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.
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.