We have performed magnetohydrodynamical simulations to study the amplification of magnetic fields in the precursors of shock waves. Strong magnetic fields are required in the precursors of the strong shocks that occur in supernova remnants. Observati
ons also suggest that magnetic field amplification takes place in the weak shocks that occur in galaxy clusters and that produce so-called radio relics. Here, we extend the study of magnetic field amplification by cosmic-ray driven turbulence to weak shocks. The amplification is driven by turbulence that is produced by the cosmic-ray pressure acting on the density inhomogeneities in the upstream fluid. The clumping that has been inferred from X-ray data for the outskirts of galaxy clusters could provide some of the seed inhomogeneities. Magnetic field power spectra and Faraday maps are produced. Furthermore, we investigate how the synchrotron emission in the shock precursor can be used to verify the existence of this instability and constrain essential plasma parameters.
The diffuse meta-galactic radiation field at ultraviolet to infrared wavelengths - commonly labeled extragalactic background light (EBL) - contains the integrated emission history of the universe. Difficult to access via direct observations, indirect
constraints on its density can be derived through observations of very-high energy (VHE; E>100 GeV) gamma-rays from distant sources: the VHE photons are attenuated via pair-production with the low energy photons from the EBL, leaving a distinct imprint in the VHE spectra measured on earth. Discoveries made with current-generation VHE observatories like H.E.S.S., MAGIC and VERITAS enabled strong constraints on the density of the EBL, especially in the near-infrared. Here, the constrains on the EBL density from such ground based VHE observations will be briefly reviewed and the potential of the next-generation instruments to improve on these limits will be discussed.
Modern astrophysics, especially at GeV energy scales and above is a typical example where several disciplines meet: The location and distribution of the sources is the domain of astronomy. At distances corresponding to significant redshift cosmologic
al aspects such as the expansion history come into play. Finally, the emission mechanisms and subsequent propagation of produced high energy particles is at least partly the domain of particle physics, in particular if new phenomena beyond the Standard Model are probed that require base lines and/or energies unattained in the laboratory. In this contribution we focus on three examples: Highest energy cosmic rays, tests of the Lorentz symmetry and the search for new light photon-like states in the spectra of active galaxies.
Observations of distant sources of high-energy (HE) gamma-rays are affected by attenuation resulting from the interaction of the gamma-rays with low energy photons from the diffuse meta-galactic radiation fields at ultraviolet (UV) to infrared (IR) w
avelengths (Extragalactic Background Light; EBL). Recently, a large data-set of HE observations from the 1st year survey of the Large Area Telescope (LAT) instrument on-board of the Fermi satellite became available, covering an energy range from 100 MeV up to 100 GeV. In this paper, the potential of such large HE data-sets to probe the density of the EBL - especially in the UV to optical - is explored. The data from the catalog is investigated for an attenuation signature in the energy range 10-100 GeV and the results are compared with the predictions from EBL model calculations. No clear signature is found. The statistics are still limited by (1) the sensitivity of Fermi/LAT to detect sources above 10 GeV, (2) the number of firmly identified sources with known redshift, both which will improve over the coming years.
The diffuse meta-galactic radiation field at ultraviolet to infrared wavelengths - commonly labeled extragalactic background light (EBL) - contains the integrated emission history of the universe. Difficult to access via direct observations indirect
constraints on its density can be derived through observations of very-high energy (VHE; E>100 GeV) gamma-rays from distant sources: the VHE photons are attenuated via pair-production with the low energy photons from the EBL, leaving a distinct imprint in the VHE spectra measured on earth. Discoveries made with current generation VHE observatories like H.E.S.S. and MAGIC enabled strong constraints on the density of the EBL especially in the near-infrared. In this article the prospect of future VHE observatories to derive new constraints on the EBL density are discussed. To this end, results from current generation instruments will be extrapolated to the future experiments sensitivity and investigated for their power to enable new methods and improved constraints on the EBL density.
The high-lights of ground-based very-high-energy (VHE, $E>100$ GeV) gamma-ray astronomy are reviewed. The summary covers both Galactic and extra-galactic sources. A total of at least 70 sources are currently known. Implications for our understanding
of the non-thermal Universe are discussed. The next generation of ground based gamma-ray instruments aims to cover the entire accessible energy range from as low as $approx 10$ GeV up to $10^5$ GeV and to improve the sensitivity by an order of magnitude in comparison with current instruments.
