Origin of the light cosmic ray component below the ankle

Abstract

The origin and nature of the ultrahigh energy cosmic rays remains a mystery. However, considerable progress has been achieved in past years due to observations performed by the Pierre Auger Observatory and Telescope Array. Above ${10}^{18}\text{ }\text{ }\mathrm{eV}$ the observed energy spectrum presents two features: a hardening of the slope at $\ensuremath{\sim}{10}^{18.6}\text{ }\text{ }\mathrm{eV}$, which is known as the ankle, and a suppression at $\ensuremath{\sim}{10}^{19.6}\text{ }\text{ }\mathrm{eV}$. The composition inferred from the experimental data, interpreted by using the current high energy hadronic interaction models, seems to be light below the ankle, showing a trend to heavier nuclei for increasing values of the primary energy. Also, the anisotropy information is consistent with an extragalactic origin of this light component that would dominate the spectrum below the ankle. Therefore, the models that explain the ankle as the transition from the galactic and extragalactic components are disfavored by present data. Recently, it has been proposed that this light component originates from the photodisintegration of more energetic and heavier nuclei in the source environment. The formation of the ankle can also be explained by this mechanism. In this work, we study in detail this general scenario, but in the context of the central region of active galaxies. In this case, the cosmic rays are accelerated near the supermassive black hole present in the central region of these types of galaxies, and the photodisintegration of heavy nuclei takes place in the radiation field that surrounds the supermassive black hole.