5,321 research outputs found
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
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
Cosmic ray driven Galactic winds
The escape of cosmic rays from the Galaxy leads to a gradient in the cosmic
ray pressure that acts as a force on the background plasma, in the direction
opposite to the gravitational pull. If this force is large enough to win
against gravity, a wind can be launched that removes gas from the Galaxy,
thereby regulating several physical processes, including star formation. The
dynamics of these cosmic ray driven winds is intrinsically non-linear in that
the spectrum of cosmic rays determines the characteristics of the wind
(velocity, pressure, magnetic field) and in turn the wind dynamics affects the
cosmic ray spectrum. Moreover, the gradient of the cosmic ray distribution
function causes excitation of Alfven waves, that in turn determine the
scattering properties of cosmic rays, namely their diffusive transport. These
effects all feed into each other so that what we see at the Earth is the result
of these non-linear effects. Here we investigate the launch and evolution of
such winds, and we determine the implications for the spectrum of cosmic rays
by solving together the hydrodynamical equations for the wind and the transport
equation for cosmic rays under the action of self-generated diffusion and
advection with the wind and the self-excited Alfven waves.Comment: 14 pages, 15 figures. Accepted for publication to MNRAS main journa
Instabilities of noncommutative two dimensional BF model
The noncommutative extension of two dimensional BF model is considered. It is
shown that the realization of the noncommutative map via the Groenewold-Moyal
star product leads to instabilities of the action, hence to a non
renormalizable theory.Comment: 9 page
The origin of the positron excess in cosmic rays
We show that the positron excess measured by the PAMELA experiment in the
region between 10 and 100 GeV may well be a natural consequence of the standard
scenario for the origin of Galactic cosmic rays. The 'excess' arises because of
positrons created as secondary products of hadronic interactions inside the
sources, but the crucial physical ingredient which leads to a natural
explanation of the positron flux is the fact that the secondary production
takes place in the same region where cosmic rays are being accelerated.
Therefore secondary positrons (and electrons) participate in the acceleration
process and turn out to have a very flat spectrum, which is responsible, after
propagation in the Galaxy, for the observed positron 'excess'. This effect
cannot be avoided though its strength depends on the values of the
environmental parameters during the late stages of evolution of supernova
remnants.Comment: 4 Pages, 2 figures. Some references and discussion adde
General Solution Of Linear Vector Supersymmetry
We give the general solution of the Ward identity for the linear vector
supersymmetry which characterizes all topological models. Such solution, whose
expression is quite compact and simple, greatly simplifies the study of
theories displaying a supersymmetric algebraic structure, reducing to a few
lines the proof of their possible finiteness. In particular, the cohomology
technology usually involved for the quantum extension of these theories, is
completely bypassed. The case of Chern-Simons theory is taken as an example.Comment: 18 pages, LaTeX, no figure
High Energy Neutrinos from Cosmic Ray Interactions in Clusters of Galaxies
The spatial clustering of galaxies in galaxy clusters implies that the
background of infrared (IR) light in the intracluster medium (ICM) may exceed
the universal background. Cosmic rays injected within the ICM propagate
diffusively and at low enough energies are trapped there for cosmological
times. The photopion production interactions of cosmic rays with the IR photons
are responsible for the generation of neutrinos whose detection may shed some
light on the origin and propagation of high energy cosmic rays in the universe.
Here we discuss our calculations of the flux of neutrinos from single clusters
as well as the contribution of photopion production in clusters of galaxies to
the diffuse neutrino background.Comment: 10 pages, 10 figures, submitted to PR
High Energy Phenomena in Clusters of Galaxies
Several phenomena in high energy astrophysics have been recently related to
clusters of galaxies and to cosmic ray interactions occurring inside these
structures. In many of these phenomena the observable effects depend on the
energy density of cosmic rays confined in the Intra Cluster (IC) medium, which
is a poorly known quantity. We propose here that useful indications about this
quantity can be obtained from present and future observations of galaxy
clusters in the radio and hard X-ray frequency ranges.Comment: 5 pages, 3 Figures, Latex (using espcrc2,epsfig), to appear in the
Proceedings of the TAUP97, Eds. A. DiCredico et al., in press. Send comments
to S.Colafrancesco: [email protected]
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