1,070 research outputs found
Cosmic Ray Transport in the Galaxy: a Review
The physics of energetic particle propagation in magnetised environments
plays a crucial role in both the processes of acceleration and transport of
cosmic rays. Recent theoretical developments in the field of cosmic ray
research have been mainly in the direction of exploring non-linear aspects of
the processes in which these particles are involved, namely the action of
cosmic rays on the environment in which the transport and/or acceleration take
place. When cosmic rays propagate outside of the acceleration region, such
action is mainly in two forms: 1) they generate hydromagnetic waves, through
streaming instabilities, leading to a dependence of the scattering properties
of the medium on the spectrum and spatial distribution of the energetic
particles, and 2) they exert a dynamical action on the plasma, which may cause
the launching of cosmic ray driven Galactic winds. In this article we discuss
these and other recent developments and how they compare with the bulk of new
observations on the spectra of primary nuclei (mainly H and He) and secondary
to primary ratios, such as the B/C ratio and the /ratio, and the
positrons ratio . We also comment on some radically new models
of the origin of CRs, in which the physical meaning of the secondary to primary
ratios is not the same as in the standard model.Comment: Solicited Review Article, accepted for publication in Advances in
Space Researc
The supernova remnant W44: a case of cosmic-Ray reacceleration
Supernova remnants (SNRs) are thought to be the primary sources of Galactic
Cosmic Rays (CRs). In the last few years, the wealth of gamma-ray data
collected by GeV and TeV instruments has provided important information about
particle energisation in these astrophysical sources, allowing us to make
progress in assessing their role as CR accelerators. In particular, the
spectrum of the gamma-ray emission detected by AGILE and Fermi-LAT from the two
middle aged Supernova Remnants (SNRs) W44 and IC443, has been proposed as a
proof of CR acceleration in SNRs. Here we discuss the possibility that the
radio and gamma-ray spectra from W44 may be explained in terms of
re-acceleration and compression of Galactic CRs. The recent measurement of the
interstellar CR flux by Voyager I has been instrumental for our work, in that
the result of the reprocessing of CRs by the shock in W44 depends on the CR
spectrum at energies that are precluded to terrestrial measurement due to solar
modulation. We introduce both CR protons and helium nuclei in our calculations,
and secondary electrons produced in situ are compared with the flux of Galactic
CR electrons reprocessed by the slow shock of this SNR.We find that the
multi-wavelength spectrum of W44 can be explained by reaccelerated particles
with no need of imposing any break on their distribution, but just a high
energy cut-off at the maximum energy the accelerator can provide.We also find
that a model including both re-acceleration and a very small fraction of
freshly accelerated particles may be more satisfactory on physical groundsComment: 11 pages, 5 figures, accepted by A&
The fate of ultrahigh energy nuclei in the immediate environment of young fast-rotating pulsars
Young, fast-rotating neutron stars are promising candidate sources for the
production of ultrahigh energy cosmic rays (UHECRs). The interest in this model
has recently been boosted by the latest chemical composition measurements of
cosmic rays, that seem to show the presence of a heavy nuclear component at the
highest energies. Neutrons stars, with their metal-rich surfaces, are
potentially interesting sources of such nuclei, but some open issues remain: 1)
is it possible to extract these nuclei from the star's surface? 2) Do the
nuclei survive the severe conditions present in the magnetosphere of the
neutron star? 3) What happens to the surviving nuclei once they enter the wind
that is launched outside the light cylinder? In this paper we address these
issues in a quantitative way, proving that for the most reasonable range of
neutron star surface temperatures (K), a large fraction of heavy
nuclei survive photo-disintegration losses. These processes, together with
curvature losses and acceleration in the star's electric potential, lead to
injection of nuclei with a chemical composition that is mixed, even if only
iron is extracted from the surface. We show that under certain conditions the
chemical composition injected into the wind region is compatible with that
required in previous work based on purely phenomenological arguments (typically
protons, CNO and Fe), and provides a
reasonable explanation of the mass abundance inferred from ultra high energy
data.Comment: 15 pages, 3 figures, accepted in JCAP, minor modification
Grammage of cosmic rays around Galactic supernova remnants
The residence time of cosmic rays (CRs) in the Galaxy is usually inferred
from the measurement of the ratio of secondary-to-primary nuclei, such as the
boron (B)/carbon (C) ratio, which provides an estimate of the amount of matter
traversed by CRs during their propagation, the so called CR grammage. However,
after being released by their parent sources, for instance supernova remnants
(SNRs), CRs must cross the disc of the Galaxy, before entering the much lower
density halo, in which they are believed to spend most of the time before
eventually escaping the Galaxy. In the near-source region, the CR propagation
is shown to be dominated by the non-linear self-generation of waves. Here we
show that due to this effect, the time that CRs with energies up to 10
TeV spend within a distance pc from the sources is much larger
than naive estimates would suggest. The corresponding grammage is close to
current estimates of the total grammage traversed throughout the whole Galaxy.
Moreover, there is an irreducible grammage that CRs traverse while trapped
downstream of the shock that accelerated them, though this contribution is
rather uncertain. We conclude that at energies TeV, the observed
grammage is heavily affected by the near-source non-linear trapping of CRs, and
at energies TeV it is affected by the source grammage. As a result,
the measurement of the B/C ratio should be used very cautiously as an indicator
of the propagation of CRs on large Galactic scales.Comment: 5 pages, 3 figures, Accepted for Publication in Phys. Rev.
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