The Pierre Auger Observatory is the largest ultra-high
energy cosmic-ray detector in the world. With the Surface
Detector array of the Observatory we can also search
for neutral particles that point back to their sources
of production. In particular, ultra-high energy neutrinos
and photons, with energies around 1 EeV and above,
can be identified, and the most stringent limits at EeV energies
are set to their fluxes from a large fraction of the sky. These
limits provide additional information on the nature of the
ultra-high energy cosmic rays which may help identifying
the sources where they are accelerated up to 100 EeV.