One of the key challenges in astroparticle physics is the identification of the sources of cosmic rays at the highest energies (above 1 EeV). In this context, the search for neutral messenger particles in the ultra-high-energy (UHE) regime is of high interest. The sources of the gravitational waves (GWs) that can be observed with the current generation of GW detectors provide extreme astrophysical environments that are most likely to be unique in the universe. Another extraordinary source candidate is the anomalous blazar TXS 0506+056 which has been found to be coincident with two periods of enhanced high-energy neutrino flux in 2014/15 and 2017 as reported by the IceCube Collaboration. Due to their distance and transient nature, the capabilities of these sources to produce UHE radiation can only be studied through neutral messengers like photons and neutrinos.
The Pierre Auger Observatory near Malargüe, Argentina, is the largest air-shower experiment for the detection of UHE cosmic rays. With its surface detector, consisting of a grid of 1660 water Cherenkov detectors covering an area of 3000 km², it has a unique exposure to UHE photons and neutrinos and has published first constraints on these particles from GW sources and TXS 0506+056.