Population Studies of Galactic Ultra-High-Energy Gamma-Ray Sources with LHAASO

Background: The LHAASO observatory has recently measured the ultra-high-energy (UHE) diffuse gamma-ray emission from our Galaxy.
This emission originates from charged particles (cosmic rays) accelerated in powerful astrophysical sources (e.g., supernova remnant) and subsequently propagating through the turbulent Galactic magnetic field. During their propagation, these particles interact with the interstellar medium, producing secondary gamma rays and neutrinos that reach Earth. Diffuse gamma-ray emission measurements require masking the emission associated with bright sources. As a result, the diffuse signal is unavoidably contaminated by the emission from unresolved sources, i.e., sources too faint to be detected individually. Quantifying this contribution is crucial for interpreting the diffuse emission and for understanding cosmic-ray propagation in the Galaxy. The LHAASO detector has also produced the first catalog of Galactic gamma-ray sources in the UHE regime. This dataset enables a population study of such extreme accelerators and offers a basis for estimating the flux from sources below the detection threshold.

Project: In this thesis project, the student will be working with the LHAASO source catalog. Using an unbinned likelihood technique, the student will analyze the flux, latitude, and longitude distributions of gamma-ray sources observed by LHAASO and derive average properties of the UHE gamma-ray source population. The student will compute the expected gamma-ray flux from the population of sources below LHAASO’s detection threshold. This contribution will then be added to existing Galactic diffuse emission models, and the combined prediction will be compared with recent observational data (e.g., from LHAASO and FermiLAT) to extract constraints on CR transport properties.