A new high-speed pattern recognition trigger for ground-based telescope arrays used in gamma ray astronomy

Modern imaging atmospheric Cerenkov telescopes (IACTs) are often configured as an array of individual telescopes, each having 500 pixels or more, where stereoscopic views of gamma ray air showers using two or more telescopes operating in unison improve the measurement of the location of the source in the night sky. The gamma-ray showers of interest have significant backgrounds, including cosmic-ray showers from protons and heavier elements, muons, and fluctuations in the night sky background that generate noise events in the photo-detectors. It is desirable to lower the thresholds on individual pixels, as this reduces the energy threshold of the instrument and facilitates observation of more distant cosmological objects. However, lowering the threshold also increases the noise and background rates. System aspects ultimately determine how low the threshold can be, including the depth of memory in the front end electronics, the speed of the data acquisition, and the sophistication of the trigger. Gamma-ray howers have a distinct but not unique signature compared to the background signals. We have developed a three-stage, high-speed trigger that can recognize patterns from gamma-ray showers and correlate them across all telescopes in the array to form a stereoscopic real-time pattern recognition trigger. Our goal is to process the ~10 MHz individual pixel rate on 500+ channels of each telescope (Level 1), and produce a camera trigger rate of 10 MHz (Level 2), and an array trigger rate of less thanl KHz (Level 3). This is a significant increase in performance over current IACTs that operate typically at a 1 KHz Level-2 and at 300~Hz event acceptance rate. We describe the architecture of this new sophisticated trigger, present first measurements of the prototype system, and describe plans to test this system in an existing IACT as a proof of principle for a future IACT array that might consist of hundreds of telescopes.