L’astronomie gamma et les défis des modèles standard du rayonnement cosmique galactique

Soutenance de HDR de Luigi Tibaldo (Salle de Conférence)

Résumé de la HDR

Cosmic rays (CRs) are particles irradiating the Earth with energies ranging from sub-MeV to above 10^20 eV and almost isotropic arrival directions. There is strong evidence that below 10^15 eV CRs are of Galactic origin. The sixty-years-old standard Galactic CR paradigm postulates that the particles are accelerated in the disk of the Milky Way, presumably in supernova remnants, and then they are diffusively confined in a kpc-sized halo for durations of several Myr. Today conventional models of Galactic CRs based on this paradigm are called into question by the exquisite detail reached by the recent measurements and by difficulties in connecting the phenomenological description of the macro-observables to the micro-physics of plasma processes driving particle acceleration and transport.

Gamma-ray observations probe CRs in remote locations via continuum gamma-ray emission produced by decay of unstable particles (mostly neutral pions) generated in inelastic CR nuclei collisions, Bremsstrahlung of CR leptons interacting with matter, and inverse-Compton scattering of CR leptons off low-energy photons. In the first part of this presentation I will present the main results of my work based on gamma-ray observations to investigate some challenges of standard Galactic CR models, placing them in the broader context of the current research in the field. I will focus on two key questions connecting gamma-ray astronomy and Galactic CR astrophysics: are supernova remnants the only sources of Galactic CRs? How well do we understand CR propagation in the Galaxy?

In the second part of the presentation I will discuss ongoing developments towards future observational facilities for gamma-ray astronomy with a focus on the Cherenkov Telescope Array Observatory (CTAO), and outline the main scientific drivers of my future research. On one hand, I plan to continue studying the origin, transport and interactions of Galactic CRs to understand how and to what degree standard models need to be extended, as well as how the mutual feedback between cosmic rays and the interstellar medium impacts both and influences galaxy evolution.

On the other hand, observations of antinuclei in CRs and of gamma rays can also probe the existence of antimatter domains in the Universe. The laws of physics known to date suggest that the Big Bang should have created equal amounts of matter and antimatter, leading to mutual annihilation. Yet, observations show that the local Universe is strongly matter-dominated and, at the scale of the entire observable Universe, a perfect symmetry between matter and antimatter is ruled out. The origin of the asymmetry remains an unsolved enigma and the last word on the existence of antimatter domains in the Universe has not been said yet. Therefore, I plan to explore new avenues to investigate antimatter in the Universe.