This is still an open question in astroparticle physics and a long-standing mystery that the Pierre Auger Observatory aims to solve.
Scientific Highlights
Understanding the origin and nature of ultra-high energy cosmic rays has been one of the driving forces of the field of astroparticle physics. A key piece of information is what type of particles cosmic rays consist of.
For the first time, the possible appearance of ultra-high energy photons from merging compact objects such as black holes was tested.
The energy spectrum and mass composition data from the Pierre Auger Observatory constrain the source properties.
In a recent review article current searches for ultra-high-energy (UHE) photons at the Pierre Auger Observatory were summarized.
This looks like the most natural question regarding ultra-high energy cosmic rays, but it’s also one of the hardest ones to be answered.