No abstract

Gamma-ray line signatures can be expected in the very-high-energy (E(γ)>100 GeV) domain due to self-annihilation or decay of dark matter (DM) particles in space. Such a signal would be readily distinguishable from astrophysical γ-ray sources that in most cases produce continuous spectra that span over several orders of magnitude in energy. Using data collected with the H.E.S.S. γ-ray instrument, upper limits on linelike emission are obtained in the energy range between ∼ 500 GeV and ∼ 25 TeV for the central part of the Milky Way halo and for extragalactic observations, complementing recent limits obtained with the Fermi-LAT instrument at lower energies. No statistically significant signal could be found. For monochromatic γ-ray line emission, flux limits of (2 × 10(-7) -2 × 10(-5)) m(-2) s(-1) sr(-1) and (1 × 10(-8) -2 × 10(-6)) m(-2) s(-1)sr(-1) are obtained for the central part of the Milky Way halo and extragalactic observations, respectively. For a DM particle mass of 1 TeV, limits on the velocity-averaged DM annihilation cross section ⟨σv⟩(χχ → γγ) reach ∼ 10(-27) cm(3)s(-1), based on the Einasto parametrization of the Galactic DM halo density profile.

The measurement of an excess in the cosmic-ray electron spectrum between 300 and 800 GeV by the ATIC experiment has -together with the PAMELA detection of a rise in the positron fraction up to ≈100 GeV -motivated many interpretations in terms of dark matter scenarios; alternative explanations assume a nearby electron source like a pulsar or supernova remnant. Here we present a measurement of the cosmic-ray electron spectrum with H.E.S.S. starting at 340 GeV. While the overall electron flux measured by H.E.S.S. is consistent with the ATIC data within statistical and systematic errors, the H.E.S.S. data exclude a pronounced peak in the electron spectrum as suggested for interpretation by ATIC. The H.E.S.S. data follow a power-law spectrum with spectral index of 3.0 ± 0.1(stat.) ± 0.3(syst.), which steepens at about 1 TeV.

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.

Aims. A detailed study of the spectrum and variability of the source HESS J1745−290 in the Galactic Center (GC) region using new data from the H.E.S.S. array of Cherenkov telescopes is presented. Flaring activity and quasi periodic oscillations (QPO) of HESS J1745−290 are investigated. Methods. The image analysis is performed with a combination of a semi-analytical shower model and the statistical moment-based Hillas technique. The spectrum and lightcurves of HESS J1745−290 are derived with a likelihood method based on a spectral shape hypothesis. Rayleigh tests and Fourier analysis of the H.E.S.S. GC signal are used to study the periodicity of the source. Results. With a three-fold increase in statistics compared to previous work, a deviation from a simple power law spectrum is detected for the first time. The measured energy spectrum over the three years 2004, 2005 and 2006 of data taking is compatible with both a power law spectrum with an exponential cut-off and a broken power law spectrum. The curvature of the energy spectrum is likely to be intrinsic to the photon source, as opposed to effects of interstellar absorption. The power law spectrum with an exponential cut-off is characterized by a photon index of 2.10 ± 0.04 stat ± 0.10 syst and a cut-off energy at 15.7 ± 3.4 stat ± 2.5 syst TeV. The broken power law spectrum exhibits spectral indices of 2.02 ± 0.08 stat ± 0.10 syst and 2.63 ± 0.14 stat ± 0.10 syst with a break energy at 2.57 ± 0.19 stat ± 0.44 syst TeV. No significant flux variation is found. Increases in the γ-ray flux of HESS J1745−290 by at least a factor of two would be required for a 3σ detection of a flare with time scales of an hour. Investigation of possible QPO activity at periods claimed to be detected in X-rays does not show any periodicities in the H.E.S.S. signal.