Echoes of the past: ultra-high energy cosmic rays accelerated by radio galaxies, scattered by starburst galaxies

Abstract

We explore the possibility that the hotspot of ultrahigh energy cosmic rays (UHECR) detected by the Telescope Array (TA) from the approximate direction of M82 and the M81 group of galaxies might be the echo of UHECR emitted by Centaurus A in an earlier more powerful phase. Echoes from other starburst galaxies or groups of galaxies may contribute to the UHECR flux at the Earth. We use an illustrative Monte Carlo model of mono-energetic UHECR transport by small angle scattering to generate synthetic sky maps. The model informs a discussion of overall energetics and time and distance scales. We find a viable echo model for the observed UHECR hotspots if the UHECR luminosity of Centaurus A 20 Myr ago was 200 times its present luminosity and if the ordered magnetic field exceeds 10-20nG out to a distance of 400-800kpc in the circumgalactic medium of M82 and other starburst galaxies.