losses.
Bremsstrahlung, synchrotron, and inverse Compton radiation of the nonthermal
electrons produce multiwavelength photon spectra in quantitative agreement with
the radio and the hard emission observed by ASCA and EGRET from IC 443. We
distinguish interclump shock wave emission from molecular clump shock wave
emission accounting for a complex structure of molecular cloud. Spatially
resolved X- and gamma- ray spectra from the supernova remnants IC 443, W44, and
3C391 as might be observed with BeppoSAX, Chandra XRO, XMM, INTEGRAL and GLAST
would distinguish the contribution of the energetic lepton component to the
gamma-rays observed by EGRET.Comment: 14 pages, 4 figure, Astrophysical Journal, v.538, 2000 (in press
Efficiency of Nonlinear Particle Acceleration at Cosmic Structure Shocks
We have calculated the evolution of cosmic ray (CR) modified astrophysical
shocks for a wide range of shock Mach numbers and shock speeds through
numerical simulations of diffusive shock acceleration (DSA) in 1D quasi-
parallel plane shocks. The simulations include thermal leakage injection of
seed CRs, as well as pre-existing, upstream CR populations. Bohm-like diffusion
is assumed. We model shocks similar to those expected around cosmic structure
pancakes as well as other accretion shocks driven by flows with upstream gas
temperatures in the range K and shock Mach numbers spanning
. We show that CR modified shocks evolve to time-asymptotic states
by the time injected particles are accelerated to moderately relativistic
energies (p/mc \gsim 1), and that two shocks with the same Mach number, but
with different shock speeds, evolve qualitatively similarly when the results
are presented in terms of a characteristic diffusion length and diffusion time.
For these models the time asymptotic value for the CR acceleration efficiency
is controlled mainly by shock Mach number. The modeled high Mach number shocks
all evolve towards efficiencies %, regardless of the upstream CR
pressure. On the other hand, the upstream CR pressure increases the overall CR
energy in moderate strength shocks (). (abridged)Comment: 23 pages, 12 ps figures, accepted for Astrophysical Journal (Feb. 10,
2005
Effects of fluoxetine on the oral environment of bulimics
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73809/1/j.1399-302X.1993.tb00545.x.pd
Spallation dominated propagation of Heavy Cosmic Rays and the Local Interstellar Medium (LISM)
Measurements of ultra heavy nuclei at GeV/n energies in the galactic cosmic
radiation address the question of the sources (nucleosynthetic s- and
r-processes). As such, the determination of CR source abundances is a promising
way to discriminate between existing nucleosynthesis models. For primary
species (nuclei present and accelerated at sources), it is generally assumed
that the relative propagated abundances, if they are close in mass, are not too
different from their relative source abundances. Besides, the range of the
correction factor associated to propagation has been estimated in weighted slab
models only. Heavy CRs that are detected near Earth were accelerated from
regions that are closer to us than were the light nuclei. Hence, the geometry
of sources in the Solar neighbourhood, and as equally important, the geometry
of gas in the same region, must be taken into account. In this paper, a two
zone diffusion model is used, and as was previously investigated for
radioactive species, we report here on the impact of the local interstellar
medium (LISM) feature (under-dense medium over a scale ~100 pc) on primary and
secondary stable nuclei propagated abundances. Going down to Fe nuclei, the
connection between heavy and light abundances is also inspected. A general
trend is found that decreases the UHCR source abundances relative to the HCR
ones. This could have an impact on the level of r-process required to reproduce
the data.Comment: 12 pages, 9 figures, accepted by A&A. Comparison with truncated
weighted slab and discussion added. Figure 8 modified. New appendix on
truncated weighted slab techniqu
- …