6,020 research outputs found

### New insights on hadron acceleration at supernova remnant shocks

We outline the main features of nuclei acceleration at supernova remnant
forward shocks, stressing the crucial role played by self-amplified magnetic
fields in determining the energy spectrum observed in this class of sources. In
particular, we show how the standard predictions of the non-linear theory of
diffusive shock acceleration has to be completed with an additional ingredient,
which we propose to be the enhanced velocity of the magnetic irregularities
particles scatter against, to reconcile the theory of efficient particle
acceleration with recent observations of gamma-ray bright supernova remnants.Comment: 7 pages, 2 figures. To apper in "Cosmic-ray induced phenomenology in
star-forming environments: Proceedings of the 2nd Session of the Sant Cugat
Forum of Astrophysics" (April 16-19, 2012), Olaf Reimer and Diego F. Torres
(eds.

### 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 $\gamma$-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 $\gamma$-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

### Non-linear diffusive acceleration of heavy nuclei in supernova remnant shocks

We describe a semi-analytical approach to non-linear diffusive shock
acceleration in the case in which nuclei other than protons are also
accelerated. The structure of the shock is determined by the complex interplay
of all nuclei, and in turn this shock structure determines the spectra of all
components. The magnetic field amplification upstream is described as due to
streaming instability of all nuclear species. The amplified magnetic field is
then taken into account for its dynamical feedback on the shock structure as
well as in terms of the induced modification of the velocity of the scattering
centers that enters the particle transport equation. The spectra of accelerated
particles are steep enough to be compared with observed cosmic ray spectra only
if the magnetic field is sufficiently amplified and the scattering centers have
high speed in the frame of the background plasma. We discuss the implications
of this generalized approach on the structure of the knee in the all-particle
cosmic ray spectrum, which we interpret as due to an increasingly heavier
chemical composition above $10^{15}$eV. The effects of a non trivial chemical
composition at the sources on the gamma ray emission from a supernova remnant
when gamma rays are of hadronic origin are also discussed.Comment: 23 pages, 5 figures, minor changes to reflect the published versio

### 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

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