One of the most pressing mysteries in astroparticle physics is the composition and origin of cosmic rays at energies around 1 EeV = 1018 eV. In contrast to charged particles, deflected in galactic and extragalactic magnetic fields, neutral particles such as neutrinos, neutrons, or photons point back to their production site. One way to learn more about sources of ultra-high energy cosmic rays is therefore to search for an accumulation of events from specific directions. This is akin to astronomical observations of a distant galaxy, say, made with an optical telescope collecting photons of visible light.
100 years after their prediction by Albert Einstein, Gravitational Waves (GW) were detected in 2015 by the LIGO detectors. With the surface detector of Auger we have searched for ultrahigh-energy neutrinos in temporal and spatial coincidence with such remarkable events.