Background rejection using image residuals from large telescopes in imaging atmospheric Cherenkov telescope arrays

Laura Olivera-Nieto, Helena X. Ren, Alison M. W. Mitchell, Vincent Marandon, Jim Hinton

Submitted on 23 November 2022


Identification of Cherenkov light generated by muons has been suggested as a promising way to dramatically improve the background rejection power of Imaging Atmospheric Cherenkov Telescope (IACT) arrays at high energies. However, muon identification remains a challenging task, for which efficient algorithms are still being developed. We present an approach in which, rather than identifying Cherenkov light from muons, we simply consider the presence of Cherenkov light other than the main shower image in IACTs with large mirror area. We show that in the case of the H.E.S.S. array of five telescopes this approach results in background rejection improvements at all energies above 1 TeV. In particular, the rejection power can be improved by a factor 34 at energies above 20 TeV while keeping 90% of the original gamma-ray efficiency.


Comment: Accepted for publication in EPJC

Subjects: Astrophysics - High Energy Astrophysical Phenomena; Astrophysics - Instrumentation and Methods for Astrophysics


Large telescope images of rejected events that would be labelled as gamma-ray candidates in the small telescope reconstruction. In both cases, the right panel shows the ImPACT prediction associated with that event, whereas the left panel shows the actual recorded event image. The event image shown in the panel labelled as "a" contains a clear muon arc, whereas the additional feature in the image in panel "b" is much smaller and has a simpler shape. However, the maximum pixel intensity in this feature is more than double what is expected from NSB noise. Note that the colorbar has been restricted so that the fainter features are visible, since the main shower is much brighter in both cases.