The role of the hadron initiated single electromagnetic subcascades in IACT observations

Abstract

The sensitivity of Imaging Air Cherenkov Telescopes (IACTs) worsens significantly at low energies because the gamma/hadron separation becomes much more complex. In this paper we study the impact of the single electromagnetic subcascade events on the efficiency of the gamma/hadron separation for a system of four IACTs using Monte Carlo simulations. The studies are done for: two different altitudes of the observatory, three different telescope sizes and two hadron interaction models (GHEISHA and FLUKA). More than 90% of the single electromagnetic proton-induced subcascade events are showers with primary energy below 200 GeV, regardless on the trigger threshold. The estimated efficiency of the gamma/hadron separation using the FLUKA model is similar to results obtained using the GHEISHA model. Nevertheless, for at least one triggered telescope only, a higher fraction of single electromagnetic subcascade events was obtained from the FLUKA model. Finally, the calculated quality factors are anti-correlated with the contributions of the false gamma-ray events in the proton initiated showers. Therefore, the occurrence of single electromagnetic subcascade events is one of the main reasons of the worsening of the primary gamma-ray selection efficiency at low energies.