The effect of night sky background on source reconstruction in atmospheric Cherenkov telescope arrays and implications for image cleaning

Abstract

Abstract Gamma-ray astronomy through imaging atmospheric Cherenkov telescope (IACTs) has enabled the study of numerous astrophysical sources in the GeV-TeV domain. The presence of night sky background (NSB) in IACT images affects the reconstruction of the gamma-ray being observed and is usually dealt through image cleaning. In this work, we propose that the image cleaning approach might be adapted to the reconstruction method and NSB levels. In particular, we study the effect of NSB on source reconstruction for gamma-rays observed through IACT arrays through Monte Carlo simulations. We find that the presence of NSB can have a smoothing effect on the shower images enabling better source reconstruction under certain circumstances. We also find that there is an optimum cleaning threshold depending on the energy of the initial gamma-ray. This threshold shows very little dependence on the NSB level or core position. Based on these results, we propose and test a hybrid image cleaning method for source reconstruction. We find that this hybrid image cleaning method results in better source reconstruction when observations are made in high NSB conditions. Whereas at low NSB levels, the application of a single image cleaning threshold yields better results. We finally conclude that this implies that different image cleaning methods might be needed to best carry out source reconstructions at different NSB levels.