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The efficiency of the $\gamma$/hadron separation worsens significantly at lowenergies for Imaging Air Cherenkov Telescopes (IACT). This observed effect waspartially explained by the occurrence of a hardly reducible hadronic background(i.e. detected images that are formed mainly by the light from a singleelectromagnetic or a single $\pi^0$ sub-cascade in the proton induced shower)[1,2]. IACTs also record events containing Cherenkov light from at most twoelectromagnetic sub-cascades, which don't have to be products of single $\pi^0$decay in the hadron initiated showers. In this paper, the impact of at most twoelectromagnetic sub-cascades events on the primary $\gamma$-ray selection wasstudied using the Monte Carlo simulations. We investigate how the totalfraction of at most two sub-cascades events in the expected total protonicbackground depends on the hadron interaction models (GHEISHA and FLUKA),trigger threshold, reflector area and altitude of the observatory. We show thatthe efficiency of the $\gamma$/hadron separation is anti-correlated with thecontribution of at most two electromagnetic sub-cascades events in the protoninitiated showers below 200 GeV. The influence of at most two electromagneticsub-cascades events on the $\gamma$-ray selection is similar to the effect ofthe single electromagnetic sub-cascade and singe $\pi^0$ events in IACTobservations. The occurrence of all these images in the data collected by IACTsreduces the efficiency of the $\gamma$/hadron separation at low energies.

Influence of the proton initiated at most two electromagnetic sub-cascades events on IACT observations