Project A3: "Cosmic-ray transport in the transition region from Galactic to extragalactic origin"

The origin of the most energetic Galactic cosmic rays (GCRs) is still not understood, and important open questions in the energy range from about 0.1 PeV to 1 EeV (10¹⁴–10¹⁸ eV) remain to be answered: the source of the highest-energy GCRs remains unidentified; the maximum energies of various acceleration mechanisms are uncertain; the transition from Galactic to extragalactic CRs (EGCRs), including several features in the cosmic-ray (CR) energy spectrum, such as the knee, the second knee, and the ankle, are not well understood. Diffusive shock acceleration (DSA) at supernova remnant (SNR) shock fronts is generally thought to be capable of accelerating GCRs up to energies of the knee at ~ 10¹⁵ eV, while an extragalactic origin is presumed to be the source for CRs with energies above the ankle at ~ 5 · 10¹⁸ eV. The origin of CRs in the energy range between the knee and ankle remains largely unknown, and an ad-hoc extra component has often been introduced in the literature for filling this gap (see, e.g. Hillas, 2005). It has also been proposed that DSA at Galactic wind termination shocks (GWTS), rather than at supernova remnant (SNR) shocks, may accelerate CRs to beyond the knee (see e.g. Bustard et al., 2017). Recently, more light has been shed onto this question by KASCADE-Grande reporting a steepening in the energy spectrum of the heavy CR component at ~ 8 · 10¹⁶ eV and at about the same energy a hardening in the spectrum of the light component (Apel et al., 2011). This observation suggests that the transition from Galactic to extragalactic CRs starts well below the ankle in the all-particle energy spectrum. Another important piece of information that has become available from various experiments, most importantly from IceCube and Auger, is the observation of intermediate and large-scale anisotropies which results from the interplay of nearby sources and CR diffusion, and possibly the onset of EGCRs. In the context of this project, the shift of the phase in right ascension pointing toward the Galactic center at PeV energies (Aartsen et al., 2016) and toward the Galactic anticenter at EeV energies (Aab et al., 2020) is of particular interest. This, together with an evidence for a dipole-like anisotropy at energies E > 8 · 10¹⁸ eV (Aab et al., 2015), suggest that the transition from Galactic to extragalactic sources is observed.

In this project, we plan to combine these and future results with a theoretical modeling of Galactic and extragalactic CR transport, with the aim to reproduce the observational features (energy spectra of elemental groups with its break points, associated changes of composition, onset of anisotropies) and to predict yet unobserved features. A central goal is to understand the influence of the Galactic wind on CRs. In particular, we will investigate how GCRs diffuse out of the Galaxy, how EGCRs will be injected, and if the GWTS is capable of accelerating particles up to the heavy knee, possibly contributing to the spectrum. Finally, by the end of Phase 1 of CIM, we will be able to quantify a possible contribution from Galactic wind termination shocks in other spiral galaxies like starbursts or Seyferts and their possible contribution to the spectrum and – by combining our results with other projects in CIM – will reach a key goal of this CRC by being able to compare the outflow structure of galaxies. Phase 2 will be dedicated to adding hadronic interactions to the picture and more systematically investigate termination shocks of external galaxies. In Phase 3, we will systematically use new observatories like CTA, SKA, and IceCube Gen-2 to draw conclusions on the origin of cosmic rays in the knee-to-ankle region.