Multi messenger astronomy with H.E.S.S : the starburst galaxy NGC 253 and the search for short time scale transients Gamma ray astronomy has proven to provide unique insights into cosmic ray accelerators in the past few decades. By combining information at the highest photon energies with the entire electromagnetic spectrum in multi wavelength studies detailed knowledge of non thermal particle populations in astronomical objects and systems has been gained: Many individual classes of gamma ray sources could be identified inside our galaxy and outside of it. Different sources were found to exhibit a wide range of temporal evolution ranging from seconds to stable behaviours over many years of observations. With the dawn of both neutrino and gravitational wave astronomy additional messengers have come into play over the last years. This development presents the advent of multi messenger astronomy: a novel approach not only to search for sources of cosmic rays but for astronomy in general. In this thesis both traditional multi wavelength studies and multi messenger studies will be presented. They were carried out with the H.E.S.S. experiment an imaging air Cherenkov telescope array located in the Khomas Highland of Namibia. H.E.S.S. has entered its second phase in 2012 with the addition of a large fifth telescope. ...