Expectation on observations of Fermi-LAT gamma-ray sources using the HADAR experiment

High altitude detection of astronomical radiation (HADAR) is an innovative array of atmospheric Cherenkov telescopes that employs pure water as its medium. By utilizing large-aperture hemispherical lenses, HADAR can capture atmospheric Cherenkov light, enabling the detection of gamma rays and cosmic rays in the energy range of 10 GeV to 10 TeV. Compared to traditional Imaging Atmospheric Cherenkov telescopes, HADAR offers distinct advantages such as a low energy threshold, high sensitivity, and a wide field of view. The telescope mainly consists of a hemispherical lens with a diameter of 5 m acting as a Cherenkov light collector, a cylindrical metal tank with a 4 m radius and 7 m height, and an imaging system at the bottom of the tank. The sky region covered by HADAR is much larger than the current generation of Imaging Atmospheric Cherenkov Telescopes. The field of view of HADAR can reach up to 60 degrees. Its continuous scanning capability allows for comprehensive observations of gamma-ray sources throughout the entire celestial sphere, making it an ideal instrument for studying transient and variable sources. In this study, the observational capabilities of HADAR are thoroughly investigated using the latest 4FGL-DR3 and 4LAC-DR3 gamma-ray source catalogs from Fermi-LAT. For extragalactic sources, the energy spectra in the high energy range have been extrapolated to the very high energy range, taking into account the absorption effect caused by extragalactic background light. By comparing the extrapolated results with existing VHE experimental data, the feasibility of this extrapolation method has been demonstrated. Through simulated analyses of the significance of these sources, it is anticipated that HADAR will detect a total of 93 gamma-ray sources with a significance exceeding 5 standard deviations during one year of operation. These sources comprise 45 galactic sources, 39 extragalactic sources, 3 sources of unknown type, and 6 unassociated sources.