A New Sum Trigger to Provide a Lower Energy Threshold for the MAGIC Telescope

We have designed and implemented a new trigger concept (hereafter called sumtrigger) for Cherenkov telescopes, which allowed us to lower the trigger threshold of the MAGIC telescope by a factor of two to 25 GeV. For the new trigger, we subdivided the camera in 24 overlapping patches, each patch consisting of 18 pixels. In each patch the clipped analog signals of all pixels are summed and the trigger decision is subsequently derived from the summed signal. The clipping of the individual analog signals before summing prevented accidental triggers from large afterpulses of the photomultipliers (PMT). Special emphasis were put in the design of the analog part of the electronics to preserve the fast characteristics of the PMT signals. The 2.6 ns FWHM of the analog signals results in an effective coincidence window of about 3 ns, thus strongly suppressing accidental triggers caused by fluctuations of the night sky background (NSB). In the sumtrigger all pixels of one patch contribute to the trigger decision. For 25 GeV gamma-ray showers, a patch size of 18 PMT maximizes the signal to noise ratio of the air shower signal over the fluctuations of the NSB. In combination with the clipping of the signals before forming the sum we achieved a factor of two lower trigger threshold of the sumtrigger compared to the standard MAGIC trigger. The sumtrigger was successfully used to detect pulsed gamma-ray emission from the Crab Pulsar with the MAGIC telescope.