Probing the sources of gravitational waves

Since almost 60 years, physicists have been puzzled about the origin of ultra-high-energy (UHE) cosmic rays (CRs). In 2015, the first direct measurement of gravitational waves (GWs) shed light on a new promising type of source candidate: the merging processes of compact binary systems (compact binary mergers, CBMs).
Since charged CRs are considerably deflected by extragalactic magnetic fields, one has to rely on
neutral messenger particles like photons or neutrinos if correlations between GWs and CRs shall be analyzed. The technique of observing a single astronomical object in the light of multiple messenger signals is called ``multimessenger astronomy'' (MMA) and witnessed a major breakthrough with the observation of a binary neutron star merger in 2017. The analysis presented in this work shall open a new window in the field of MMA by searching for photons with energies beyond 10 EeV from transient point sources.
For the first time, constraints on UHE photons from CBMs are derived using data collected by the Pierre Auger Observatory and public data on GWs from the LIGO and Virgo detectors. Air shower events that have been found to be coincident with a GW are analyzed separately and the sensitivity of the Auger Observatory surface detector to a potential photon signal is evaluated. In preparation for future GW observations, a dedicated analysis strategy is developed aiming to optimize the physics results within the capabilities of Auger. Furthermore, also the atypical blazar TXS 0506+056 is analyzed for UHE photons during two periods of enhanced high-energy neutrino emission.
Finally and as an addition to this thesis, a first step is taken to lower the energy threshold of the Auger surface detector for the search for photons from transient point sources. For this purpose a new photon-discriminating air shower observable is being developed and analyzed in a simulation study.