135 research outputs found
Diffuse Galactic Gamma Rays from Shock-Accelerated Cosmic Rays
A shock-accelerated particle flux \propto p^-s, where p is the particle
momentum, follows from simple theoretical considerations of cosmic-ray
acceleration at nonrelativistic shocks followed by rigidity-dependent escape
into the Galactic halo. A flux of shock-accelerated cosmic-ray protons with s ~
2.8 provides an adequate fit to the Fermi-LAT gamma-ray emission spectra of
high-latitude and molecular cloud gas when uncertainties in nuclear production
models are considered. A break in the spectrum of cosmic-ray protons claimed by
Neronov, Semikoz, & Taylor (PRL, 108, 051105, 2012) when fitting the gamma-ray
spectra of high-latitude molecular clouds is a consequence of using a
cosmic-ray proton flux described by a power law in kinetic energy.Comment: Version to correspond to published letter in PRL; corrected Fig.
Particle Diffusion and Acceleration by Shock Wave in Magnetized Filamentary Turbulence
We expand the off-resonant scattering theory for particle diffusion in
magnetized current filaments that can be typically compared to astrophysical
jets, including active galactic nucleus jets. In a high plasma beta region
where the directional bulk flow is a free-energy source for establishing
turbulent magnetic fields via current filamentation instabilities, a novel
version of quasi-linear theory to describe the diffusion of test particles is
proposed. The theory relies on the proviso that the injected energetic
particles are not trapped in the small-scale structure of magnetic fields
wrapping around and permeating a filament but deflected by the filaments, to
open a new regime of the energy hierarchy mediated by a transition compared to
the particle injection. The diffusion coefficient derived from a quasi-linear
type equation is applied to estimating the timescale for the stochastic
acceleration of particles by the shock wave propagating through the jet. The
generic scalings of the achievable highest energy of an accelerated ion and
electron, as well as of the characteristic time for conceivable energy
restrictions, are systematically presented. We also discuss a feasible method
of verifying the theoretical predictions. The strong, anisotropic turbulence
reflecting cosmic filaments might be the key to the problem of the acceleration
mechanism of the highest energy cosmic rays exceeding 100 EeV (10^{20} eV),
detected in recent air shower experiments.Comment: 39 pages, 2 figures, accepted for publication in Ap
Index
The interest in relativistic beam-plasma instabilities has been greatly rejuvenated over the past two decades by novel concepts in laboratory and space plasmas. Recent advances in this long-standing field are here reviewed from both theoretical and numerical points of view. The primary focus is on the two-dimensional spectrum of unstable electromagnetic waves growing within relativistic, unmagnetized, and uniform electron beam-plasma systems. Although the goal is to provide a unified picture of all instability classes at play, emphasis is put on the potentially dominant waves propagating obliquely to the beam direction, which have received little attention over the years. First, the basic derivation of the general dielectric function of a kinetic relativistic plasma is recalled. Next, an overview of two-dimensional unstable spectra associated with various beam-plasma distribution functions is given. Both cold-fluid and kinetic linear theory results are reported, the latter being based on waterbag and Maxwell–Jüttner model distributions. The main properties of the competing modes (developing parallel, transverse, and oblique to the beam) are given, and their respective region of dominance in the system parameter space is explained. Later sections address particle-in-cell numerical simulations and the nonlinear evolution of multidimensional beam-plasma systems. The elementary structures generated by the various instability classes are first discussed in the case of reduced-geometry systems. Validation of linear theory is then illustrated in detail for large-scale systems, as is the multistaged character of the nonlinear phase. Finally, a collection of closely related beam-plasma problems involving additional physical effects is presented, and worthwhile directions of future research are outlined.Original Publication: Antoine Bret, Laurent Gremillet and Mark Eric Dieckmann, Multidimensional electron beam-plasma instabilities in the relativistic regime, 2010, Physics of Plasmas, (17), 12, 120501-1-120501-36. http://dx.doi.org/10.1063/1.3514586 Copyright: American Institute of Physics http://www.aip.org/</p
On The Origin of Very High Energy Cosmic Rays
We discuss the most recent developments in our understanding of the
acceleration and propagation of cosmic rays up to the highest energies. In
particular we specialize our discussion to three issues: 1) developments in the
theory of particle acceleration at shock waves; 2) the transition from galactic
to extragalactic cosmic rays; 3) implications of up-to-date observations for
the origin of ultra high energy cosmic rays (UHECRs).Comment: Invited Review Article to appear in Modern Physics Letters A, Review
Sectio
The origin of ultra high energy cosmic rays
We briefly discuss some open problems and recent developments in the
investigation of the origin and propagation of ultra high energy cosmic rays
(UHECRs).Comment: Invited Review Talk at TAUP 2005 (Zaragoza - September 10-14, 2005).
7 page
High Energy Cosmic Rays From Supernovae
Cosmic rays are charged relativistic particles that reach the Earth with
extremely high energies, providing striking evidence of the existence of
effective accelerators in the Universe. Below an energy around
eV cosmic rays are believed to be produced in the Milky Way while above that
energy their origin is probably extragalactic. In the early '30s supernovae
were already identified as possible sources for the Galactic component of
cosmic rays. After the '70s this idea has gained more and more credibility
thanks to the the development of the diffusive shock acceleration theory, which
provides a robust theoretical framework for particle energization in
astrophysical environments. Afterwards, mostly in recent years, much
observational evidence has been gathered in support of this framework,
converting a speculative idea in a real paradigm. In this Chapter the basic
pillars of this paradigm will be illustrated. This includes the acceleration
mechanism, the non linear effects produced by accelerated particles onto the
shock dynamics needed to reach the highest energies, the escape process from
the sources and the transportation of cosmic rays through the Galaxy. The
theoretical picture will be corroborated by discussing several observations
which support the idea that supernova remnants are effective cosmic ray
factories.Comment: Final draft of a chapter in "Handbook of Supernovae" edited by Athem
W. Alsabti and Paul Murdi
Cosmic-ray-induced ionization in molecular clouds adjacent to supernova remnants - Tracing the hadronic origin of GeV gamma radiation
Energetic gamma rays (GeV to TeV photon energy) have been detected toward
several supernova remnants (SNR) associated with molecular clouds. If the gamma
rays are produced mainly by hadronic processes rather than leptonic processes
like bremsstrahlung, then the flux of energetic cosmic ray (CR) nuclei (>1 GeV)
required to produce the gamma rays can be inferred at the site where the
particles are accelerated in SNR shocks. It is of great interest to understand
the acceleration of the CR of lower energy (<1 GeV) accompanying the energetic
component. These particles of lower energy are most effective in ionizing
interstellar gas, leaving an observable imprint on the interstellar ion
chemistry. A correlation of energetic gamma radiation with enhanced
interstellar ionization can thus support the hadronic origin of the gamma rays
and constrain the acceleration of ionizing CR in SNR. We propose a method to
test the hadronic origin of GeV gamma rays from SNR associated with a molecular
cloud. We use observational gamma ray data for each of these SNR known,
modeling the observations to obtain the underlying proton spectrum assuming
that the gamma rays are produced by pion decay. Assuming that the acceleration
mechanism does not only produce high energy protons, but also low energy
protons, this proton spectrum at the source is then used to calculate the
ionization rate of the molecular cloud. Ionized molecular hydrogen triggers a
chemical network forming molecular ions. The relaxation of these ions results
in characteristic line emission, which can be predicted. We show that the
ionization rate for at least two objects is more than an order of magnitude
above Galactic average for molecular clouds, hinting at an enhanced formation
rate of molecular ions. There will be interesting opportunities to measure
crucial molecular ions in the infrared and submillimeter-wave parts of the
spectrum.Comment: published in Astronomy and Astrophysics, 13 pages, 19 figure
Cosmic-ray acceleration in supernova remnants: non-linear theory revised
A rapidly growing amount of evidences, mostly coming from the recent
gamma-ray observations of Galactic supernova remnants (SNRs), is seriously
challenging our understanding of how particles are accelerated at fast shocks.
The cosmic-ray (CR) spectra required to account for the observed phenomenology
are in fact as steep as , i.e., steeper than the
test-particle prediction of first-order Fermi acceleration, and significantly
steeper than what expected in a more refined non-linear theory of diffusive
shock acceleration. By accounting for the dynamical back-reaction of the
non-thermal particles, such a theory in fact predicts that the more efficient
the particle acceleration, the flatter the CR spectrum. In this work we put
forward a self-consistent scenario in which the account for the magnetic field
amplification induced by CR streaming produces the conditions for reversing
such a trend, allowing --- at the same time --- for rather steep spectra and CR
acceleration efficiencies (about 20%) consistent with the hypothesis that SNRs
are the sources of Galactic CRs. In particular, we quantitatively work out the
details of instantaneous and cumulative CR spectra during the evolution of a
typical SNR, also stressing the implications of the observed levels of
magnetization on both the expected maximum energy and the predicted CR
acceleration efficiency. The latter naturally turns out to saturate around
10-30%, almost independently of the fraction of particles injected into the
acceleration process as long as this fraction is larger than about .Comment: 24 pages, 5 figures, accepted for publication in JCA
Radio observations of ZwCl 2341.1+0000: a double radio relic cluster
Context: Hierarchal models of large scale structure (LSS) formation predict
that galaxy clusters grow via gravitational infall and mergers of (smaller)
mass concentrations, such as clusters and galaxy groups. Diffuse radio
emission, in the form of radio halos and relics, is found in clusters
undergoing a merger, indicating that shocks or turbulence associated with the
merger are capable of accelerating electrons to highly relativistic energies.
Here we report on radio observations of ZwCl 2341.1+0000, a complex merging
structure of galaxies located at z=0.27, using Giant Metrewave Radio Telescope
(GMRT) observations.
Aims: The main aim of the observations is to study the nature of the diffuse
radio emission in the galaxy cluster ZwCl 2341.1+0000.
Methods: We have carried out GMRT 610, 241, and 157 MHz continuum
observations of ZwCl 2341.1+0000. The radio observations are combined with
X-ray and optical data of the cluster.
Results: The GMRT observations show the presence of a double peripheral radio
relic in the cluster ZwCl 2341.1+0000. The spectral index is -0.49 \pm 0.18 for
the northern relic and -0.76 \pm 0.17 for the southern relic respectively. We
have derived values of 0.48-0.93 microGauss for the equipartition magnetic
field strength. The relics are probably associated with an outwards traveling
merger shock waves.Comment: 14 pages, 10 figures, accepted for publication in A&A on July 30,
200
The Fermi Bubble as a Source of Cosmic Rays in the Energy Range > 10E15 eV
The {\it Fermi} Large Area Telescope has recently discovered two giant
gamma-ray bubbles which extend north and south of the Galactic center with
diameters and heights of the order of kpc. We suggest that the
periodic star capture processes by the Galactic supermassive black hole Sgr
A, with a capture rate of
yr and an energy release of erg per capture, can
result in hot plasma injecting into the Galactic halo at a wind velocity of
cm s. The periodic injection of hot plasma can produce a
series of shocks. Energetic protons in the bubble are re-accelerated when they
interact with these shocks. We show that for energy larger than
eV, the acceleration process can be better described by the stochastic
second-order Fermi acceleration.
We propose that hadronic cosmic rays (CRs) within the ``knee'' of the
observed CR spectrum are produced by Galactic supernova remnants distributed in
the Galactic disk. Re-acceleration of these particles in the Fermi Bubble
produces CRs beyond the knee. With a mean CR diffusion coefficient in this
energy range in the bubble cm s, we can
reproduce the spectral index of the spectrum beyond the knee and within. The
conversion efficiency from shock energy of the bubble into CR energy is about
10\%. This model provides a natural explanation of the observed CR flux,
spectral indices, and matching of spectra at the knee.Comment: 43 pages, 8 figues, to be published in the Astrophysical Journal;
version 2, 45 pages, 8 figures, added references and corrected typo
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