ED Sciences Physiques et de l'Ingénieur
Study of novae as cosmic-ray accelerators with Fermi-LAT and H.E.S.S.
by Paul FAUVERGE (Laboratoire de Physique des 2 Infinis de Bordeaux)
The defense will take place at 14h00 - Salle Marie CURIE LP2i Bordeaux 19 Chemin du Solarium 33170 Gradignan
in front of the jury composed of
- Kumiko KOTERA - Directrice de recherche - Sorbonne Université - Rapporteur
- Vincent TATISCHEFF - Directeur de recherche - Université Paris-Saclay - Rapporteur
- Christine GRAUBY-HEYWANG - Professeure - Université de Bordeaux - Examinateur
- François BRUN - Ingénieur - CEA - Examinateur
Novae are violent phenomena that occur in binary systems where a white dwarf accretes matter from its companion star. When the conditions required for the onset of hydrogen fusion are met, a thermonuclear explosion is triggered, leading to the ejection of matter. Although they have long been observed at other wavelengths, it was only in the early 2010s, thanks to the Large Area Telescope onboard the Fermi satellite (Fermi-LAT), that novae were identified as sources of gamma-ray emission. During these explosions, several ejecta components can coexist, with different velocities. Their interaction leads to the formation of internal shocks, capable of accelerating particles to relativistic energies. These particles can then produce gamma-ray photons through different mechanisms and contribute, to a lesser extent, to the cosmic-ray flux observed on Earth. In contrast to supernova remnants, where acceleration processes develop over timescales of several thousand years, novae provide laboratories allowing the study of these phenomena in quasi real time. This thesis aims to characterize the gamma-ray emission of novae by combining the analysis of Fermi-LAT data and very-high-energy observations obtained with H.E.S.S. A dedicated analysis method for transient sources has been developed and validated, including background modeling, source localization as well as the extraction of spectral and temporal properties. The Fermi-LAT analysis of four classical novae, combined with multi-wavelength data, highlights spectral and temporal properties that are, in most cases, consistent with a scenario dominated by hadronic processes, in which accelerated protons interact with the dense ejecta material. For three of them, simultaneous observations with H.E.S.S. reveal the determining role of time-dependent effects, in particular the absorption of very-high-energy gamma-ray photons through pair production in the days following the explosion. A significant result of this work is the identification of a first hint of very-high-energy emission by H.E.S.S. associated with a classical nova a few days after its discovery, thus providing direct access to the most energetic component of these explosions. Overall, these results support a predominantly hadronic interpretation of the gamma-ray emission from novae. Their rapid evolution and observational accessibility make them privileged objects for constraining models of particle acceleration and radiation in dense and transient environments. The prospects offered by future observations, in particular with the next generation of Cherenkov telescopes (CTAO), thanks to an energy threshold of a few tens of GeV and improved sensitivity, will make it possible to extend these studies to a larger number of novae and to better constrain the physical processes at work in these explosions.