Multi-messenger probe of Cosmic Ray Origins – MICRO

Identifying the sources of Ultra-High Energy Cosmic Rays (UHECR) is one of the most pressing questions in high-energy astrophysics. The advent of high-statistics and high-quality data, most prominently obtained by the Pierre Auger Observatory, has radically changed our understanding of the high-energy Universe, though still without disclosing the cosmic-ray sources. The proposed project addresses this question and aims at identifying source classes that correlate best with existing observational data (direction, energy distribution, and primary mass). A novelty of the proposed approach will be a complete study of bursting sources starting from the modeling of selected source classes, including hadronic interactions within the source, and over the propagation down to Earth, to predicting the UHECR sky as a function of energy and primary mass. Ultra-high-energy sources should be able to confine cosmic rays within a sufficiently magnetized and large region to accelerate them up to the highest observed energies, which imposes in turn a minimum magnetic luminosity. Few, if any, astrophysical sources are able to sustain such a luminosity in the electromagnetic band over a long period of time. This pushes the proponents of the MICRO project to investigate further bursting sources hosted by AGN and starburst galaxies. Intermediate-scale anisotropies of UHECRs can inform us on the direction and on the flux of nearby or most luminous source candidates relatively to an isotropic background built up by fainter objects. The latter component can be estimated from constraints on the luminosity functions of source candidates as a function of redshift. The absolute UHECR flux of each resolved source can be in turn determined relatively to its contribution to the all-sky UHECR spectrum, emphasizing the importance of joint constraints from spectral and anisotropy observables. Besides constraints from arrival directions and the all-sky spectrum, composition informs us on the distance distribution of the sources, as the energy-loss length of an UHECR depends on its nature. Thus, the combined fit of transient source models to arrival direction, spectral, and composition data would constrain the direction, distance, and absolute flux of the source candidates. The objectives specifically addressed in the MICRO project will provide important answers to the leading question of identifying the sources of UHECRs (i) how do burst-like signatures (GRBs, AGN-flares) fit the cosmic-ray data, (i) how can we constrain the 3D distribution of sources from available UHECR observables, and (iii) could astrophysical high-energy neutrinos, some high-energy gamma rays, and UHECRs come from the same bursting sources.
The MICRO consortium comprises four Institutions with experienced PIs. They bring in the complementary expertise that is needed to successfully address the ambitious goals of the novel project within a time period of three years.