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

Abstract

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 γ/hadron separation in IACTs becomes much more difficult below 100 GeV [1]. A system of two large telescopes may eventually be triggered by hadronic events containing Cherenkov light from only one electromagnetic subcascade or two γ subcascades, which are products of the single π 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 γ-rays and they have small sizes (mainly below 400 photoelectrons (p.e.)) which correspond to an energy of primary γ-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 γ-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.PACS numbers: 95.55. Ka; 95.55.Vj; 95.75.Mn; 95.85.Pw; 95.85.Ry

The background from single pi0 events in the IACT observations

“…2b) when trigger threshold increases from 3 to 5 phe.. Note that the presented values, that were calculated for 220 m 2 telescopes at altitude of 2.2 km, are consistent with the results obtained for a slightly smaller trigger area of the camera [8].…”

“…Third, a primary electron or positron initiated shower can trigger a IACTs system [7]. Fourthly, the system of IACTs may be triggered by Cherenkov photons produced by electrons and positrons from at most two electromagnetic subcascades, which are a products of a single π 0 decay in the hadron initiated shower [8,9].…”

Spatial likelihood analysis for MAGIC telescope data

“…For weak or extended sources, data collected with IACTs are strongly dominated by an irreducible background (Maier & Knapp 2007;Sobczyńska 2009Sobczyńska , 2015Sitarek et al 2018) that cannot be separated, as mentioned in Sect. 1.…”

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

“…[3]). The decrease of the γ/hadron separation efficiency at low energies may be explained by several reasons: a) due to the larger fluctuations of the Cherenkov light density at ground [4] one may expect larger fluctuations of the image parameters; b) the geomagnetic field influences on the image parameters [5,6]; c) the existence of the images of the primary electron or positron [7]; d) IACTs may be triggered by Cherenkov photons produced by charged particles from a single or at most two electromagnetic sub-cascades (which are a products of a single π 0 decay) in the hadron initiated shower [1,2,8].…”

“…This observed effect was partially explained by the occurrence of a hardly reducible hadronic background (i.e. detected images that are formed mainly by the light from a single electromagnetic or a single π 0 sub-cascade in the proton induced shower) [1,2]. IACTs also record events containing Cherenkov light from at most two electromagnetic sub-cascades, which don't have to be products of single π 0 decay in the hadron initiated showers.…”