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Periodic Report Summary 1 - COSMICMAG (Evaluation of the Uncertainties of the Galactic Magnetic Field to Elucidate the Origin of Ultra-High Energy Cosmic Rays)

Summary description of the project objectives

COSMICMAG aims at the development of a new analysis approach for charged particle astronomy to find the astrophysical sources of cosmic particles with extreme energies.

During the recent years, there has been a rapid progress in our understanding of (GMF) and the collected statistics of ultra-high energy cosmic ray events. It should therefore be possible to correct the measured arrival directions of cosmic rays on Earth for deflections in the GMF and to significantly improve the search for their astrophysical sources in this way. The research objectives of COSMICMAG are therefore:

1) A thorough evaluation of the uncertainties of the GMF with the aim of producing sky maps of the deflection uncertainty of cosmic rays given their arrival direction and rigidity (= energy divided by charge).
2) Development of an optimal analysis method for correlation studies that takes into account the deflection uncertainties originating from the uncertainties in both GMF and the estimated rigidities.
3) Application of the optimal data analysis method to data of the Pierre Auger Observatory.
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Description of the work performed since the beginning of the project

The first three months of the project were dedicated to the transfer of knowledge from the Outgoing Host. All numerical tools needed for the fit of the Galactic Magnetic Field by Jansson&Farrar have been transferred to the researcher and their use has been extensively discussed with Prof. Farrar and Dr. Jansson.

Within the next two months the existing framework for the estimation of the model parameters was optimized. The code was re-implemented to perform the minimization using gradient optimization instead of Markov-Chain Monte Carlo. These changes allow a full minimization of the model parameters within several minutes instead of the several days of computing time needed in the original implementation. Thus rapid tests of different magnetic field
parametrizations are possible.

The first half of 2015 was dedicated to study the effect of magnetic field confinement in the vicinity of the sources of ultra-high energy cosmic rays. The correspondingly increased residence time in the photon fields of the source environment leads to a higher probability for photo-nuclear interactions and nuclues breakup close to the source. A numerical calculation of the propagation of cosmic rays through these photon fields was developed in collaboration with Prof. Farrar and Prof. Anchordoqui. The study lead to a novel understanding of the spectrum and composition of cosmic rays above 10^18 eV and provides a new explanation for the spectral feature in the ultra-high energy cosmic ray flux known as the ankle. Results have been published in Phys.Rev.D.

During the second half of 2015, a computational framework ("RUQI") for the calculation of observables related to the Galactic magnetic field was developed. This development was neccessary since the existing "Hammurabi" framework has several shortcomings concerning computational speed and accuracy and flexibility for choosing different astrophysical models and observer positions. The new framework was benchmarked against the existing one. The differences found could be traced back to a wrong synchrotron calculation in Hammurabi by a factor of two, and a fix of the problem was communicated to the developers of Hammurabi.

Throughout the first two years, different astrophysical data sets useful for the study of magnetic fields were investigated and interfaced to the optimization procedure. This includes the new Planck data on synchrotron emission, the original unbinned catalogue of extragalactic rotation measures used by Jannson&Farrar, H_alpha surveys, dust reddening maps, dispersion and rotation measures of pulsars, extragalactic scattering measures and locations of HII
regions and superbubbles.

For the last half year, the work has been mainly focused on developing a model of thermal electrons that is needed to interpret the rotation measures to infer the Galactic magnetic field and its uncertainties. An optimization procedure was set up to tune the model to the dispersion measures of pulsars and to extragalactic scattering measures.

In accordance with the original workplan, about 20% of the time was invested into work related to the fellow's responsibilities as Science Coordinator of the Pierre Auger Collaboration and Deputy Spokesperson of the NA61/SHINE experiment. Moreover, the compatibility of the mass composition measurements from the Pierre Auger and Telescope Array Collaborations was studied within the Joint Working Group of the two experiments. This work is relevant for the COSMICMAG project, since a good mass (and thus charge) estimate of cosmic rays is needed to back-track their trajectories through the Galactic magnetic field.

The teaching activities outlined in the workplan were fulfilled by giving a graduate lecture on High Energy Astrophysics in the fall semester of 2015.

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Description of the main results achieved so far

a) Origin of the ankle in the ultrahigh energy cosmic ray spectrum, and of the extragalactic protons below it: The insights about the effects of cosmic-ray propagation near their sources lead to a novel understanding of the spectrum and composition of cosmic rays above 10^18 eV and provides a new explanation for the spectral feature in the ultra-high energy cosmic ray flux known as the ankle. In contrast to the two previous approaches, the model is compatible with all currently available cosmic ray data and it provides predictions for neutrino fluxes that can be detected by next-generation astrophysical neutrino detectors. The results were published in Phys.Rev. D92 (2015) no.12, 123001.

b) A computational framework to simulate astrophysical observables related to the Galactic magnetic field: The developed RUQI framework is a C++ package for the calculation rotation measures, dispersion measures, emission measures, scattering measures and synchrotron emission in galaxies. It supports adaptive line-of-sight integration, dynamic loading of astrophysical models and arbitrary observer positions. A first public release will be made available after the end of the project.

c) Preliminary results on the impact of the distribution of thermal electrons within the Galaxy on the estimate of the Galactic magnetic field: Previous attempts to model the Galactic magnetic field relied on the NE2001 model of thermal electrons to interpret the rotation measures of pulsars and extragalactic sources. Our preliminary model of thermal electrons uses a greatly enlarged data set of dispersion and scattering measures to constrain the spacial distribution of thermal electrons within the Galaxy. In addition catalogues of HII regions and superbubbles are included for a realistic modeling of known inhomogeneities of the warm ionized medium. Preliminary studies show that the impact of different modeling approaches on the inferred Galactic magnetic field parameters are significant. The propagation of these differences to the arrival direction of back-tracked cosmic rays with rigidity of 60 EV results in angular differences of up to 10 degree in extreme cases.
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Expected final results and their potential impact and use

The main final result of the project will be an update of the magnetic field model of Jansson&Farrar. The most important part of this update will be the evaluation of the uncertainties of the estimated magnetic field due to the underlying assumption of the functional form, thermal electrons densities and cosmic-ray electron densities. The refined model and its uncertainties will have a variety of applications in astrophysics ranging from the understanding of the formation of astrophysical magnetic fields to the interpretation of synchrotron emission from supernova remnants.

Within the context of the COSMICMAG project, which is mainly motivated by the study of cosmic rays, the model uncertainties will be propagated to obtain posterior distributions for the uncertainty of the deflection of cosmic rays in the Galactic magnetic field depending on their arrival direction. These uncertainty maps will have a large impact on the study of the origin of cosmic rays, because they will allow us to assign an event-by-event uncertainty to the direction from which the cosmic ray entered the Galaxy. With this new information, the analysis of the arrival direction of ultra-high energy cosmic rays can be optimized. Together with the charge estimate provided by the data of the upgraded Pierre Auger Observatory, it might be possible to unveil the origin of ultra-high energy cosmic rays.

The developed model for the density of thermal electrons will be published separately and could supersede the 15-year-old NE2001 model. An improved description of thermal electrons within the Galaxy is needed for many applications in astrophysics. Among other uses, the thermal electron density is essential to derive the distances of pulsars from their dispersion measure, since the majority of pulsars is too far to estimate their distance via parallax measurements. It is also used to estimate the foreground contamination of the dispersion and scattering measure of extragalactic radio signals. In particular, an improved model will be useful to study the nature of the enigmatic Fast Radio Bursts, a recently discovered high-energy astrophysical phenomenon of presumably extragalactic origin.

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

Reported by

Karlsruher Institut fuer Technologie
Germany

Subjects

Life Sciences
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