In 2007 a prototype of an analog Sum-Trigger for air Cherenkov telescopes was installed in the MAGIC I telescope lowering the trigger threshold from 55 GeV to 25 GeV and led to the detection of pulsed gamma-radiation from the Crab pulsar. However, that first prototype requires intensive maintenance and manual adjustment of each trigger channel and thus strongly limits its applicability for new high-resolution cameras comprising a larger number of pixels in the trigger area. Hence, a new setup with fully automatic setting of the trigger parameters has recently been designed. To minimize system costs and complexity, the improved version utilizes a sequential rate counting procedure to determine and compensate the gain and delay shifts of all channels caused by aging and changing operation conditions of the camera's photomultipliers (PMTs). The key element of the new system is a continuously tunable, passive delay line for fast analog signals enabling precise equalization of the PMT's signal transit times. When only a few small signals form the trigger, as in low energy air showers, precise temporal overlap in the analog sum is particularly essential for efficient detection. Unlike current commercial devices with bandwidths below 200 MHz and discrete delay setting, this new delay line allows smooth variation of each channel's delay within a range of 6 ns while maintaining a bandwidth of above 300 MHz, thus preserving the system's minimal pulse width of 2.6 ns. In 2010 a fully working new Sum-Trigger test setup comprising eight pixels has been built and temporarily installed at the MAGIC I telescope. It has been shown that the new design meets or even surpasses the requirements of a new analog trigger system with an anticipated energy threshold below 25 GeV.

A Tunable Delay Line for Fast Analog Pulses as Key Element of a New Sum-Trigger for Cherenkov Telescopes

For the first time, pulsed γ-rays above 25 GeV from the Crab pulsar have been detected with the MAGIC telescope [1]. All Crab pulsar emission models predict that the energy spectrum of the pulsed emission drops off sharply somewhere between a few GeV and a few tens of GeV. The lack of experimental data at the relevant energies prevente d so far the discrimination between the different theoretical scenarios of γ-ray emission in pulsars. The MAGIC measurements reveal that the drop-off in the emitted radiation occurs at relativ ely high energies, which indicates that the emission must occur far out in the Crab pulsar’s magnetos phere. All models in which the emitting region is located close to the Crab pulsar’s surfac e (e.g., the so-called polar cap model) are ruled out by the MAGIC results.

First detection of the Crab pulsar above 25 GeV with the MAGIC Telescope

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First detection of the Crab pulsar above 25 GeV with the MAGIC Telescope