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Evaluation of the Uncertainties of the Galactic Magnetic Field to Elucidate the Origin of Ultra-High Energy Cosmic Rays

Final Report Summary - COSMICMAG (Evaluation of the Uncertainties of the Galactic Magnetic Field to Elucidate the Origin of Ultra-High Energy Cosmic Rays)

Cosmic rays are the highest energy messengers of astrophysical phenomena in the Universe. The sources of these particles are unknown and it is one of the great puzzles of modern astrophysics how they are accelerated to macroscopic energies of >10^20 eV. A main obstacle for the identification of the astrophysical sources of these ultrahigh-energy particles is that their arrival directions measured at Earth are affected by the Galactic magnetic field (GMF) that bends the trajectory of charged cosmic rays as they traverse the Galaxy.

The main scientific goal of the COSMICMAG project was therefore to study the GMF and to provide estimates on the uncertainty on the distortion of the arrival directions of cosmic rays caused by an incomplete knowledge of the GMF.

In collaboration with Prof. G.R. Farrar (NYU) the following work has been carried out to achieve the project's objectives:

* a new computational framework was developed to calculate astrophysical observables (rotation
measures and synchrotron emission) for a given model of the GMF.
* the existing GMF model by Jansson & Farrar (2012) was improved by new parametric
descriptions of the different components of the GMF.
* new data on the diffuse synchrotron radiation from the Galaxy measured by the Planck
satellite was studied
* the influence of auxiliary model assumptions on the interpretation of the data was studied.
This includes the density and spatial distribution of thermal and cosmic-ray electrons as
well as correlations between the thermal electrons and the magnetic field.

These studies led to an ensemble of models of the GMF that are compatible with current astrophysical observations. This ensemble of models can be considered as a estimate of the uncertainty of our knowledge of the GMF. It can be used to propagate this uncertainty to any kind of calculation involving the magnetic field of the Galaxy, by repeating the calculation for each of the models.
In the absence of further input to select among or discard some of the GMF model variations, the variations of the results gives an estimate of the propagated uncertainty. The main objective of the project was accomplished by using these ensembles to construct sky maps of the deflection uncertainty of cosmic rays given their arrival direction and rigidity R (= energy divided by charge). These maps show that

* at large rigidities (R > 60 EV) the overall amount of deflection and correspondingly also the
model differences are small, confirming the long-speculated possibility of charged particle
astronomy with protons at ultrahigh energies.
* for rigidities as low as 20 EV, the different deflections are mostly confined within well-defined
regions. It is thus plausible that a correction for the spatially varying average deflection based
on all models, can still be used to enhance the capabilities to identify the sources of ultrahigh
energy cosmic rays.
* at low rigidities (~10 EV) the differences in the backtracked directions start to diverge
considerably, but even in this case, a more sophisticated analysis can reduce the current
uncertainties in studies of the source direction of ultrahigh energy cosmic ray nuclei.

Further results regarding the origin of ultrahigh-energy cosmic rays were obtained in collaboration with Prof. G.R. Farrar (NYU) and Prof. L.A. Anchordoqui (CUNY). We studied the influence of photo-nuclear interaction of cosmic-ray nuclei in photon fields of the source environment. Our results provide a novel explanation of the long-standing open problem of cosmic-ray physics, the interpretation of the "ankle" in the cosmic-ray flux, a hardening of the observed energy spectrum of cosmic rays at around an energy of 10^18.5 eV.

The target audience, for which our results are relevant, is first and foremost the scientific community conducting basic research in the field of astroparticle physics. Our model for the origin of the ankle is included in recent text books of the field ("Cosmic Rays and Particle Physics" by T.K. Gaisser, R. Engel and E. Resconi and "Astroparticle Physics: Theory and Phenomenology " by G. Sigl) and the deflection maps will e.g. be used by the Joint Working Group on Arrival Directions of Cosmic Rays of the Pierre Auger, IceCube and Telescope Array collaborations. The ensemble of GMF model variations will have many other applications in astrophysics, e.g. for the study of the propagation of low-energy cosmic rays in the Galaxy.

More information about the COSMICMAG project is available at the project's web page,
https://web.ikp.kit.edu/munger/CosmicMag/