Tremendous advances in our knowledge of the origin of Galactic cosmic-rays (CR) have been made during the last decades. In particular, shock acceleration theory and observations with X-ray and gamma-ray have confirmed that the remnants of supernova explosions do accelerate particles. But so far, no evidence supports their ability to produce particles up to the knee energy of CRs around the PeV range. Gamma-rays in the very high energy (VHE, around the TeV) and ultra-high energy (UHE, beyond 100 TeV) domains are the key observables to search for extreme Galactic accelerators. 
The unprecedented sensitivity in the UHE regime provided by the recently completed LHAASO observatory (and other planned facilities such as CTAO, HAWC or SWGO) is essential to identify Galactic sources of >PeV cosmic rays. The first sky survey conducted with the LHAASO facility has revealed 43 sources in the UHE domain in the northern hemisphere, some of them significantly emitting beyond the PeV. The detection of such a large number of potential PeVatrons by LHAASO came as a surprise, as the number of active hadronic PeVatrons in the Galaxy is expected to be limited to a few only if they are supernova remnants. Similarly, even if the astrophysical neutrinos diffuse background is established, no neutrino source has been detected in the Galaxy so far. A large fraction of the UHE gamma-ray sources could therefore be leptonic PeVatrons, which was not expected a decade ago because it was thought that the Klein-Nishina effect would have suppressed the IC above ~50 TeV assuming a standard diffuse acceleration mechanism spectrum.  Actually, nearly all the LHAASO UHE sources are located close to powerful pulsars. Based on general considerations on the energy budget and acceleration constraints, it has been proposed that a large fraction of the Galactic UHE sources could be produced by the nebulae of young and energetic pulsars.  

We propose to put this idea to a test in this PhD project, thanks to a detailed multi-instrument spectro-morphological characterization of the gamma-ray emission of some of these PeVatron candidates as well as their associated X-ray emission.

The student will take advantage of the large dataset of H.E.S.S. over the Galactic plane as well as the collaboration with HAWC and existing Fermi data to perform a broad-band joint analysis (from few  GeV to 100 TeV) of several PeVatron candidates using the Gammapy library. Such joint studies are essential to properly determine the source morphology and its possible energy dependence and separate the emission from foreground contamination. In addition, X-ray observations will be used  to constrain the synchrotron emission produced by UHE electrons to probe the leptonic or hadronic origin of VHE and UHE emissions. To do so, multi-zone kinetic modeling particle population in the leptonic PeVatrons will be developed and tested against the data. 
 
The supervising team at APC (B. Khelifi, R. Terrier) is heavily involved in HESS and CTAO experiments as well as in the development of the Gammapy library. It has built a strong collaboration with Q. Remy (MPI-K, Heidelberg), which is a HAWC collaboration member to perform joint analyses of HESS and HAWC data.  Long term visits there will be necessary during the PhD.