1,568 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.
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 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
Non-linear diffusive shock acceleration with free escape boundary
We present here a semi-analytical solution of the problem of particle
acceleration at non-linear shock waves with a free escape boundary at some
location upstream. This solution, besides allowing us to determine the spectrum
of particles accelerated at the shock front, including the shape of the cutoff
at some maximum momentum, also allows us to determine the spectrum of particles
escaping the system from upstream. This latter aspect of the problem is crucial
for establishing a connection between the accelerated particles in
astrophysical sources, such as supernova remnants, and the cosmic rays observed
at the Earth. An excellent approximate solution, which leads to a
computationally fast calculation of the structure of shocks with an arbitrary
level of cosmic ray modification, is also obtained.Comment: 11 pages, 2 figures, Accepted for publication in APh
Strong evidences of hadron acceleration in Tycho's Supernova Remnant
Very recent gamma-ray observations of G120.1+1.4 (Tycho's) supernova remnant
(SNR) by Fermi-LAT and VERITAS provided new fundamental pieces of information
for understanding particle acceleration and non-thermal emission in SNRs. We
want to outline a coherent description of Tycho's properties in terms of SNR
evolution, shock hydrodynamics and multi-wavelength emission by accounting for
particle acceleration at the forward shock via first order Fermi mechanism. We
adopt here a quick and reliable semi-analytical approach to non-linear
diffusive shock acceleration which includes magnetic field amplification due to
resonant streaming instability and the dynamical backreaction on the shock of
both cosmic rays (CRs) and self-generated magnetic turbulence. We find that
Tycho's forward shock is accelerating protons up to at least 500 TeV,
channelling into CRs about the 10 per cent of its kinetic energy. Moreover, the
CR-induced streaming instability is consistent with all the observational
evidences indicating a very efficient magnetic field amplification (up to ~300
micro Gauss). In such a strong magnetic field the velocity of the Alfv\'en
waves scattering CRs in the upstream is expected to be enhanced and to make
accelerated particles feel an effective compression factor lower than 4, in
turn leading to an energy spectrum steeper than the standard prediction
{\propto} E^-2. This latter effect is crucial to explain the GeV-to-TeV
gamma-ray spectrum as due to the decay of neutral pions produced in nuclear
collisions between accelerated nuclei and the background gas. The
self-consistency of such an hadronic scenario, along with the fact that the
concurrent leptonic mechanism cannot reproduce both the shape and the
normalization of the detected the gamma-ray emission, represents the first
clear and direct radiative evidence that hadron acceleration occurs efficiently
in young Galactic SNRs.Comment: Minor changes. Accepted for publication in Astronomy & Astrophysic
On the escape of particles from cosmic ray modified shocks
Stationary solutions to the problem of particle acceleration at shock waves
in the non-linear regime, when the dynamical reaction of the accelerated
particles on the shock cannot be neglected, are known to show a prominent
energy flux escaping from the shock towards upstream infinity. On physical
grounds, the escape of particles from the upstream region of a shock has to be
expected in all those situations in which the maximum momentum of accelerated
particles, , decreases with time, as is the case for the Sedov-Taylor
phase of expansion of a shell Supernova Remnant, when both the shock velocity
and the cosmic ray induced magnetization decrease. In this situation, at each
time , particles with momenta larger than leave the system from
upstream, carrying away a large fraction of the energy if the shock is strongly
modified by the presence of cosmic rays. This phenomenon is of crucial
importance for explaining the cosmic ray spectrum detected at Earth. In this
paper we discuss how this escape flux appears in the different approaches to
non-linear diffusive shock acceleration, and especially in the quasi-stationary
semi-analytical kinetic ones. We apply our calculations to the Sedov-Taylor
phase of a typical supernova remnant, including in a self-consistent way
particle acceleration, magnetic field amplification and the dynamical reaction
on the shock structure of both particles and fields. Within this framework we
calculate the temporal evolution of the maximum energy reached by the
accelerated particles and of the escape flux towards upstream infinity. The
latter quantity is directly related to the cosmic ray spectrum detected at
Earth.Comment: Version accepted for publication in MNRA
Comparison of Different Methods for Nonlinear Diffusive Shock Acceleration
We provide a both qualitative and quantitative comparison among different
approaches aimed to solve the problem of non-linear diffusive acceleration of
particles at shocks. In particular, we show that state-of-the-art models
(numerical, Monte Carlo and semi-analytical), even if based on different
physical assumptions and implementations, for typical environmental parameters
lead to very consistent results in terms of shock hydrodynamics, cosmic ray
spectrum and also escaping flux spectrum and anisotropy. Strong points and
limits of each approach are also discussed, as a function of the problem one
wants to study.Comment: 26 pages, 4 figures, published version (references updated
The contribution of supernova remnants to the galactic cosmic ray spectrum
The supernova paradigm for the origin of galactic cosmic rays has been deeply
affected by the development of the non-linear theory of particle acceleration
at shock waves. Here we discuss the implications of applying such theory to the
calculation of the spectrum of cosmic rays at Earth as accelerated in supernova
remnants and propagating in the Galaxy. The spectrum is calculated taking into
account the dynamical reaction of the accelerated particles on the shock, the
generation of magnetic turbulence which enhances the scattering near the shock,
and the dynamical reaction of the amplified field on the plasma. Most
important, the spectrum of cosmic rays at Earth is calculated taking into
account the flux of particles escaping from upstream during the Sedov-Taylor
phase and the adiabatically decompressed particles confined in the expanding
shell and escaping at later times. We show how the spectrum obtained in this
way is well described by a power law in momentum with spectral index close to
-4, despite the concave shape of the instantaneous spectra of accelerated
particles. On the other hand we also show how the shape of the spectrum is
sensible to details of the acceleration process and environment which are and
will probably remain very poorly known.Comment: 19 pages, 8 figures, published version (references updated
Hadronic versus leptonic origin of gamma-ray emission from supernova remnants
GeV and TeV emission from the forward shocks of supernova remnants (SNRs)
indicates that they are capable particle accelerators, making them promising
sources of Galactic cosmic rays (CRs). However, it remains uncertain whether
this -ray emission arises primarily from the decay of neutral pions
produced by very high energy hadrons, or from inverse-Compton and/or
bremsstrahlung emission from relativistic leptons. By applying a semi-analytic
approach to non-linear diffusive shock acceleration (NLDSA) and calculating the
particle and photon spectra produced in different astrophysical environments,
we parametrize the relative strength of hadronic and leptonic emission. We show
that, even if CR acceleration is likely to occur in all SNRs, the observed
photon spectra may instead primarily reflect the environment surrounding the
SNR, specifically the ambient density and radiation field. We find that the
most hadronic-appearing spectra are young and found in environments of high
density but low radiation energy density. This study aims to guide the
interpretation of current -ray observations and single out the best
targets of future campaigns.Comment: 9 pages, 6 figures, submitted to Ap
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