The Origin of Galactic Cosmic Rays

Motivated by recent measurements of the major components of the cosmic radiation around 10 TeV/nucleon and above, we discuss the phenomenology of a model in which there are two distinct kinds of cosmic ray accelerators in the galaxy. Comparison of the spectra of hydrogen and helium up to 100 TeV per nucleon suggests that these two elements do not have the same spectrum of magnetic rigidity over this entire region and that these two dominant elements therefore receive contributions from different sources.Comment: To be published in Physical Review D, 13 pages, with 3 figures, uuencode

Sterile neutrino production via active-sterile oscillations: the quantum Zeno effect

We study several aspects of the kinetic approach to sterile neutrino production via active-sterile mixing. We obtain the neutrino propagator in the medium including self-energy corrections up to $\mathcal{O}(G^2_F)$, from which we extract the dispersion relations and damping rates of the propagating modes. The dispersion relations are the usual ones in terms of the index of refraction in the medium, and the damping rates are $\Gamma_1(k) = \Gamma_{aa}(k) \cos^2\theta_m(k); \Gamma_2(k) = \Gamma_{aa}(k) \sin^2\theta_m(k)$ where $\Gamma_{aa}(k)\propto G^2_F k T^4$ is the active neutrino scattering rate and $\theta_m(k)$ is the mixing angle in the medium. We provide a generalization of the transition probability in the \emph{medium from expectation values in the density matrix}: $P_{a\to s}(t) = \frac{\sin^22\theta_m}{4}[e^{-\Gamma_1t} + e^{-\Gamma_2 t}-2e^{-{1/2}(\Gamma_1+\Gamma_2)t} \cos\big(\Delta E t\big)]$ and study the conditions for its quantum Zeno suppression directly in real time. We find the general conditions for quantum Zeno suppression, which for $m_s\sim \textrm{keV}$ sterile neutrinos with $\sin2\theta \lesssim 10^{-3}$ \emph{may only be} fulfilled near an MSW resonance. We discuss the implications for sterile neutrino production and argue that in the early Universe the wide separation of relaxation scales far away from MSW resonances suggests the breakdown of the current kinetic approach.Comment: version to appear in JHE

A new estimate of the extragalactic radio background and implications for ultra-high-energy gamma-ray propagation

We make a new estimate of the extragalactic radio background down to kHz frequencies based on the observed luminosity functions and radio spectra of normal galaxies and radio galaxies. We have constructed models for the spectra of these two classes of objects down to low frequencies based on observations of our Galaxy, other normal galaxies and radio galaxies. We check that the models and evolution of the luminosity functions give source counts consistent with data and calculate the radio background expected from kHz to GHz frequencies. The motivation for this calculation is that the propagation of ultra-high energy gamma-rays in the universe is limited by photon-photon pair production on the radio background. Electromagnetic cascades involving photon-photon pair production and subsequent synchrotron radiation in the intergalactic magnetic field may develop. Such gamma-rays may be produced in acceleration sites of ultra-high energy cosmic rays, as a result of interactions with the microwave background, or emitted as a result of decay or annihilation of topological defects. We find that photon-photon pair production on the radio background remains the dominant attenuation process for gamma-rays from $3 \times 10^{10}$ GeV up to GUT scale energies.Comment: LaTeX, 21 pages, including 10 postscript figures, tar'd and gzip'

Turbulent diffusion and drift in galactic magnetic fields and the explanation of the knee in the cosmic ray spectrum

We reconsider the scenario in which the knee in the cosmic ray spectrum is explained as due to a change in the escape mechanism of cosmic rays from the Galaxy from one dominated by transverse diffusion to one dominated by drifts. We solve the diffusion equations adopting realistic galactic field models and using diffusion coefficients appropriate for strong turbulence (with a Kolmogorov spectrum of fluctuations) and consistent with the assumed magnetic fields. We show that properly taking into account these effects leads to a natural explanation of the knee in the spectrum, and a transition towards a heavier composition above the knee is predicted.Comment: 17 pp., 6 figures; revised version with minor changes. To appear in JHE

Ultra-High Energy Cosmic Rays from Neutrino Emitting Acceleration Sources?

We demonstrate by numerical flux calculations that neutrino beams producing the observed highest energy cosmic rays by weak interactions with the relic neutrino background require a non-uniform distribution of sources. Such sources have to accelerate protons at least up to 10^{23} eV, have to be opaque to their primary protons, and should emit the secondary photons unavoidably produced together with the neutrinos only in the sub-MeV region to avoid conflict with the diffuse gamma-ray background measured by the EGRET experiment. Even if such a source class exists, the resulting large uncertainties in the parameters involved in this scenario does currently not allow to extract any meaningful information on absolute neutrino masses.Comment: 6 pages, 4 figures, RevTeX styl

Radio detection of cosmic ray air showers with LOPES

In the last few years, radio detection of cosmic ray air showers has experienced a true renaissance, becoming manifest in a number of new experiments and simulation efforts. In particular, the LOPES project has successfully implemented modern interferometric methods to measure the radio emission from extensive air showers. LOPES has confirmed that the emission is coherent and of geomagnetic origin, as expected by the geosynchrotron mechanism, and has demonstrated that a large scale application of the radio technique has great potential to complement current measurements of ultra-high energy cosmic rays. We describe the current status, most recent results and open questions regarding radio detection of cosmic rays and give an overview of ongoing research and development for an application of the radio technique in the framework of the Pierre Auger Observatory.Comment: 8 pages; Proceedings of the CRIS2006 conference, Catania, Italy; to be published in Nuclear Physics B, Proceedings Supplement

Extragalactic Sources for Ultra High Energy Cosmic Ray Nuclei

In this article we examine the hypothesis that the highest energy cosmic rays are complex nuclei from extragalactic sources. Under reasonable physical assumptions, we show that the nearby metally rich starburst galaxies (M82 and NGC 253) can produce all the events observed above the ankle. This requires diffusion of particles below $10^{20}$ eV in extragalactic magnetic fields $B \approx 15$ nG. Above $10^{19}$ eV, the model predicts the presence of significant fluxes of medium mass and heavy nuclei with small rate of change of composition. Notwithstanding, the most salient feature of the starburst-hypothesis is a slight anisotropy induced by iron debris just before the spectrum-cutoff.Comment: To appear in Phys. Rev. D, reference adde

A lower bound on the local extragalactic magnetic field

Assuming that the hard gamma-ray emission of Cen A is a result of synchrotron radiation of ultra-relativistic electrons, we derive a lower bound on the local extragalactic magnetic field, $B> 10^{-8}$ G. This result is consistent with (and close to) upper bounds on magnetic fields derived from consideration of cosmic microwave background distortions and Faraday rotation measurements.Comment: Includes extensive discussion of particle acceleration above 10^20 eV in the hot spot-like region of Cen

The clustering of ultra-high energy cosmic rays and their sources

The sky distribution of cosmic rays with energies above the 'GZK cutoff' holds important clues to their origin. The AGASA data, although consistent with isotropy, shows evidence for small-angle clustering, and it has been argued that such clusters are aligned with BL Lacertae objects, implicating these as sources. It has also been suggested that clusters can arise if the cosmic rays come from the decays of very massive relic particles in the Galactic halo, due to the expected clumping of cold dark matter. We examine these claims and show that both are in fact not justified.Comment: 13 pages, 8 figures, version in press at Phys. Rev.

Propagation of ultra-high energy protons in regular extragalactic magnetic fields

We study the proton flux expected from sources of ultra high energy cosmic rays (UHECR) in the presence of regular extragalactic magnetic fields. It is assumed that a local source of ultra-high energy protons and the magnetic field are all in a wall of matter concentration with dimensions characteristic of the supergalactic plane. For a single source, the observed proton flux and the local cosmic ray energy spectrum depend strongly on the strength of the field, the position of the observer, and the orientation of the field relative to the observer's line of sight. Regular fields also affect protons emitted by sources outside the local magnetic fields structure. We discuss the possibility that such effects could contribute to an explanation of the excess of UHECR above $5.10^{19}$ eV, and the possibility that sources of such particles may be missed if such magnetic fields are not taken into account.Comment: 6 pages, 4 figures Comments for revised version: 12 pages, 12 figures. Enlarged discussion of effects on cosmic ray spectrum. Additional discussion focussing on spatial and temporal boundary condition