Galactic magnetism is a growing field of research in astrophysics. The crucial role that magnetism plays in the ecosystem of a galaxy has been known for years, but only recently advances in detection methods, technology and computer power have made big leaps forward in understanding galactic magnetic fields possible (e.g., Sun et al. 2008, Jansson & Farrar 2012; Haverkorn 2015). As a result, it has become clear that simple models of large-scale galactic magnetic fields following galactic spiral arms are hugely inadequate. Recent data from large multi-wavelength radio-polarimetric surveys have allowed refinement of these models including e.g. anisotropic turbulence and/or vertical field components. However, these models are still data starved and inclusion of magnetic field information through other sources than radio polarimetry is highly desired.

Ultra-high energy cosmic rays (UHECR) reach up to energies of ~10¹¹ GeV, many orders of magnitude above those achievable by particle accelerators. Both theoretical considerations and also increasing observational evidence points to an extragalactic origin of these particles (Stanev et al. 1995, Pierre Auger Collaboration 2007, 2015). Due to interactions with the cosmic microwave background these particles cannot travel far (the well known “GZK effect”), and only very few known astrophysical objects are capable to produce them. Nevertheless, it has not been possible so far to identify the population of UHECR sources, mainly because of the unknown deflections they suffer when traversing the magnetic fields in and around our Galaxy. This uncertainty is owing to (a) the usually very simple models for the GMF used in the UHECR community, and (b) the statistical uncertainty of reconstructing charge and energy of the individual particles from the air showers initiated by them, which are detected in UHECR observatories. Combining the newest insights in Galactic magnetism and the full properties of individual air showers will greatly improve the computed UHECR deflections by the Milky Way magnetic field. But this argument can be reversed: comparing “magnetically corrected” arrival directions of UHECR with probable source scenarios as a function of GMF parameters can provide constraints on GMF models, in particular on larger scales and in the halo of our Galaxy, scales which are diffcult to probe by analysing polarized synchrotron radiation or Faraday rotation measures.

Several activities have been pushed forward in the last years to tackle the connection between UHECR and the GMF: Starting with an informal meeting involving both communities at Ringberg Castle in 2009, Enßlin and co-workers have developed the Hammurabi Code (Waelkens et al. 2009) which allows both to constrain GMF models from astronomical data and to calculate UHECR deflections predicted by these models, and continued the research activity in the subject by developing non-parametric Bayesian methods and software tools (Enßlin et al. 2009, Selig et al. 2013) to achieve a 3D-tomography of our Galaxy, including its magnetic field. Based on the Hammurabi code and available data from WMAP and pulsar rotation measures, Jansson & Farrar (2012) developed an optimal parametrized model of the Galactic magnetic field, and calculated UHECR deflections in this field to constrain the contribution of the radio galaxy Centaurus A to the UHECR flux (Farrar et al. 2013, Keivani et al. 2015). Within a recently awarded Marie-Curie Fellowship, members of our proposed team will collaborate to extend this approach to utilize information in individual air showers on mass and energy of the primary particles.

Given this status of the research field, it is time to undergo a critical review of the applied models and methods by an international distinguished expert team. In particular, it would be important to discuss in which respect the current direction of research, i.e., using improved models of the GMF derive conclusions on UHECR models, can be reversed and to use existing theoretical and experimental knowledge on UHECR origin as a constraint for improved modelling of the GMF. Here is where our project sets in.

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