The topological second-level trigger of the HESS phase 2 telescope

HESS is an array of atmospheric Cherenkov telescopes dedicated to GeV–TeV γ-ray astronomy. The original array has been in operation since the beginning of 2004. It is composed of four 12-m diameter telescopes. The installation of a fifth 28-m diameter telescope is being completed. This telescope will operate both in stereoscopic mode and in monoscopic mode i.e. without a coincident detection on the smaller telescopes. A second-level trigger system is needed to suppress spurious triggers of the 28-m telescope when operated in monoscopic mode. This paper gives the motivation and principle of the second-level trigger. The principle of operation is illustrated by an example algorithm. The hardware implementation of the second level trigger system of HESS phase 2 is described and its expected performances are then evaluated.