As ultra-high energy photons ( EeV and beyond ) propagate from their sources of production to Earth , radiation-matter interactions can occur , leading to an effective screening of the incident flux .
In this energy domain , photons can undergo e ^ { + } / e ^ { - } pair production when interacting with the surrounding geomagnetic field , which in turn can produce a cascade of electromagnetic particles called preshower .
Such cascade can initiate air showers in the Earth ’ s atmosphere that gamma-ray telescopes , such as the next-generation gamma-ray observatory Cherenkov Telescope Array , can detect through Cherenkov emission .
In this paper , we study the feasibility of detecting such phenomena using Monte-Carlo simulations of nearly horizontal air showers for the example of the La Palma site of the Cherenkov Telescope Array .
We investigate the efficiency of multivariate analysis in correctly identifying preshower events initiated by 40 EeV photons and cosmic ray dominated background simulated in the energy range 10 TeV – 10 EeV .
The effective areas for such kind of events are also investigated and event rate predictions related to different ultra-high energy photons production models are presented .
While the expected number of preshowers from diffuse emission of UHE photon for 30 hours of observation is estimated around 3.3 \times 10 ^ { -5 } based on the upper limits put by the Pierre Auger Observatory , this value is at the level of 2.7 \times 10 ^ { -4 } ( 5.7 \times 10 ^ { -5 } ) when considering the upper limits of the Pierre Auger Observatory ( Telescope Array ) on UHE photon point sources .
However , UHE photon emission may undergo possible ” boosting ” due to gamma-ray burst , increasing the expected number of preshower events up to 0.17 and yielding a minimum required flux of \sim 0.2 \mathrm { km ^ { -2 } yr ^ { -1 } } to obtain one preshower event , which is about a factor 10 higher than upper limits put by the Pierre Auger Observatory and Telescope Array ( 0.034 and 0.019 \mathrm { km ^ { -2 } yr ^ { -1 } } , respectively ) .