Cosmic ray acceleration in W51C observed with the MAGIC telescopes

Abstract

We present the observations of the W51 complex with the MAGIC telescopes. After collecting more than 50 h of good quality data, we are able to measure the energy spectrum between 75 GeV and 5.5 TeV, and to fit it with a single power law with an index of 2. 58 ± 0. 07stat± 0. 22syst. We look for underlying structures by means of detailed morphological studies that allow us to locate the majority of the emission around the interaction point between W51C and W51B, where shocked molecular and atomic gas is found. We also find a feature extending towards the pulsar wind nebula candidate CXO J192318. 5 + 140305. The possible contribution of this pulsar wind nebula candidate, assuming a point-like source adds up to 20 % of the overall emission. The broad band spectral energy distribution of the whole gamma-ray source can be explained with a simple hadronic model that implies proton acceleration at least up to 100 TeV. The outcome of the modeling, together with the morphology of the source, suggests that we indeed observe ongoing acceleration of ions in the interaction zone between supernova remnant and cloud.