D. Hildebrand

Design and Operation of FACT -- The First G-APD Cherenkov Telescope

The First G-APD Cherenkov Telescope (FACT) is designed to detect cosmic gamma-rays with energies from several hundred GeV up to about 10 TeV using the Imaging Atmospheric Cherenkov Technique. In contrast to former or existing telescopes, the camera of the FACT telescope is comprised of solid-state Geiger-mode Avalanche Photodiodes (G-APD) instead of photomultiplier tubes for photo detection. It is the first full-scale device of its kind employing this new technology. The telescope is operated at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands, Spain) since fall 2011. This paper describes in detail the design, construction and operation of the system, including hardware and software aspects. Technical experiences gained after one year of operation are discussed and conclusions with regard to future projects are drawn.

Calibration and performance of the photon sensor response of FACT — the first G-APD Cherenkov telescope

The First G-APD Cherenkov Telescope (FACT) is the first in-operation test of the performance of silicon photo detectors in Cherenkov Astronomy. For more than two years it is operated on La Palma, Canary Islands (Spain), for the purpose of long-term monitoring of astrophysical sources. For this, the performance of the photo detectors is crucial and therefore has been studied in great detail. Special care has been taken for their temperature and voltage dependence implementing a correction method to keep their properties stable. Several measurements have been carried out to monitor the performance. The measurements and their results are shown, demonstrating the stability of the gain below the percent level. The resulting stability of the whole system is discussed, nicely demonstrating that silicon photo detectors are perfectly suited for the usage in Cherenkov telescopes, especially for long-term monitoring purpose.

A novel camera type for very high energy gamma-ray astronomy based on Geiger-mode avalanche photodiodes

Geiger-mode avalanche photodiodes (G-APD) are promising new sensors for light detection in atmospheric Cherenkov telescopes. In this paper, the design and commissioning of a 36-pixel G-APD prototype camera is presented. The data acquisition is based on the Domino Ring Sampling (DRS2) chip. A sub-nanosecond time resolution has been achieved. Cosmic-ray induced air showers have been recorded using an imaging mirror setup, in a self-triggered mode. This is the first time that such measurements have been carried out with a complete G-APD camera.

FACT - The first G-APD Cherenkov telescope (first results)

In October 2011, the first air-Cherenkov telescope utilizing Geiger-mode avalanche photodiodes commenced operations. The silicon-based devices display several advantages compared to classical photomultiplier tubes allowing for a more compact camera design of higher reliability, lower power consumption and bias voltage, and better prospects for improving the photon detection efficiency. Here, the first physics results are presented from a few months of data taking. Although still preliminary, the results already show a superb fidelity of the data, demonstrating the potential of avalanche photodiodes for ground-based gamma ray astronomy. The stability and high sensitivity are ideal for remote monitoring observations of variable gamma-ray sources.

FACT - The G-APD revolution in Cherenkov astronomy

Since two years, the FACT telescope is operating on the Canary Island of La Palma. Apart from its purpose to serve as a monitoring facility for the brightest TeV blazars, it was built as a major step to establish solid state photon counters as detectors in Cherenkov astronomy. The camera of the First G-APD Cherenkov Telesope comprises 1440 Geiger-mode avalanche photo diodes (G-APD), equipped with solid light guides to increase the effective light collection area of each sensor. Since no sense-line is available, a special challenge is to keep the applied voltage stable although the current drawn by the G-APD depends on the flux of night-sky background photons significantly varying with ambient light conditions. Methods have been developed to keep the temperature and voltage dependent response of the G-APDs stable during operation. As a cross-check, dark count spectra with high statistics have been taken under different environmental conditions. In this presentation, the project, the developed methods and the experience from two years of operation of the first G-APD based camera in Cherenkov astronomy under changing environmental conditions will be presented.

Long term VHE gamma ray monitoring of bright blazars with a dedicated Cherenkov telescope

We intend to set up an imaging air Cherenkov telescope with low cost, but high performance design for remote operation. The goal is to dedicate this gamma-ray telescope to long-term monitoring observations of nearby, bright blazars at very high energies (VHE). We will (i) search for orbital modulation of the blazar emission due to supermassive black hole binaries, (ii) study the statistics of flares and their physical origin, and (iii) correlate the data with observations of flares with higher sensitivity telescopes such as MAGIC, VERITAS, and H.E.S.S. Common observations with theWhipple 10m-monitoring telescope will be the first step towards a future 24 h-monitoring of selected sources. This idea was presented for the first time in [1]. The telescope design is based on a full technological upgrade of one of the former telescopes of the HEGRA collaboration, still located at the Observatorio del Roque de los Muchachos on the Canarian Island of La Palma (Spain). After this upgrade, the telescope will be operated robotic, its sensitivity will greatly be improved and a much lower energy threshold below 350GeV will be achieved.

Long‐term monitoring of bright blazars with a dedicated Cherenkov telescope

Blazar observations in VHE Gamma‐rays show intensity variations on time scales of minutes to years, most frequently with variability times of about one day. They could be caused by the interaction of relativistic jets with the surroundings, but also carry the signature of internal processes of the central engine, possibly binary systems of supermassive black holes. Ultimately, long‐term monitoring with 24‐hour coverage is needed in addition to the shorter high sensitivity exposures provided by telescopes such as MAGIC, VERITAS and H.E.S.S., in order to study the physical origin of such flaring activity. This can be achieved with a global network of small robotic Cherenkov telescopes. As a first step, we plan to set up a fully dedicated small Cherenkov telescope and carry out joint observations with the Whipple 10 m monitor telescope. The new low cost, but high performance telescope will be the upgrade of one of the former HEGRA telescopes, still located at the Observatorio del Roque de los Muchachos on th...

FACT - Calibration of Imaging Atmospheric Cerenkov Telescopes with Muon Rings

M. Nothe∗, a M. L. Ahnen b, M. Balbo c, M. Bergmann d , C. Bockermann e, A. Biland b, T. Bretz b, K. A. Brugge a, J. Buss a, D. Dorner d , S. Einecke a, J. Freiwald a, C. Hempfling d , D. Hildebrand b, G. Hughes b, W. Lustermann b, K. Mannheim d , K. Meier d , K. Morik e, S. Muller b, D. Neise b, A. Neronov c, A.-K. Overkemping a, A. Paravac d , F. Pauss b, W. Rhode a, F. Temme a, J. Thaele a, S. Toscano c, P. Vogler b, R. Walter c, and A. Wilbert d Email: maximilian.noethe@tu-dortmund.de

FACT-Tools: Streamed Real-Time Data Analysis

K. A. Brügge b∗, M. L. Ahnena, M. Balboc, M. Bergmannd , A. Bilanda, C. Bockermanne, T. Bretza, J. Bussb, D. Dornerd , S. Eineckeb, J. Freiwaldb, C. Hempflingd , D. Hildebranda, G. Hughesa, W. Lustermanna, K. Mannheimd , K. Meierd , K. Morike, S. Müllera, D. Neisea, A. Neronovc, M. Nötheb, A.-K. Overkempingb, A. Paravacd , F. Paussa, W. Rhodeb, F. Temmeb, J. Thaeleb, S. Toscanoc, P. Voglera, R. Walterc, and A. Wilbertd Email: kai.bruegge@tu-dortmund.de

FACT - Status and Experience from Three Years Operation of the First SiPM Camera

A. Biland∗a, M. L. Ahnena, M. Balbob, M. Bergmannc, T. Bretza,1, K. A. Brugged , J. Bussd , D. Dornerc, S. Einecked , J. Freiwaldd , C. Hempflingc, D. Hildebranda, G. Hughesa, W. Lustermanna, K. Mannheimc, K. Meierc, S. Mullera, D. Neisea, A. Neronovb, M. Nothed , A.-K. Overkempingd , A. Paravacc, F. Paussa, W. Rhoded , F. Temmed , J. Thaeled , S. Toscanob, P. Voglera, R. Walterb, and A. Wilbertc aETH Zurich, Institute for Particle Physics Otto-Stern-Weg 5, 8093 Zurich, Switzerland bUniversity of Geneva, ISDC Data Center for Astrophysics Chemin d’Ecogia 16, 1290 Versoix, Switzerland cUniversitat Wurzburg, Institute for Theoretical Physics and Astrophysics Emil-Fischer-Str. 31, 97074 Wurzburg, Germany dTU Dortmund, Experimental Physics 5 Otto-Hahn-Str. 4, 44221 Dortmund, Germany 1also at RWTH Aachen E-mail: biland@phys.ethz.ch