History of UHECR production in Centaurus A

The origin of ultra-high-energy cosmic rays (UHECRs) remains a puzzle more than 50 years after their discovery. Yet, the well-established excess of events toward the radio galaxy Centaurus A (Cen A) raises the possibility that it is the first identifiable UHECR source. Although Cen A has been considered as a UHECR source, its present jet activity seems unable to explain the most energetic events. Recently, the lobes of radio galaxies have been proposed as potential reservoirs of UHECRs, from which particles accelerated in previous episodes of higher activity slowly escape. In this work, we investigated whether the past activity episodes of Cen A could be related to the UHECR excess observed in the Centaurus region. By modeling the UHECR propagation in the Giant Lobes of Cen A, we tested the reservoir hypothesis and its connection with the activity history of Cen A. By analyzing the energies of the events and the overall mass composition of UHECRs, we find that activity within the last ∼30 Myr is required to explain the excess. This period aligns closely with the timescale where the transition regions and the Giant Lobes must be energized, as revealed by radio and γ ray observations.