333 research outputs found
Dark matter distribution in the universe and ultra-high energy cosmic rays
Two of the greatest mysteries of modern physics are the origin of the dark
matter in the universe and the nature of the highest energy particles in the
cosmic ray spectrum. We discuss here possible direct and indirect connections
between these two problems, with particular attention to two cases: in the
first we study the local clustering of possible sources of ultra-high energy
cosmic rays (UHECRs) driven by the local dark matter overdensity. In the second
case we study the possibility that UHECRs are directly generated by the decay
of weakly unstable super heavy dark matter.Comment: 15 pages, 7 figures. Invited Talk at the "International Workshop on
observing UHECRs from space and earth", August 9-12, 2000, Metepec, Puebla
(Mexico
Opening the ultra high energy cosmic ray window from the top
While several arguments can be proposed against the existence of particles
with energy in excess of eV in the cosmic ray spectrum,
these particles are actually observed and their origin seeks for an
explanation. After a description of the problems encountered in explaining
these ultra-high energy cosmic rays (UHECRs) in the context of astrophysical
sources, we will review the so-called {\it Top-Down} (TD) Models, in which
UHECRs are the result of the decay of very massive unstable particles, possibly
created in the Early Universe. Particular emphasis will be given to the
signatures of the TD models, likely to be accessible to upcoming experiments
like Auger.Comment: 13 pages, 3 figures. Invited Talk at the Vulcano Workshop `Frontier
Objects in Astrophysics and Particle Physics', May 22-27, 200
Origin of very high and ultra high energy cosmic rays
While there is some level of consensus on a Galactic origin of cosmic rays up
to the knee ( eV) and on an extragalactic origin of
cosmic rays with energy above eV, the debate on the genesis of
cosmic rays in the intermediate energy region has received much less attention,
mainly because of the ambiguity intrinsic in defining such a region. The energy
range between eV and eV is likely to be the place
where the transition from Galactic to extragalactic cosmic rays takes place.
Hence the origin of these particles, though being of the highest importance
from the physics point of view, it is also one of the most difficult aspects to
investigate. Here I will illustrate some ideas concerning the sites of
acceleration of these particles and the questions that their investigation may
help answer, including the origin of \underline{ultra} high energy cosmic rays.Comment: Solicited Review Paper to appear in 'Comptes Rendus Physique
Multiwavelength observations of clusters of galaxies and the role of cluster mergers
Some clusters of galaxies have been identified as powerful sources of
non-thermal radiation, from the radio to X-ray wavelengths. The classical
models proposed for the explanation of this radiation usually require large
energy densities in cosmic rays in the intracluster medium and magnetic fields
much lower that those measured using the Faraday rotation. We study here the
role that mergers of clusters of galaxies may play in the generation of the
non-thermal radiation, and we seek for additional observable consequences of
the model. We find that if hard X-rays and radio radiation are respectively
interpreted as inverse Compton scattering (ICS) and synchrotron emission of
relativistic electrons, large gamma ray fluxes are produced, and for the Coma
cluster, where upper limits are available, these limits are exceeded. We also
discuss an alternative and testable model that naturally solves the problems
mentioned above.Comment: 8 pages, 4 figures. Contributed Talk at the Vulcano Workshop
`Frontier Objects in Astrophysics and Particle Physics', May 22-27, 200
Cosmic Ray Acceleration in Supernova Remnants
We review the main observational and theoretical facts about acceleration of
Galactic cosmic rays in supernova remnants, discussing the arguments in favor
and against a connection between cosmic rays and supernova remnants, the
so-called supernova remnant paradigm for the origin of Galactic cosmic rays.
Recent developments in the modeling of the mechanism of diffusive shock
acceleration are discussed, with emphasis on the role of 1) magnetic field
amplification, 2) acceleration of nuclei heavier than hydrogen, 3) presence of
neutrals in the circumstellar environment. The status of the supernova-cosmic
ray connection in the time of Fermi-LAT and Cherenkov telescopes is also
discussed.Comment: Invited Plenary review talk at ICATPP 2010, Villa Olmo, Como 7-8
October 201
Origin of Galactic Cosmic Rays
The origin of the bulk of cosmic rays (CRs) observed at Earth is the topic of
a century long investigation, paved with successes and failures. From the
energetic point of view, supernova remnants (SNRs) remain the most plausible
sources of CRs up to rigidity ? 10^6-10^7 GV. This confidence somehow resulted
in the construction of a paradigm, the so-called SNR paradigm: CRs are
accelerated through diffusive shock acceleration in SNRs and propagate
diffusively in the Galaxy in an energy dependent way. Qualitative confirmation
of the SNR acceleration scenario has recently been provided by gamma ray and
X-ray observations. Diffusive propagation in the Galaxy is probed
observationally through measurement of the secondary to primary nuclei flux
ratios (such as B/C). There are however some weak points in the paradigm, which
suggest that we are probably missing some physical ingredients in our models.
The theory of diffusive shock acceleration at SNR shocks predicts spectra of
accelerated particles which are systematically too hard compared with the ones
inferred from gamma ray observations. Moreover, hard injection spectra
indirectly imply a steep energy dependence of the diffusion coefficient in the
Galaxy, which in turn leads to anisotropy larger than the observed one.
Moreover recent measurements of the flux of nuclei suggest that the spectra
have a break at rigidity ? 200 GV, which does not sit well with the common
wisdom in acceleration and propagation. In this paper I will review these new
developments and suggest some possible implications.Comment: Invited Review Talk in SciNeGHE 2012, 20-22 June 2012, Lecce (Italy
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