The Topo-trigger: a new concept of stereo trigger system for imaging atmospheric Cherenkov telescopes

TL;DR

The paper introduces the Topo-trigger, a topological stereo trigger system for IACTs like MAGIC, which significantly reduces accidental triggers and enhances gamma-ray detection efficiency, thereby lowering the energy threshold.

Contribution

It presents a novel topological trigger algorithm that improves trigger rejection rates and increases collection area in atmospheric Cherenkov telescopes.

Findings

Rejects 85% of accidental triggers

Preserves 99% of gamma-ray events

Increases collection area by 10-20%

Abstract

Imaging atmospheric Cherenkov telescopes (IACTs) such as the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes endeavor to reach the lowest possible energy threshold. In doing so the trigger system is a key element. Reducing the trigger threshold is hampered by the rapid increase of accidental triggers generated by ambient light, the so-called Night Sky Background (NSB). In this paper we present a topological trigger, dubbed Topo-trigger, which rejects events on the basis of their relative orientation in the telescope cameras. We have simulated and tested the trigger selection algorithm in the MAGIC telescopes. The algorithm was tested using MonteCarlo simulations and shows a rejection of 85% of the accidental stereo triggers while preserving 99 % of the gamma rays. A full implementation of this trigger system would achieve an increase in collection area between 10 and 20% at the energy threshold. The analysis energy threshold of the instrument is expected to decrease by ~8 %. The selection algorithm was tested on real MAGIC data taken with the current trigger configuration and no $\gamma$-like events were found to be lost.