Dark matter gamma-ray line searches toward the Galactic Center halo with H.E.S.S. I

The presence of dark matteris nowadays widely supported by a large body of astronomical and cosmological observations. A large amount of dark matteris expected to be present in the central region of the Milky Way. Very-high-energy (>100 GeV) g-rays can be produced in the annihilationof dark matterparticles. The H.E.S.S. array of Imaging Atmospheric Cherenkov Telescopes is a powerful tools to observe the Galactic Centre trying to detect g-rays from dark matter annihilation. A new search for a dark mattersignal has been carried out on the full H.E.S.S.-I data set of 2004-2014 observations. A 2D-binned likelihood method has been applied to exploit the spectral and spatial properties of signal and background. Updated constraints are derived on the velocity-weighted annihilationcross section for signals from prompt annihilationof dark matterparticles into two photons. The larger statistics from the 10-year Galactic Centerdataset of H.E.S.S.-I together with the 2D-analysis technique allows to significantly improve the previous limits.