Speaker
Description
A key question in the field of UHECR physics is whether the observed flux is produced by a few prominent local sources or a large population of sources with similar contributions to the total flux. We constrain the ensemble of sources by investigating how diverse sources can be in terms of the maximum cosmic ray rigidity while remaining compatible with observations. We additionally study if the observed flux can be explained by two distinct source populations -- one emitting heavier cosmic rays and one producing only UHE protons. To this end, we use 1D CRPropa simulations to predict the UHECR spectrum and composition at Earth after accounting for interactions during propagation. Injection is modelled according to a simple phenomenological description but we allow for non-identical sources or a superposition of two distinct source populations. Our results show that sources contributing to the observed flux must be effectively identical in terms of maximum rigidity, although a large population of low-rigidity "hidden" sources can exist. If a two-population scenario is considered then a proton fraction of up to order of 10% is still compatible with observations.
