A Bayesian View on the Galactic Magnetic Field: From Faraday Rotation to Ultra-High Energy Cosmic Ray Deflections

Our Galaxy, the Milky Way, contains a magnetic field with a complicated morphology.  Because of the complexity of the field and difficulties in measuring magnetic fields, modelling this field and revealing its origin is an extremely daunting task.  However, a realistic Galactic magnetic field (GMF) model would be extremely rewarding: magnetic field knowledge is essential for understanding the Milky Way's ecosystem and evolution.  Also, it would elucidate the origin and evolution of galactic magnetism.  Lastly, such a model is essential for extragalactic studies for which the GMF is an interfering foreground; in particular, a realistic GMF model would finally allow extremely energetic charged cosmic particles, named Ultra-High Energy Cosmic Rays (UHECR) to be traced back through space in order to discover their sources in the Universe.

Within an International Team hosted by the International Space Science Institute (ISSI) in Bern, we brought together experts from the three communities of Bayesian inference, UHECR theory and observations, and GMF data and modelling.  The main results of this effort are (a) the development of a realistic GMF model based on magneto-hydrodynamical dynamo equations, (b) implementation of the software package IMAGINE,  a Bayesian sampler for determining the parameters of this and other GMF models, and (c) a path to using potential large scale anisotropies in UHECR arrival directions and in the local large-scale structure of the Universe (UHECR sources) to to trace back the origin of UHECRs through the GMF and extragalactic space.

The goal of this workshop will be to define and initiate scientific applications of IMAGINE, to prepare it for general use by the community. Obvious examples for first applications are the quantitative comparison of various GMF models in the literature, and their uncertainties and limits in comparison to the physics-based model founded on magneto-hydrodynamical dynamo equations.  Data sets considered to constrain GMF models shall include the most recent results on Faraday rotation measures, synchrotron emission from the Galaxy, and polarised dust emission and absorption.  Particular emphasis will be given to the discussion of UHECR deflections with a view on the information content of potential anisotropies in the measured arrival directions with respect to the UHECR source scenarios and GMF modelling.