A Topological Array Trigger for VERITAS

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an array of four imaging atmospheric-Cherenkov telescope. A fast topological trigger system is being built as an upgrade to VERITAS (pending funding) with the scientific goal of reducing the energy threshold for detection of gamma rays. The current trigger system results in a threshold of around 130 GeV, a reflection of the background rates from cosmic rays and night-sky fluctuations. The topological trigger is being designed to further suppress both cosmic rays and night-sky accidentals. At the single-telescope trigger, the rate of night-sky fluctuations will be reduced by a factor of 10 through a narrower coincidence gate. Suppression of cosmic rays will occur at the array level by comparing the image parameters of at least 3 telescopes against a look-up table of simulated gamma rays. The topological trigger uses field-programmable gate arrays (FPGAs) and will be adaptable to different observing modes and special physics triggers, e.g. pulsars and bursts. The trigger design and expected performance are presented. This new trigger system could also find application in the planned Advanced Gamma-Ray Imaging System (AGIS).