Detection of the Crab Pulsar above 25 GeV with the MAGIC Telescope

The 17 m diameter Cherenkov telescope MAGIC, located on the Canary Island of La Palma, has the lowest trigger threshold among operating Cherenkov telescopes; this makes it a very promising tool for the detection of the pulsed gamma-ray (γ-ray) emission from ground. Recently, a new analog summation trigger concept, which allows reducing the threshold even further, has been designed and implemented. The analog signals of patches in the camera, comprising 18 pixels each, are summed up in the analog mode. In order to prevent triggers from photomultiplier (PMT) afterpulses, the individual pixel signals are clipped at a preset amplitude level and then added up. The average FWHM of the time duration of signals induced by cosmic rays is 2.6 ns. This allows one to keep the coincidence time window very small and thus to strongly suppress accidental triggers. In the usual digital trigger designs, the signal should be above a certain threshold from each individual pixel to contribute to the trigger. In the summation trigger all low amplitude signals in a given patch in the camera are summed up and fully contribute to the trigger.The new trigger concept provides a low cost trigger for Cherenkov telescopes and it is very effective and stable. This trigger system for the MAGIC telescope was installed in October 2007 and was successfully used in the detection of the Crab Pulsar above 25 GeV.