Coordinator: Jörg Rachen
Project Outline
The origin of ultra-high energy cosmic rays is considered a mystery. Nevertheless, during the decades the existence of these particle has been known, a large number of source scenarios has been suggested to explain them. The most famous way to depict their basic properties has been developed my Michael Hillas (1984), who plotted the “usual suspects” of UHECR source candidates into a diagram of magnetic field strength vs. radius, today known as the “Hillas plot”. The interesting thing about the Hillas plot is hereby, that over almost 20 orders of magnitude in scale, the product of these quantities is constant by order of magnitude for all dynamically relevant cosmic ray sources. Relating it to the maximum energy of the particles which can be achieved in any electromagnetically based acceleration process, Emax = e B R, it is found that this value exactly corresponds to the maximum energy of cosmic rays observed.
This intriguing correspondence leads to the attempt to search for some fundamental properties in our Universe which may explain it. A very basic approach uses the observed self-similarity of structure formation in the Universe: relating the released gravitational potential energy in collapses of matter on all scales to a “non-thermal cascade” of turbulence, magnetic field generation in dynamos, and cosmic ray acceleration, one finds not only the right order of magnitude for the cosmic ray maximum energy, but also of the total power injected as cosmic rays into the Universe, explaining its observed flux level. The astonishing constancy of this process in scale is then reduced to the astonishing constancy of of the mass-to-radius ratio of thermalized objects, which is ~ 105-106 on all scales, from stars to clusters of galaxies.
The purpose of this approach is that it allows to connect cosmic ray emissivity and maximum energy to basic properties of universal large-scale structure formation. Using recent reconstructions of the observed large-scale structure of dark matter halos based on structure formation simulations (Merson et al. 2016) and merging them with cosmic ray propagation simulations (Alves Batista et al. 2015), allows to develop a “most natural guess” of UHECR arrival directions. In a Bayesian language, this can be used as a prior to assign plausibility values to UHECR deflections, and thus constrain properties of the Galactic magnetic field. Future developments in the understanding of baryonic processes in structure formation (Schaal et al. 2016), and comparison to multi-messenger tracers of UHECR sources will help to improve this approach in the future.
Project Status
The basic idea has been presented at the 28th Texas Symposium on Relativistic Astrophysics in Geneva, and since then on various seminars and colloquia in Hamburg, Berlin, Krakow and Newcastle. A journal paper, including an application to large scale structure simulations to predict a prior for UHECR arrival direction is in preparation and shall be published during 2017.