Population Synthesis of Galactic Very-High-Energy Gamma-Ray Sources

• Applicant: Dr. Kathrin Egberts
• Subject Area: Astrophysics and Astronomy
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 500120112

Cosmic rays, very energetic charged particles, are ubiquitous in our Milky Way. They interact with matter and radiation fields they encounter, producing energetic gamma-rays. These gamma-rays can be used to study the origin and propagation of the original cosmic rays. The field of very-high-energy (VHE) gamma-ray astronomy has experienced an explosive development in recent years. In the Galactic Plane, the High Energy Stereoscopic System (H.E.S.S.) detected 78 objects in its Galactic Plane Survey and a faint component of large-scale diffuse emission. The 78 detected gamma-ray sources can be expected to represent only the tip of the iceberg of the Galactic VHE gamma-ray source population and our understanding so far is limited to the information that comes with these sources. Neither the entire population nor the measurement of large-scale diffuse emission with unknown contributions of truly diffuse emission and unresolved sources is understood. Crucial for this understanding are the sources that lie below the detection threshold and are therefore unresolved.The goal of this project is the comprehensive characterisation of the overall population of VHE gamma-ray emitters and the application to the measurement of large-scale diffuse emission with H.E.S.S. to disentangle the contribution of unresolved gamma-ray sources and truly diffuse emission. For this purpose, an existing population synthesis code, developed by the applicant, will be expanded to adequately describe the population of Galactic VHE gamma-ray sources on the basis of the data from the H.E.S.S. Galactic Plane Survey. Particular focus is put on the realistic description of the H.E.S.S. data by including the complex dependencies of the instrument sensitivity, and by tackling the problems of source confusion and incomplete information of sources (i.e. lacking distance estimates, lacking extent information of point-like sources). Inclusion of spectral information allows the comparison with other instruments in neighbouring energy bands like the Fermi Large Area Telescope and the High-Altitude Water Cherenkov Observatory. Due to the complementarity of these instruments this has the potential to add valuable information to the model and, at the same time, it guarantees a consistent picture throughout different wavelengths. The approach will be expanded and tested by adding physics of the individual object classes. The characterisation of the source population will then be used to simulate the Galaxy in VHE gamma-rays. These simulations will be applied to a more precise measurement of large-scale diffuse emission including the identification of the contributions of unresolved gamma-ray sources. For the first time, this will allow a meaningful interpretation of the VHE diffuse gamma-ray emission in terms of cosmic-ray propagation physics. In addition, the simulations will allow to investigate the challenges and limitations that the forthcoming Cherenkov Telescope Array will face.

• DFG Programme: Research Grants
• International Connection: Austria
• Cooperation Partner: Professor Dr. Ralf Kissmann