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Talk by Gwenael Giacinti at CRATER2018, Gran Sasso Science Institute, L'Aquila

Observations of extended gamma-ray emission around Galactic cosmic-ray (CR) sources can be used as novel probes of interstellar magnetic fields. Using very-high-energy gamma-ray data from the HAWC Observatory, we place constraints on the properties of the magnetic turbulence within $\approx 25$ pc from Geminga. We inject and propagate individual CR electrons in 3D realizations of turbulent magnetic fields, calculate the resulting gamma-ray emission, and compare with HAWC measurements of this region. We find that HAWC data is compatible with expectations for Kolmogorov or Kraichnan turbulence, and can be well fitted for reasonable coherence lengths and strengths of the turbulence, despite implying a CR diffusion coefficient significantly smaller than those suggested by Galactic CR propagation codes. The best fit is found for a coherence length $L_{\rm c} \approx 1$ pc and a magnetic field strength $B_{\rm rms} \approx 3 ��$G, and the preferred value for $L_{\rm c}$ increases with $B_{\rm rms}$. Moreover, the apparent lack of strong asymmetries in the observed emission allows us to constrain the coherence length to $L_{\rm c} \lesssim 5$ pc in this region., submitted to MNRAS

We present an overview of the current status of the fields of ultra-high energy cosmic rays (UHECR), extragalactic and Galactic magnetic fields (GMF). At the highest energies, the UHECR composition is controversial. Presently, no source has been detected. In Chapter 3, we develop a new method to search in the data for sources of ultra-high energy protons or light nuclei. We test the efficiency of this method. We are able to reconstruct the positions of detected sources with a good precision. In Chapter 4, we propose a new method to detect sources of ultra-high energy heavy nuclei. We apply this method to the 69 events with energies larger than 55 EeV published by the Pierre Auger Collaboration, and detect a set of events for which the reconstructed source lies at a few degrees from the Virgo cluster. This may be a hint at the Virgo cluster as a nuclei source. It will be possible to test this possibility with the next generation of UHECR experiments. In Chapter 5, we study the propagation in recent GMF models of heavy nuclei with energies larger than 55 EeV. We show that sources of ultra-high energy heavy nuclei which are located in nearly half of the sky would have their fluxes magnified or demagnified at Earth by more than a factor three, due to magnetic lensing effects in the GMF. We point out the presence of "blind regions". We also study the possibility of "UHECR astronomy" for such a composition, and we prove that all UHECR above 55 EeV cannot have been emitted by a same nearby source, such as Cen A., Nous présentons l'état des connaissances dans le domaine des rayons cosmiques d'ultra-haute énergie (RCUHE) et des Champs Magnétiques Galactiques (CMC) et extragalactiques. Aux plus hautes énergies, la composition des RCUHE est controversée. A l'heure actuelle, aucune source n'a été détectée. Dans le Chapitre 3, nous mettons au point une nouvelle méthode de recherche dans les données de sources de protons ou de noyaux légers d'ultra-haute énergie. Nous testons l'efficacité de cette méthode. Nous arrivons à reconstruire la position des sources détectées avec une bonne précision. Dans le Chapitre 4, nous proposons une nouvelle méthode de détection de sources de noyaux lourds d'ultra-haute énergie. Nous l'appliquons aux 69 événements d'énergies supérieures à 55 EeV publiés par la Collaboration Pierre Auger et détectons un ensemble d'événements pour lequels la source est reconstruite à quelques degrés de l'amas de la Vierge. L'amas de la Vierge est peut-être une source de noyaux. Cette possibilité pourra être testée avec la prochaine génération d'expériences. Dans le Chapitre 5, nous étudions la propagation de noyaux lourds d'énergies supérieures à 55 EeV dans des modèles récents du CMG. Nous montrons que les sources de noyaux lourds d'ultra-haute énergie localisées dans près de la moitié du ciel voient leurs flux amplifiés ou atténués à la Terre de plus d'un facteur trois, à cause de lentilles magnétiques dans le CMG. Nous présentons l'existence de "régions aveugles". Nous étudions aussi la possibilité d'"astronomie en RCUHE" pour cette composition et nous prouvons que les RCUHE au-delà de 55 EeV n'ont pas tous été émis par une même source proche comme Cen A.

In this work, we present detailed simulations for propagation of ultra-high energy (UHE) heavy nuclei, with E > 60 EeV, within recent Galactic Magnetic Field (GMF) models. We investigate the impacts of the regular and turbulent components of the GMF. We show that with UHE heavy nuclei, there is no one-to-one correspondence between the arrival directions of cosmic rays (CR) measured at Earth and the direction of their extragalactic sources. Sources can have several distorted images on the sky. We compute images of galaxy clusters and of the supergalactic plane in recent GMF models and show the challenges, and possibilities, of "UHECR astronomy" with heavy nuclei. Finally, we present a quantitative study of the impact of the GMF on the (de-)magnification of source fluxes, due to magnetic lensing effects. We find that for 60 EeV iron nuclei, sources located in up to about one fifth of the sky would have their fluxes so strongly demagnified that they would not be detectable at Earth, even by the next generation of UHECR experiments., 4 pages (2 columns), 5 figures. To appear in Proceedings of the 32nd International Cosmic Ray Conference, Beijing, China, 11-18 August 2011