The background from single electromagnetic subcascades for a stereo system of air Cherenkov telescopes

TL;DR

This study investigates how background events from single electromagnetic subcascades affect the sensitivity of stereo air Cherenkov telescopes like MAGIC at energies below 200 GeV, using Monte Carlo simulations.

Contribution

It provides a detailed analysis of electromagnetic subcascade backgrounds and their impact on gamma/hadron separation in stereo IACT systems at low energies.

Findings

Background from electromagnetic subcascades is significant below 200 GeV.

Sensitivity deterioration occurs due to background events surviving simple cuts.

The background impacts the detection of gamma-ray signals from sources like the Crab Nebula.

Abstract

The MAGIC experiment, a very large Imaging Air Cherenkov Telescope (IACT) with sensitivity to low energy (E < 100 GeV) VHE gamma rays, has been operated since 2004. It has been found that the gamma/hadron separation in IACTs becomes much more difficult below 100 GeV [Albert et al 2008] A system of two large telescopes may eventually be triggered by hadronic events containing Cherenkov light from only one electromagnetic subcascade or two gamma subcascades, which are products of the single pi^0 decay. This is a possible reason for the deterioration of the experiment's sensitivity below 100 GeV. In this paper a system of two MAGIC telescopes working in stereoscopic mode is studied using Monte Carlo simulations. The detected images have similar shapes to that of primary gamma-rays and they have small sizes (mainly below 400 photoelectrons (p.e.)) which correspond to an energy of primary gamma-rays below 100 GeV. The background from single or two electromagnetic subcascdes is concentrated at energies below 200 GeV. Finally the number of background events is compared to the number of VHE gamma-ray excess events from the Crab Nebula. The investigated background survives simple cuts for sizes below 250 p.e. and thus the experiment's sensitivity deteriorates at lower energies.