Gamma-ray line signatures can be expected in the very-high-energy ($E_{\gamma }>100\mathrm{GeV}$) domain due to self-annihilation or decay of dark matter (DM) particles in space. Such a signal would be readily distinguishable from astrophysical $\gamma$-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. $\gamma$-ray instrument, upper limits on linelike emission are obtained in the energy range between $\sim 500\mathrm{GeV}$ and $\sim 25\mathrm{TeV}$ 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 $\gamma$-ray line emission, flux limits of $(2\times {10}^{-7}–2\times {10}^{-5}){\mathrm{m}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ and $(1\times {10}^{-8}–2\times {10}^{-6}){\mathrm{m}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ 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 $⟨\sigma v{⟩}_{\chi \chi \to \gamma \gamma }$ reach $\sim {10}^{-27}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$, based on the Einasto parametrization of the Galactic DM halo density profile.

• Received 15 October 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.041301