Abstract. The production of K + and of K − mesons in heavy-ion collisions at beam energies of 1 to 2 AGeV has systematically been investigated with the Kaon Spectrometer KaoS. The ratio of the K + production excitation function for Au+Au and for C+C reactions increases with decreasing beam energy, which is expected for a soft nuclear equation-of-state. A comprehensive study of the K + and of the K − emission as a function of the size of the collision system, of the collision centrality, of the kaon energy, and of the polar emission angle has been performed. The K − /K + ratio is found to be nearly constant as a function of the collision centrality and can be explained by the dominance of strangeness exchange. On the other hand the spectral slopes and the polar emission patterns are different for K − and for K + . Furthermore the azimuthal distribution of the particle emission has been investigated. K + mesons and pions are emitted preferentially perpendicular to the reaction plane as well in Au+Au as in Ni+Ni collisions. In contrast for K − mesons in Ni+Ni reactions an in-plane flow was observed for the first time at these incident enegies.

Aims. Results obtained in very-high-energy (VHE; E ≥ 100 GeV) γ-ray observations performed with the H.E.S.S. telescope array are used to investigate particle acceleration processes in the vicinity of the young massive stellar cluster Westerlund 1 (Wd 1). Methods. Imaging of Cherenkov light from γ-ray induced particle cascades in the Earth's atmosphere is used to search for VHE γ rays from the region around Wd 1. Possible catalogued counterparts are searched for and discussed in terms of morphology and energetics of the H.E.S.S. source. Results. The detection of the degree-scale extended VHE γ-ray source HESS J1646-458 is reported based on 45 h of H.E.S.S. observations performed between 2004 and 2008. The VHE γ-ray source is centred on the nominal position of Wd 1 and detected with a total statistical significance of ∼20σ. The emission region clearly extends beyond the H.E.S.S. point-spread function (PSF). The differential energy spectrum follows a power law in energy with an index of Γ = 2.19 ± 0.08 stat ± 0.20 sys and a flux normalisation at 1 TeV of Φ 0 = (9.0 ± 1.4 stat ± 1.8 sys ) × 10 −12 TeV −1 cm −2 s −1 . The integral flux above 0.2 TeV amounts to (5.2 ± 0.9) × 10 −11 cm −2 s −1 . Conclusions. Four objects coincident with HESS J1646-458 are discussed in the search of a counterpart, namely the magnetar CXOU J164710.2−455216, the X-ray binary 4U 1642-45, the pulsar PSR J1648-4611 and the massive stellar cluster Wd 1. In a single-source scenario, Wd 1 is favoured as site of VHE particle acceleration. Here, a hadronic parent population would be accelerated within the stellar cluster. Beside this, there is evidence for a multi-source origin, where a scenario involving PSR J1648-4611 could be viable to explain parts of the VHE γ-ray emission of HESS J1646-458.

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The accretion of matter onto a massive black hole is believed to feed the relativistic plasma jets found in many active galactic nuclei (AGN). Although some AGN accelerate particles to energies exceeding 10(12) electron volts and are bright sources of very-high-energy (VHE) gamma-ray emission, it is not yet known where the VHE emission originates. Here we report on radio and VHE observations of the radio galaxy Messier 87, revealing a period of extremely strong VHE gamma-ray flares accompanied by a strong increase of the radio flux from its nucleus. These results imply that charged particles are accelerated to very high energies in the immediate vicinity of the black hole.

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. An annihilation signal of dark matter is searched for from the central region of the Milky Way. Data acquired in dedicated on-off observations of the Galactic center region with H.E.S.S. are analyzed for this purpose. No significant signal is found in a total of ∼9 h of on-off observations. Upper limits on the velocity averaged cross section, hσvi, for the annihilation of dark matter particles with masses in the range of ∼300 GeV to ∼10 TeV are derived. In contrast to previous constraints derived from observations of the Galactic center region, the constraints that are derived here apply also under the assumption of a central core of constant dark matter density around the center of the Galaxy. Values of hσvi that are larger than 3 × 10 −24 cm 3 =s are excluded for dark matter particles with masses between ∼1 and ∼4 TeV at 95% C.L. if the radius of the central dark matter density core does not exceed 500 pc. This is the strongest constraint that is derived on hσvi for annihilating TeV mass dark matter without the assumption of a centrally cusped dark matter density distribution in the search region. Introduction.-The formation of the large scale structure of the universe as well as the dynamics of galaxy clusters and individual galaxies strongly suggest the presence of dark matter on the respective length scale [1]. Many extensions of the standard model of particle physics predict a stable particle without electromagnetic coupling whose presence can account for the missing mass that is apparent in astrophysical environments [1]. The annihilation of dark matter particles is expected to produce photons with energies up to the mass of the dark matter particles [2]. The detection of γ rays from a given direction can thus indirectly probe the presence of dark matter particles along the corresponding line of sight.The central region of the Milky Way is of particular interest for indirect searches for annihilating dark matter because the squared dark matter density integrated over the line of sight towards the target region (i.e., the astrophysical or J factor) is expected to be large [3]. The J factor for observations of the Galactic center region depends strongly on the dark matter density distribution within the Milky Way. Simulations of the dynamics of the dark matter content of galaxies predict to universal dark matter density distributions. Towards the center of the galaxies, the influence of baryons on the distribution of dark matter is not yet resolved. The formation of pronounced den...