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The W51 complex hosts the supernova remnant W51C which is known to interactwith the molecular clouds in the star forming region W51B. In addition, apossible pulsar wind nebula CXO J192318.5+140305 was found likely associatedwith the supernova remnant. Gamma-ray emission from this region was discoveredby Fermi/LAT (between 0.2 and 50 GeV) and H.E.S.S. (>1 TeV). The spatialdistribution of the events could not be used to pinpoint the location of theemission among the pulsar wind nebula, the supernova remnant shell and/or themolecular cloud. However, the modeling of the spectral energy distributionpresented by the Fermi/LAT collaboration suggests a hadronic emissionmechanism. We performed observations of the W51 complex with the MAGICtelescopes for more than 50 hours. The good angular resolution in the medium(few hundred GeV) to high (above 1 TeV) energies allow us to performmorphological studies. We detect an extended emission of very-high-energy gammarays, with a significance of 11 standard deviations. We extend the spectrumfrom the highest Fermi/LAT energies to \sim 5 TeV and find that it follows asingle power law with an index of 2.58 \pm 0.07stat \pm 0.22syst . The mainpart of the emission coincides with the shocked cloud region, while we find afeature extending towards the pulsar wind nebula. The possible contribution ofthe pulsar wind nebula, assuming a point-like source, shows no dependence onenergy and it is about 20% of the overall emission. The broad band spectralenergy distribution can be explained with a hadronic model that implies protonacceleration above 100 TeV. This result, together with the morphology of thesource, tentatively suggests that we observe ongoing acceleration of ions inthe interaction zone between supernova remnant and cloud. These results shedlight on the long-standing problem of the origin of galactic cosmic rays.

Morphological and spectral properties of the W51 region measured with the MAGIC telescopes