The impact of clouds on image parameters in IACT at very high energies

Dorota Sobczyńska,W Bednarek

doi:10.1051/epjconf/20158903009

Abstract

The effective observation time with the Cherenkov telescopes arrays is limited to clear sky conditions due to considerable absorption of Cherenkov light by the possible presence of clouds. However below the cloud altitude the primary particles with high energies can still produce enough Cherenkov photons to allow the detection by the large telescopes. In this paper, using the standard CORSIKA code, we investigate the changes of image parameters due to the absorption of Cherenkov radiation by the cloud (for γ-ray and proton showers with various energies – from 2 TeV to 100 TeV and from 10 TeV to 200 TeV, respectively). We consider the clouds with different transmissions located at various altitudes above the ground level (between 8 km and 3 km). We show that, for both simulated primary particles at fixed energy, the WIDTH and the DIST distributions are shifted towards larger values in the presence of clouds in comparison to the clear sky simulations. This shift decreases with the cloud altitude. The LENGTH distributions are shifted towards smaller values for images of primary γ-rays, while for primary protons this shift is not observed. We conclude that the large Cherenkov telescopes with large camera FOV could be used for observation of γ-ray showers with high energies in the presence of clouds.

Highlights

The imaging air Cherenkov technique (IACT) has been successfully used to record γ -rays from cosmic sources.The two dimensional angular distribution of the Cherenkov light produced by the shower is measured using IACT
We conclude that large mirror Cherenkov telescopes with large field of view (FOV) camera should allow for detection of very high energy γ -ray showers
In the presence of clouds the images contain less Cherenkov light and they are less concentrated in comparison to clear sky simulations

Summary

Introduction

The imaging air Cherenkov technique (IACT) has been successfully used to record γ -rays from cosmic sources. The reconstructed energy of the shower and estimated fluxes of primary particles (from the HESS data) can be corrected for the presence of low-level aerosols [6]. The applied method is based on the correction of the image parameter called SIZE. It has been shown in [9, 10], that other Hillas parameters can be affected by fully opaque clouds. The γ ray selection, based on the presented image parameters, should be possible for the data collected under cloud cover. We conclude that large mirror Cherenkov telescopes with large field of view (FOV) camera should allow for detection of very high energy γ -ray showers. FOV are required because the images may be significantly shifted outwards the camera center

Monte Carlo simulations

Results and discussion

Conclusions