Prospective study on observations of gamma-ray emission from active galactic nuclei using the HADAR experiment

The High Altitude Detection of Astronomical Radiation (HADAR) experiment is a refracting terrestrial telescope array based on the atmospheric Cherenkov imaging technique. It is a hybrid array consisting of four water-lens telescopes and a surrounding scintillation detector array for observing Cherenkov light induced by 10 GeV–10 TeV cosmic rays and gamma rays in the atmosphere. The water-lens 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, and even the CTA. The field-of-view (FOV) of HADAR can reach up to 60 degrees. The HADAR experiment possesses the advantages of a large field-of-view and low energy threshold, so it can continuously scan wide portions of the sky and easily observe extragalactic gamma-ray sources. The majority of the extragalactic gamma-ray sources detected at very high energy (VHE) energies are active galactic nuclei (AGNs). In this study, we present the potential of using the HADAR experiment for detecting AGN. Based on the AGN catalog sources of the Fermi Large Area Telescope (Fermi-LAT), the observed energy is extrapolated to the VHE range. The VHE gamma rays propagating over cosmological distances can interact with the low-energy of the extragalactic background light (EBL) and produce electron-positron pairs. Therefore, we consider the absorption effects of different EBL models when calculating the expected gamma ray spectra of the AGN sample. We select the sample with redshift measurements and locations inside the FOV of HADAR from 4LAC catalog. In total, there are 375 BL Lacertae objects (BL Lacs) and 289 flat-spectrum radio quasars (FSRQs) satisfying the selection conditions. The integral gamma ray spectra are derived and compared with the sensitivity curve of HADAR, the number of sources with fluxes above the sensitivity of HADAR is counted. Further, we calculate the statistical significance of HADAR for AGN source observation based on the equi-zenith angle sky scanning analysis method. The simulation results reveal that a total of 31 sources of Fermi-LAT AGN can be detected by HADAR with a significance greater than five standard deviations over a one-year survey period, most of which are BL Lacs.