M. Shayduk

Long-term lightcurves from combined unified very high energy γ-ray data

Now at Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, GermanyReceived ; acceptedABSTRACTContext. Very high-energy (VHE, E>100GeV) γ-ray data are a valuable input for multi-wavelength and multi-messenger (e.g. com-bination with neutrino data) studies.Aims. We aim at the conservation and homogenization of historical, current, and future VHE γ-ray-data on active galactic nuclei(AGN).Methods. We have collected lightcurve data taken by major VHE experiments since 1991 and combined them into long-termlightcurves for several AGN, and now provide our collected datasets for further use. Due to the lack of common data formats inVHE γ-ray astronomy, we have defined relevant datafields to be stor ed in standard data formats. The time variability of the combinedVHE lightcurve data was investigated, and correlation with archival X-ray data collected byRXTE/ASM tested.Results. The combination of data on the prominent blazar Mrk421 from different experiments yields a lightcurve spanning morethan a decade. From this combined dataset we derive an integral baseline flux from Mrk421 that must be lower than 33% of the C rabNebula flux above 1TeV. The analysis of the time variability y ields log-normal flux variations in the VHE-data on Mrk421.Conclusions. Existing VHE data contain valuable information concerning the variability of AGN and can be an important in-gredient for multi-wavelength or multi-messenger studies. In the future, upcoming and planned experiments will provide moredata from many transient objects, and the interaction of VHE astronomy with classical astronomy will intensify. In this con-text a unified and exchangeable data format will become incre asingly important. Our data collection is available at the url:http://nuastro-zeuthen.desy.de/magicexperiment/projects/lightcurvearchive/indexeng.html.Key words. Gamma-rays: observations – Galaxies: active – Galaxies: individual: Mrk421 – Galaxies: individual: Mrk501

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.

A new trigger provides a lower energy threshold for the MAGIC Cherenkov 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. This improves 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, which was previously thought only possible with gamma-ray detectors in space.

NectarCAM : a camera for the medium size telescopes of the Cherenkov Telescope Array

NectarCAM is a camera proposed for the medium-sized telescopes of the Cherenkov Telescope Array (CTA) covering the central energy range of ~100 GeV to ~30 TeV. It has a modular design and is based on the NECTAr chip, at the heart of which is a GHz sampling Switched Capacitor Array and a 12-bit Analog to Digital converter. The camera will be equipped with 265 7-photomultiplier modules, covering a field of view of 8 degrees. Each module includes the photomultiplier bases, high voltage supply, pre-amplifier, trigger, readout and Ethernet transceiver. The recorded events last between a few nanoseconds and tens of nanoseconds. The camera trigger will be flexible so as to minimize the read-out dead-time of the NECTAr chips. NectarCAM is designed to sustain a data rate of more than 4 kHz with less than 5\% dead time. The camera concept, the design and tests of the various subcomponents and results of thermal and electrical prototypes are presented. The design includes the mechanical structure, cooling of the electronics, read-out, clock distribution, slow control, data-acquisition, triggering, monitoring and services.

A versatile digital camera trigger for telescopes in the Cherenkov Telescope Array

This paper describes the concept of an FPGA-based digital camera trigger for imaging atmospheric Cherenkov telescopes, developed for the future Cherenkov Telescope Array (CTA). The proposed camera trigger is designed to select images initiated by the Cherenkov emission of extended air showers from very-high energy (VHE, E > 20 GeV) photons and charged particles while suppressing signatures from background light. The trigger comprises three stages. A rst stage employs programmable discriminators to digitize the signals arriving from the camera channels (pixels). At the second stage, a grid of low-cost FPGAs is used to process the digitized signals for camera regions with 37 pixels. At the third stage, trigger conditions found independently in any of the overlapping 37-pixel regions are combined into a global camera trigger by few central FPGAs. Trigger prototype boards based on Xilinx FPGAs have been designed, built and tested and were shown to function properly. Using these components a full camera trigger with a power consumption and price per channel of about 0.5 W and 19 e, respectively, can be built. With the described design the camera trigger algorithm can take advantage of pixel information in both the space and the time domain allowing, for example, the creation of triggers sensitive to the time-gradient of a shower image; the time information could also be exploited to online adjust the time window of the acquisition system for pixel data. Combining the results of the parallel execution of dierent trigger algorithms (optimized, for example, for the lowest and highest energies, respectively) on each FPGA

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

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 cameras 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 PMTs 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 channels delay within a range of 6 ns while maintaining a bandwidth of above 300 MHz, thus preserving the systems 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.

Light Sensor Candidates for the Cherenkov Telescope Array

We report on the characterization of candidate light sensors for use in the next-generation Imaging Atmospheric Cherenkov Telescope project called Cherenkov Telescope Array, a major astro-particle physics project of about 100 telescopes that is currently in the prototyping phase. Our goal is to develop with the manufacturers the best possible light sensors (highest photon detection efficiency, lowest crosstalk and afterpulsing). The cameras of those telescopes will be based on classical super-bi-alkali Photomultiplier tubes but also Silicon Photomultipliers are candidate light sensors. A full characterisation of selected sensors was done. We are working in close contact with several manufacturers, giving them feedback and suggesting improvements.

Fast readout of multi-channel detectors by using a CCD/CMOS camera

The multi-channel detectors are essential part of modern experiments. The number of readout channels for every future detector is usually much higher than before. Usually the number of channels of a detector constrains the minimal data acquisition system (DAQ) requirements and the total price of the readout system. For the future groundbased gamma-ray instruments like for example, the Cerenkov Telescope Array (CTA) the DAQ system should fulfill several stringent requirements: low power consumption, compactness, ability to record a rate of 1kHz for several thousand channels and affordable cost per channel. We present here an alternative to the conventional ADC system. The analog pulses from the light sensors are converted into optical signals and fed into the optical fibers, which are bundled as a fiber optic plate (FOP). The externally triggered high-speed CMOS camera coupled to a fast Gated Image Intensifier makes a photo of the FOP, when the optical signal flashes the FOP. The Gated Image Intensifier serves as an ultra-fast optical shutter and event delimiter.