555 research outputs found
Diffusive propagation of UHECR and the propagation theorem
We present a detailed analytical study of the propagation of ultra high
energy (UHE) particles in extragalactic magnetic fields. The crucial parameter
which affects the diffuse spectrum is the separation between sources. In the
case of a uniform distribution of sources with a separation between them much
smaller than all characteristic propagation lengths, the diffuse spectrum of
UHE particles has a {\em universal} form, independent of the mode of
propagation. This statement has a status of theorem. The proof is obtained
using the particle number conservation during propagation, and also using the
kinetic equation for the propagation of UHE particles. This theorem can be also
proved with the help of the diffusion equation. In particular, it is shown
numerically, how the diffuse fluxes converge to this universal spectrum, when
the separation between sources diminishes. We study also the analytic solution
of the diffusion equation in weak and strong magnetic fields with energy losses
taken into account. In the case of strong magnetic fields and for a separation
between sources large enough, the GZK cutoff can practically disappear, as it
has been found early in numerical simulations. In practice, however, the source
luminosities required are too large for this possibility.Comment: 16 pages, 13 eps figures, discussion of the absence of the GZK
cut-off in strong magnetic field added, a misprint in figure 6 corrected,
version accepted for publication in Ap
Signatures of Topological Defects
We argue that due to various restrictions cosmic strings and monopole-string
networks are not likely to produce the observed flux of ultra-high energy
cosmic rays (UHECR). Among the topological defects studied so far, the most
promising UHECR sources are necklaces and monopolonia. Other viable sources
which are similar to topological defects are relic superheavy particles. All
these sources have an excess of pions (and thus photons) over nucleons at
production. We demonstrate that in the case of necklaces the diffuse proton
flux can be larger than photon flux, due to absorption of the latter on
radiobackground, while monopolonia and relic particles are concentrated in the
Galactic halo, and the photon flux dominates. Another signature of the latter
sources is anisotropy imposed by asymmetric position of the sun in the Galactic
halo. In all cases considered so far, including necklaces, photons must be
present in ultra-high energy radiation observed from topological defects, and
experimental discrimination between photon-induced and proton-induced extensive
air showers can give a clue to the origin of ultra-high energy cosmic rays.Comment: version accepted for publication in Phys. Rev. D. No changes in the
conclusions and in figure
On the viability of holistic cosmic-ray source models
We consider the energy spectrum of cosmic-rays (CRs) from a purely
phenomenological point of view and investigate the possibility that they all be
produced by the same type of sources with a single power-law spectrum, in
E^{-x}, from thermal to ultra-high energies. We show that the relative fluxes
of the Galactic (GCR) and extra-galactic (EGCR) components are compatible with
such a holistic model, provided that the index of the source spectrum be x
\simeq 2.23\pm 0.07. This is compatible with the best-fit indices for both GCRs
and EGCRs, assuming that their source composition is the same, which is indeed
the case in a holistic model. It is also compatible with theoretical
expectations for particle acceleration at relativistic shocks.Comment: 5 pages, 1 figure, Accepted for publication in Astronomy and
Astrophysic
Galactic Anisotropy as Signature of ``Top-Down'' Mechanisms of Ultra-High Energy Cosmic Rays
We show that ``top-down'' mechanisms of Ultra-High Energy Cosmic Rays which
involve heavy relic particle-like objects predict Galactic anisotropy of
highest energy cosmic rays at the level of minimum . This anisotropy
is large enough to be either observed or ruled out in the next generation of
experiments.Comment: 8 pages, 1 figure, LaTeX. Final version appeared in Pisma Zh. Eksp.
Teor. Fi
Electroweak jet cascading in the decay of superheavy particles
We study decays of superheavy particles into leptons. We show that they
initiate cascades similar to QCD parton jets, if m_X\gsim 10^6 GeV.
Electroweak cascading is studied and the energy spectra of the produced leptons
are calculated in the framework of a broken SU(2) model of weak interactions.
As application, important for the Z-burst model for ultrahigh energy cosmic
rays, we consider decays of superheavy particles coupled on tree-level only to
neutrinos and derive stringent limit for these decays from the observed diffuse
extragalactic -ray flux.Comment: 4 pages, 1 eps figur
Bounds on the cosmogenic neutrino flux
Under the assumption that some part of the observed highest energy cosmic
rays consists of protons originating from cosmological distances, we derive
bounds on the associated flux of neutrinos generated by inelastic processes
with the cosmic microwave background photons. We exploit two methods. First, a
power-like injection spectrum is assumed. Then, a model-independent technique,
based on the inversion of the observed proton flux, is presented. The inferred
lower bound is quite robust. As expected, the upper bound depends on the
unknown composition of the highest energy cosmic rays. Our results represent
benchmarks for all ultrahigh energy neutrino telescopes.Comment: 12 pages, 6 figure
Cosmic Necklaces and Ultrahigh Energy Cosmic Rays
Cosmic necklaces are hybrid topological defects consisting of monopoles and
strings, with two strings attached to each monopole. We argue that the
cosmological evolution of necklaces may significantly differ from that of
cosmic strings. The typical velocity of necklaces can be much smaller than the
speed of light, and the characteristic scale of the network much smaller than
the horizon. We estimate the flux of high-energy protons produced by monopole
annihilation in the decaying closed loops. For some reasonable values of the
parameters it is comparable to the observed flux of ultrahigh-energy cosmic
rays.Comment: 10 pages, Revtex, 1 figur
Unstable superheavy relic particles as a source of neutrinos responsible for the ultrahigh-energy cosmic rays
Decays of superheavy relic particles may produce extremely energetic
neutrinos. Their annihilations on the relic neutrinos can be the origin of the
cosmic rays with energies beyond the Greisen-Zatsepin-Kuzmin cutoff. The red
shift acts as a cosmological filter selecting the sources at some particular
value z_e, for which the present neutrino energy is close to the Z pole of the
annihilation cross section. We predict no directional correlation of the
ultrahigh-energy cosmic rays with the galactic halo. At the same time, there
can be some directional correlations in the data, reflecting the distribution
of matter at red shift z=z_e. Both of these features are manifest in the
existing data. Our scenario is consistent with the neutrino mass reported by
Super-Kamiokande and requires no lepton asymmetry or clustering of the
background neutrinos.Comment: 3 pages, revtex; references adde
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