Magnetic-field-line random walk in turbulence: a two-point correlation function description

The standard theory of field-line random walk is based on the description of turbulence in the wave number space. The latter description takes into account the details of turbulence at small scales. Such details, however, are not important in the theory of random walking magnetic field lines. In the present paper we therefore use simple decorrelation models to estimate the field-line diffusion coefficient. Previous results are recovered as special limits. It is shown that a full analytical description is possible and that the only parameter controlling the field-line diffusion coefficient is the Kubo number.

Search for a dark matter annihilation signal from the galactic center halo with H.E.S.S

A search for a very-high-energy (VHE; ≥100  GeV) γ-ray signal from self-annihilating particle dark matter (DM) is performed towards a region of projected distance r∼45-150  pc from the Galactic center. The background-subtracted γ-ray spectrum measured with the High Energy Stereoscopic System (H.E.S.S.) γ-ray instrument in the energy range between 300 GeV and 30 TeV shows no hint of a residual γ-ray flux. Assuming conventional Navarro-Frenk-White and Einasto density profiles, limits are derived on the velocity-weighted annihilation cross section (σv) as a function of the DM particle mass. These are among the best reported so far for this energy range and in particular differ only little between the chosen density profile parametrizations. In particular, for the DM particle mass of ∼1  TeV, values for (σv) above 3×10(-25)  cm(3) s(-1) are excluded for the Einasto density profile.

Applicability of the Taylor-Green-Kubo formula in particle diffusion theory

Diffusion coefficients of particles can be defined as time integrals over velocity correlation functions, or as mean square displacements divided by time. In the present paper it is demonstrated that these two definitions are not equivalent. An exact relation between mean square displacements and velocity correlations is derived. As an example of the applicability of these results so-called drift coefficients of energetic particles are discussed. It is explained why different previous approaches in drift theory provided contradicting results.

Analytical description for field-line wandering in strong magnetic turbulence

We investigate analytically the random walk of magnetic field lines. In previous analytical treatments of field line wandering or random walk, it was assumed that the turbulent magnetic field is much weaker than the mean field. In the present paper, we develop an improved analytical method to describe the stochastic properties of turbulent magnetic fields. This approach is an extension of the standard theory of field line wandering and can be applied to weak as well as to strong magnetic turbulence.

Energy spectrum of cosmic-ray electrons at TeV energies

The very large collection area of ground-based gamma-ray telescopes gives them a substantial advantage over balloon or satellite based instruments in the detection of very-high-energy (>600 GeV) cosmic-ray electrons. Here we present the electron spectrum derived from data taken with the High Energy Stereoscopic System (H.E.S.S.) of imaging atmospheric Cherenkov telescopes. In this measurement, the first of this type, we are able to extend the measurement of the electron spectrum beyond the range accessible to direct measurements. We find evidence for a substantial steepening in the energy spectrum above 600 GeV compared to lower energies.

Limits on an energy dependence of the speed of light from a flare of the active galaxy PKS 2155-304

In the past few decades, several models have predicted an energy dependence of the speed of light in the context of quantum gravity. For cosmological sources such as active galaxies, this minuscule effect can add up to measurable photon-energy dependent time lags. In this Letter a search for such time lags during the High Energy Stereoscopic System observations of the exceptional very high energy flare of the active galaxy PKS 2155-304 on 28 July 2006 is presented. Since no significant time lag is found, lower limits on the energy scale of speed of light modifications are derived.

Fast Variability of Tera–Electron Volt γ Rays from the Radio Galaxy M87

The detection of fast variations of the tera-electron volt (TeV) (10(12) eV) gamma-ray flux, on time scales of days, from the nearby radio galaxy M87 is reported. These variations are about 10 times as fast as those observed in any other wave band and imply a very compact emission region with a dimension similar to the Schwarzschild radius of the central black hole. We thus can exclude several other sites and processes of the gamma-ray production. The observations confirm that TeV gamma rays are emitted by extragalactic sources other than blazars, where jets are not relativistically beamed toward the observer.