The diffuse extragalactic background light consists of the sum of the starlight emitted by galaxies through the history of the Universe, and it could also have an important contribution from the first stars, which may have formed before galaxy formation began. Direct measurements are difficult and1 not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light 1 . An alternative approach 2-5 is to study the absorption features imprinted on the γ-ray spectra of distant extragalactic objects by interactions of those photons with the background light photons 6 . Here we report the discovery of γ-ray emission from the blazars 7 H 2356−309 and 1ES 1101−232, at redshifts z=0.165 and z=0.186, respectively. Their unexpectedly hard spectra provide an upper limit on the background light at optical/near-infrared wavelengths that appears to be very close to the lower limit given by the integrated light of resolved galaxies 8 . The background flux at these wavelengths accordingly seems to be strongly dominated by the direct starlight from galaxies, thus excluding a large contribution from other sources -in particular from the first stars formed 9 . This result also indicates that intergalactic space is more transparent to γ-rays than previously thought.The observations were carried out with the High Energy Stereoscopic System 10 (H.E.S.S. ), a system of four imaging atmospheric Cherenkov telescopes operating at energies E ≥ 0.1 TeV. These two blazars are at present the most distant sources for which spectra have been measured at these energies (Tab. 1).Intergalactic absorption is caused by the process of photon-photon collision and pair production. The original spectrum emitted by the source (which we call "intrinsic") is modified such that the observed flux E) , where the optical depth τ (E) depends on the Spectral Energy Distribution (SED) of the Extragalactic Background Light (EBL) (Fig. 1). Details are provided in the Supplementary Notes and Figures. For any reasonable range of fluxes at ultraviolet (UV) and optical/near-infrared wavelengths (O-NIR), τ (E) -and thus absorption -is larger at 1 TeV with respect to 0.2 TeV. This difference makes the observed spectrum steeper (that is, Γ obs > Γ int , for a power-law model dN/dE ∝ E −Γ ) The spectral change ∆Γ=Γ obs − Γ int scales linearly with the EBL normalization, and becomes more pronounced at larger redshifts. Thus more distant objects provide a more sensitive diagnostic tool.In general, if the intrinsic spectrum were sufficiently well known, τ (E) -and thus the EBL SEDcould be effectively measured by comparing intrinsic with observed spectra. Blazars, however, are characterized by a wide range of possible spectra, and the present understanding of their radiation processes is not yet complete enough to reliably predict their intrinsic γ-ray spectra. But for these two sources, with O-NIR fluxes at the level of the "direct" estimates, the intrinsic spectra needed to reproduce the H.E.S.S. data become extremely...

The nonthermal particle production in contemporary starburst galaxies and in galaxy clusters is estimated from the Supernova rate, the iron content, and an evaluation of the dynamical processes which characterize these objects. The prlrnary energy derives from SN explosions of massive stars. The nonthermal energy is tramfformed by various secondary processes, like acceleration of particles by Supernova ttemnants as well as diffusion and/or convection in galactic winds. If convection dominates, the energy spectrum of nonthermal particles will remain hard. At greater distances from the galaxy almost the entire enthalpy of thermal gas and Cosmic Rays will be converted into wind kinetic energy, implying a fatal adiabatic energy loss for the nonthermal component. If this wind is strong enough then it will end in a strong termination shock, producing a new generation of nonthermal particles which are subsequently released without significant adiabatic losses into the external medium. In clusters of galaxies this should only be the case for early type galaxies, in agreement with observations. Clusters should also accurmflate their nontherreal component over their entire history and energize it by gravitational contraction. The pion decay T-ray fluxes of nearby contemporary starburst galaxies is quite small. However rich clusters should be extended sources of very high energy 7-rays~, detectable by the next generation of systems of air Cherenkov telescopes. Such observations will provide an independent empirical method to investigate these objects and their cosmological history.

Abstract. The time-dependent nonlinear kinetic theory for cosmic ray (CR) acceleration in supernova remnants (SNRs) is applied studying the properties of the synchrotron emission from SNRs, in particular, the surface brightness-diameter (Σ − D) relation. Detailed numerical calculations are performed for the expected range of the relevant physical parameters, namely the ambient density and the supernova explosion energy. The magnetic field in SNRs is assumed to be significantly amplified by the efficiently accelerating nuclear CR component. Due to the growing number of accelerated CRs the expected SNR luminosity increases during the free expansion phase, reaches a peak value at the beginning of the Sedov phase and then decreases again, since in this stage the overall CR number remains nearly constant, whereas the effective magnetic field diminishes with time. The theoretically predicted brightness-diameter relation in the radio range in the Sedov phase is close to Σ R ∝ D −17/4 . It fits the observational data in a very satisfactory way. The observed spread of Σ R at a given SNR size D is the result of the spread of supernova explosion energies and interstellar medium densities.

Abstract.We report the detection of a point-like source of very high energy (VHE) γ-rays coincident within 1 of Sgr A * , obtained with the HESS array of Cherenkov telescopes. The γ-rays exhibit a power-law energy spectrum with a spectral index of −2.2 ± 0.09 ± 0.15 and a flux above the 165 GeV threshold of (1.82 ± 0.22) × 10 −7 m −2 s −1 . The measured flux and spectrum differ substantially from recent results reported in particular by the CANGAROO collaboration.

Abstract. It is shown that the nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants (SNRs) describes the shell-type nonthermal X-ray morphology of Cas A, obtained in Chandra observations, in a satisfactory way. The set of empirical parameters, like distance, source size and total energy release, is the same which reproduces the dynamical properties of the SNR and the spectral characteristics of the emission produced by CRs. The extremely small spatial scales in the observed morphological structures at hard X-ray energies are due to a large effective magnetic field B d ∼ 500 µG in the interior which is at the same time not only required but also sufficient to achieve the excellent agreement between the spatially integrated radio and X-ray synchrotron spectra and their calculated form. The only reasonably thinkable condition for the production of such a large effective field strength is a very efficiently accelerated nuclear CR component. Therefore the Chandra data confirm first of all the inference that Cas A indeed accelerates nuclear CRs with the high efficiency required for Cas A to be considered as a member of the main class of Galactic CR sources and, secondly, that the nonlinear kinetic theory of CR acceleration in SNRs is a reliable method to determine the magnetic field value in SNRs.