Implications of gamma-ray observations on proton models of UHECR

Understand

The origin of ultra high energy cosmic rays (UHECR) is still unknown.

• However, great progress has been achieved in past years due to the good quality and large statistics in experimental data collected by the current observatories.
• The data of the Pierre Auger Observatory show that the composition of the UHECRs becomes progressively lighter starting from $10^{17}$ eV up to $\sim 10^{18.3}$ eV and then, beyond that energy, it becomes increasingly heavier.
• These analyses are subject to important systematic uncertainties due to the use of hadronic interaction models that extrapolate lower energy accelerator data to the highest energies.