70 research outputs found
Fermi acceleration at supernova remnant shocks
We investigate the physics of particle acceleration at non-relativistic
shocks exploiting two different and complementary approaches, namely a
semi-analytic modeling of cosmic-ray modified shocks and large hybrid (kinetic
protons/fluid electrons) simulations. The former technique allows us to extract
some information from the multi-wavelength observations of supernova remnants,
especially in the gamma-ray band, while the latter returns fundamental insights
into the details of particle injection and magnetic field amplification via
plasma instabilities. In particular, we present the results of large hybrid
simulations of non-relativistic shocks, discussing the properties of the
transition from the thermal to the non-thermal component, the spectrum of which
turns out to be the power-law predicted by first-order Fermi acceleration.
Along with a rather effective magnetic field amplification, we find that more
than 20% of the bulk energy is converted in non-thermal particles, altering
significantly the dynamics of the shock and leading to the formation of a
precursor.Comment: 4 pages, 1 figure - Proceedings of the 5th International Symposium on
High-Energy Gamma-Ray Astronomy - Heidelberg, Germany, July 9-13th, 201
Cosmic-ray Acceleration and Propagation
The origin of cosmic rays (CRs) has puzzled scientists since the pioneering
discovery by Victor Hess in 1912. In the last decade, however, modern
supercomputers have opened a new window on the processes regulating
astrophysical collisionless plasmas, allowing the study of CR acceleration via
first-principles kinetic simulations. At the same time, a new-generation of
X-ray and -ray telescopes has been collecting evidence that Galactic
CRs are accelerated in the blast waves of supernova remnants (SNRs). I present
state-of-the-art particle-in-cells simulations of non-relativistic shocks, in
which ion and electron acceleration efficiency and magnetic field amplification
are studied in detail as a function of the shock parameters. I then discuss the
theoretical and observational counterparts of these findings, comparing them
with predictions of diffusive shock acceleration theory and with
multi-wavelength observations of young SNRs. I especially outline some major
open questions, such as the possible causes of the steep CR spectra inferred
from -ray observations of SNRs and the origin of the knee in the
Galactic CR spectrum. Finally, I put such a theoretical understanding in
relation with CR propagation in the Galaxy in order to bridge the gap between
acceleration in sources and measurements of CRs at Earth.Comment: 24 pages, 7 figures, Invited Review Talk at the 34th International
Cosmic Ray Conference, The Hague, The Netherland
Simulations of Ion Acceleration at Non-relativistic Shocks. I. Acceleration Efficiency
We use 2D and 3D hybrid (kinetic ions - fluid electrons) simulations to
investigate particle acceleration and magnetic field amplification at
non-relativistic astrophysical shocks. We show that diffusive shock
acceleration operates for quasi-parallel configurations (i.e., when the
background magnetic field is almost aligned with the shock normal) and, for
large sonic and Alfv\'enic Mach numbers, produces universal power-law spectra
proportional to p^(-4), where p is the particle momentum. The maximum energy of
accelerated ions increases with time, and it is only limited by finite box size
and run time. Acceleration is mainly efficient for parallel and quasi-parallel
strong shocks, where 10-20% of the bulk kinetic energy can be converted to
energetic particles, and becomes ineffective for quasi-perpendicular shocks.
Also, the generation of magnetic turbulence correlates with efficient ion
acceleration, and vanishes for quasi-perpendicular configurations. At very
oblique shocks, ions can be accelerated via shock drift acceleration, but they
only gain a factor of a few in momentum, and their maximum energy does not
increase with time. These findings are consistent with the degree of
polarization and the morphology of the radio and X-ray synchrotron emission
observed, for instance, in the remnant of SN 1006. We also discuss the
transition from thermal to non-thermal particles in the ion spectrum
(supra-thermal region), and we identify two dynamical signatures peculiar of
efficient particle acceleration, namely the formation of an upstream precursor
and the alteration of standard shock jump conditions.Comment: 21 pages, 14 figures, Minor changes reflecting the version accepted
to Ap
Simulations of Ion Acceleration at Non-relativistic Shocks. III. Particle Diffusion
We use large hybrid (kinetic protons-fluid electrons) simulations to
investigate the transport of energetic particles in self-consistent
electromagnetic configurations of collisionless shocks. In previous papers of
this series, we showed that ion acceleration may be very efficient (up to
in energy), and outlined how the streaming of energetic particles
amplifies the upstream magnetic field. Here, we measure particle diffusion
around shocks with different strengths, finding that the mean free path for
pitch-angle scattering of energetic ions is comparable with their gyroradii
calculated in the self-generated turbulence. For moderately-strong shocks,
magnetic field amplification proceeds in the quasi-linear regime, and particles
diffuse according to the self-generated diffusion coefficient, i.e., the
scattering rate depends only on the amount of energy in modes with wavelengths
comparable with the particle gyroradius. For very strong shocks, instead, the
magnetic field is amplified up to non-linear levels, with most of the energy in
modes with wavelengths comparable to the gyroradii of highest-energy ions, and
energetic particles experience Bohm-like diffusion in the amplified field. We
also show how enhanced diffusion facilitates the return of energetic particles
to the shock, thereby determining the maximum energy that can be achieved in a
given time via diffusive shock acceleration. The parametrization of the
diffusion coefficient that we derive can be used to introduce self-consistent
microphysics into large-scale models of cosmic ray acceleration in
astrophysical sources, such as supernova remnants and clusters of galaxies.Comment: 8 pages, 7 figures, Minor changes reflecting the version accepted to
Ap
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