4,583 research outputs found
Spectra of accelerated particles at supernova shocks in the presence of neutral hydrogen: the case of Tycho
The presence of neutral hydrogen in the shock proximity changes the structure
of the shock and affects the spectra of particles accelerated through the
first-order Fermi mechanism. This phenomenon has profound implications for the
interpretation of the multifrequency spectra of radiation from supernova
remnants. Neutrals that undergo charge exchange with hot ions downstream of the
shock may result in fast neutrals moving towards the upstream gas, where they
can suffer additional charge exchange or ionisation reactions, thereby
depositing energy and momentum upstream. Here we discuss the implications of
this neutral return flux, which was already predicted in our previous work on
neutral mediated supernova shocks, and show how the spectra of accelerated
particles turn out to be appreciably steeper than , thereby affecting
the gamma ray spectra from supernova remnants in general and from Tycho
specifically. The theory that describes non-linear diffusive shock acceleration
in the presence of neutral hydrogen has been developed in recent years. Here we
use a semi-analytical theory developed in previous work and specialise our
predictions to the case of the Tycho supernova shock, where there is evidence
that the spectrum of the accelerated cosmic rays is steeper than expected from
the traditional theory of diffusive shock acceleration. We show that, if the
fraction of neutral hydrogen in the vicinity of the Tycho supernova shock is,
as suggested by observations, ~70-90, then spectra of accelerated protons
steeper than may be a natural consequence of charge exchange reactions
and the associated neutral return flux. The spectral shape is affected by this
phenomenon for particles with energies below ~100-1000 GeV, for which the
diffusion length is less than or at most comparable to the pathlength of charge
exchange and ionisation upstream of the shock.Comment: 6 pages, 3 figures. Accepted for publication by A&
Cosmic Rays, Radio Halos and Non-Thermal X-ray Emission in Clusters of Galaxies
We calculate the flux of radio, hard X-ray and UV radiation from clusters of
galaxies as produced by synchrotron emission and Inverse Compton Scattering of
electrons generated as secondaries in cosmic ray interactions in the
intracluster medium. Both the spatial distribution of cosmic rays due to their
diffusion and the spatial distribution of the intracluster gas are taken into
account. Our calculations are specifically applied to the case of the Coma
cluster. The fluxes and spectra of the radio halo emission and of the hard
X-ray excess from Coma can be explained in this model if an average magnetic
field is assumed. However, such a low value for the
intracluster magnetic field implies a large cosmic ray energy density which in
turn is responsible, through neutral pion decay, for a gamma ray flux above 100
MeV which exceeds the EGRET upper limit. This gamma ray bound can be relaxed if
the hard X-ray excess and the radio halo emission from Coma are not due to the
same population of electrons. We finally stress the unique role that the new
generation gamma ray satellites will play to discriminate among different
models for the non thermal emission in clusters of galaxies.Comment: 25 pages, 3 Figures, Latex (using epsfig,elsart), to appear in
Astroparticle Physics. Astroparticle Physics, in pres
Escape of cosmic rays from the Galaxy and effects on the circumgalactic medium
The escape of cosmic rays from the Galaxy is expected to shape their spectrum
inside the Galaxy. Yet, this phenomenon is very poorly understood and, in the
absence of a physical description, it is usually modelled as free escape from a
given boundary, typically located at a few kpc distance from the Galactic disc.
We show that the assumption of free escape leads to the conclusion that the
cosmic ray current propagating in the circumgalactic medium is responsible for
a non resonant cosmic ray induced instability that in turn leads to the
generation of a magnetic field of strength Gauss on a
scale kpc around our Galaxy. The self-generated diffusion produces
large gradients in the particle pressure that induce a displacement of the
intergalactic medium with velocity km/s. Cosmic rays are then
carried away by advection. If the overdensity of the intergalactic gas in a
region of size kpc around our Galaxy is with respect to
the cosmological baryon density , then the flux of high
energy neutrinos as due to pion production becomes comparable with the flux of
astrophysical neutrinos recently measured by IceCube.Comment: Accepted for publication in Phys. Rev. Letter
Small Scale Anisotropies of UHECRs from Super-Heavy Halo Dark Matter
The decay of very heavy metastable relics of the Early Universe can produce
ultra-high energy cosmic rays (UHECRs) in the halo of our own Galaxy. In this
model, no Greisen-Zatsepin-Kuzmin cutoff is expected because of the short
propagation distances. We show here that, as a consequence of the hierarchical
build up of the halo, this scenario predicts the existence of small scale
anisotropies in the arrival directions of UHECRs, in addition to a large scale
anisotropy, known from previous studies. We also suggest some other observable
consequences of this scenario which will be testable with upcoming experiments,
as Auger, EUSO and OWL.Comment: Contribution given at ICRC 2001 - August 7-15, 2001 - Hambur
On the radial distribution of Galactic cosmic rays
The spectrum and morphology of the diffuse Galactic gamma-ray emission
carries valuable information on cosmic ray (CR) propagation. Recent results
obtained by analyzing Fermi-LAT data accumulated over seven years of
observation show a substantial variation of the CR spectrum as a function of
the distance from the Galactic Center. The spatial distribution of the CR
density in the outer Galaxy appears to be weakly dependent upon the
galactocentric distance, as found in previous studies as well, while the
density in the central region of the Galaxy was found to exceed the value
measured in the outer Galaxy. At the same time, Fermi-LAT data suggest a
gradual spectral softening while moving outward from the center of the Galaxy
to its outskirts. These findings represent a challenge for standard
calculations of CR propagation based on assuming a uniform diffusion
coefficient within the Galactic volume. Here we present a model of non-linear
CR propagation in which transport is due to particle scattering and advection
off self-generated turbulence. We find that for a realistic distribution of CR
sources following the spatial distribution of supernova remnants and the space
dependence of the magnetic field on galactocentric distance, both the spatial
profile of CR density and the spectral softening can easily be accounted for.Comment: 6 pages, 3 figures. Accepted for publivation to MNRAS letter
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