Status report of the 17 m telescope MAGIC

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Abstract—The MAGIC telescope with its 17 m diameter mirror is today the largest operating sinlge dish Imaging Cherenkov Telescope. It is located on the Canary Island la Palma, at 2200 m above sea level, as part of the Roque de los Muchachos European Northern Observatory. It was inaugurated in October 2003, and it is taking data since autumn 2004. A second telescope of the same type is under construction for stereoscopic observations.

I. I NTRODUCTION

The 17-meter diameter MAGIC cosmic gamma-ray telescope (located at La Palma, 28.8

N, 17.8

W, 2220 m asl) is currently the largest and technologically most advanced imaging atmospheric Cherenkov telescope (IACT), Figure 1, [1-2].

Figure 1: The MAGIC telescope, summer 2004 Together with other new IACTs and satellite-borne instruments, the MAGIC Telescope is aiming to open and explore a new window in the spectrum of cosmic

electromagnetic radiation, between 30 and 250 GeV. MAGIC has a particularly important mission—to uncover the lower half of this interval, below 100 GeV, which may be decisive for several important scientific topics: the physics of various galactic and extragalactic gamma-ray sources, extragalactic background light (EBL), gamma ray bursts (GRB), pulsars, detection of dark matter, tests of quantum gravity, and the longstanding problem of the origin of cosmic rays. This important energy window has been ‘hostile’ to experimental research, because the energy has been too high for the small space-borne experiments, and at the same time too low to allow the IACTs to detect a significant number of Cherenkov photons[3-4].

Thanks to its 241 m

mirror area and numerous techniques introduced for the first time in this field, MAGIC now provides high enough sensitivity in order to reach a detection threshold of only 30 GeV [5]. We started with regular data

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taking in August 2004, and soon observed several galactic and extragalactic sources with high significance. The ON- OFF and the wobble mode observations have been used.

Physics results of the first year observations with MAGIC are shown in another paper from these proceedings [6-8].

II.P ROPERTIES OF THE MAGIC TELESCOPE

The essential parameters of the MAGIC telescope are[5]:

1) a 17 m diameter mirror with f/D=1,05.

2) a multi-segmented mirror (964 elements) of 241 m

total area.

3) An isochronous parabolic mirror profile to minimize the time spread of the Cherenkov photons.

4) A camera of 3.75º field of view, divided to an inner , higher resolution section of 397 pixels of 0.1º each and an outer section of 180 pixels of 0.2º each.

5) A dedicated PMT in the center of the camera with its associated electronics is devoted for optical observations of pulsed emission from gamma sources[9].

6) An operation range of 360º in azimuth and 105º in declination.

7) A present energy threshold at trigger level of 50 GeV.

The MAGIC telescope is based on a large number of novel elements:

1. A lightweight 17 m mirror support space frame, based on carbon fiber epoxy tubes comprising negligible thermal expansion, superior damping of oscillations, and reduced deformations compared to standard designs (steel, aluminum).

2. Novel lightweight, diamond-turned aluminum mirrors with internal heating for the prevention of dew and ice deposition [10-11].

3. A novel type of light concentrator, which provides increased quantum efficiency in synergy with new hemispherical PMTs, and a novel wavelength shifter coating which in conjunction grant a ~20% increase in photon detection efficiency [12]. Compared to classical Cherenkov telescope constructions, the overall photoelectron yield compares to a state of the art telescope of around 300 m

mirror area, i.e.

2.5-3 times larger than that of the nearest competing IACTs.

4. New hemispherical PMTs with only 6 dynodes and low gain (2x10

). Thanks to the low gain, these PMTs work efficiently at high levels of night sky background, including moonlight, dusk and dawn.

This new technique was successfully tested with one of the HEGRA telescopes during the observation of an intense flare of Mkn 501 in 1997. The technique, currently exclusively applied in MAGIC, is very important because it nearly doubles the observation time, though with a slightly higher detection threshold.

5. A novel large bandwidth analog optical fiber transmission system, designed for the transmission

Status report of the 17 m telescope MAGIC Maria Victoria Fonseca for the MAGIC Collaboration

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of the short ( < 2 ns) PMT pulses to the 100 m far counting house [13]. Apart from providing low signal distortion, this system significantly reduces the camera weight and electrically decouples the electronics in the counting house from the camera mast (very important for lighting protection and low cross talk).

6. A DAQ based on FADCs, for optimal data

collection close to the threshold. Initially a 300 MHz FADC system was used and lately a 2 GHz FADC system was implemented [14].

7. A high rate DAQ, capable of recording at a steady rate of up to 1 KHz and 10 KHz, for limited time, or restricted camera area readout, respectively.

8. A multilevel trigger system that can easily be reprogrammed according to different measurement objectives.

9. Telescope drive system that allows for rapid repositioning of the telescope to any point in the sky within <25 seconds. This feature is of crucial importance for the measurement of counterparts of gamma ray bursts (GRBs) upon rapid notification by SWIFT or other satellite borne experiment [15].

10. A novel active mirror control system that provides fast correction of the global mirror shape for the inevitable deformations of the 17 m mirror support space frame. Each of the 241 mirror panels is adjustable through a pair of actuators, and controlled by laser spot monitoring on a special screen in front of the telescope camera.

11. Two small optical telescopes, the 60 cm KVA telescope provided by the MAGIC collaboration partner from the University of Tuorla, and a 12”

Meade telescope are operated in the so-called slave mode to allow for concurrent optical observations.

Further details can be found in references [16-25] and also in The MAGIC Collaboration web pages:

http://wwwmagic.mppmu.mpg.de/ ; http://www.magic.iac.es/

III. MAGIC II: a second telescope for MAGIC The MAGIC collaboration is currently constructing a second telescope (MAGIC II) at La Palma [26]. It is located at a distance of 85 m from the first MAGIC telescope (Figure 2).

Figure 2.- The structure of the MAGIC II telescope seen in the foreground has been installed in December 2005, without mirrors and camera. The MAGIC I telescope is in the background.

The main observation mode will be the coincidence mode or stereo mode. This allows an improved reconstruction of the air shower, resulting in a better angular and energy

resolution. More importantly it improves the power to reject background induced by hadronic cosmic rays. In particular single track events, e.g. muons, can effectively be rejected since they are mostly seen by only one telescope. Currently the analysis of low energy events is limited by a low signal to background ratio. Therefore the improved background rejection power of a stereo system is also expected to decrease the energy threshold.

Figure 3: The sensitivity curve of MAGIC I for point like sources, (obtained from Monte Carlo calculations) as a function of energy. Sensitivity curves for HESS, MAGIC II and VERITAS are also given.

In stereo mode, only those gamma-ray showers whose Cherenkov light pool illuminates both telescopes can be used for the analysis. Nevertheless, the combined sensitivity of both telescopes in coincidence mode will be increased by a factor of nearly two over the sensitivity of a single telescope (figure 3). This has to be compared to an increase in

sensitivity by only a factor of 2 when operating both telescopes independently.

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MAGIC II will be largely a clone of the first MAGIC, reducing time, manpower and money necessary for its construction.

Nevertheless, several improvements will be introduced in MAGIC II:

The camera will have a larger number of small 0.1º pixels, and the trigger area will be increased by 72%. This will increase the effective field of view, which will improve the observation of extended sources and the potential of sky scans.

Initially the camera will be equipped with photomultipliers (PMTs). However it is planned to partially replace the PMTs with very high quantum efficiency GaAsP hybrid photodetectors (HPDs) [27]. The overall light detection efficiency will be increased by a factor of 2, equivalent to an increase of the mirror area by a factor of 2. This will decrease the energy threshold by a similar factor.

The complete signal processing chain from the light sensor to the digitizer is optimized for large bandwidth. The pulses will be sampled and digitized by a 2 GigaSample/s switch capacitor ring sampler [28]. The shape of the 1-2 ns short Cherenkov light pulses emitted by gamma-ray showers can thus be recorded with high resolution. This is expected to further improve the background rejection power due to the different time structure of hadron, muon and gamma events.

As can be seen in figure 2, the frame of MAGIC II has been already installed in December 2005. It is planned to complete the installation and to start data taking during the second half of 2007. This will allow combined observations with the GLAST satellite telescope which observes in the energy range below 100 GeV and which will be launched in 2007.

Simultaneous observations of MAGIC and GLAST will allow a precise cross calibration of both instruments, and will extend the energy spectrum of the combined observation to about 5 orders of magnitude.

III. A NEXT GENERATION GROUND - BASED GAMMA RAY FACILITY :CTA

The MAGIC and HESS collaborations are working together in the planning of new facility, one or several Cherenkov telescopes Arrays or CTAs, for the next generation of ground-based gamma-ray astronomy. Imaging atmospheric Cherenkov telescopes have proven an extremely successful approach to gamma-ray astronomy in the energy range above a few tens of GeVs. The proposed facility will consist of arrays of telescopes, aiming to: 1) increase the sensitivity by another order of magnitude for deep observations, 2) boost significantly the detection area and hence the detection rates, particularly important for transient phenomena at the highest energies, 3) increase the angular resolution and hence the ability to resolve the morphology of extended sources, 4) provide wide and uniform energy coverage from some tens of GeVs to 100 TeV, and 5) enhance the all sky survey and monitoring capability.

These features will allow the exploration of non-thermal processes in the Universe, in close cooperation with and complementing observatories in other wavelength ranges of

electromagnetic radiation, and for other messenger types.

The low energy threshold and the significantly improved sensitivity will dramatically increase the number of sources (more than 500 sources in northern and southern hemispheres) and the quality of the data. Given the wealth of sources in the central region of our Galaxy and the richness of their morphological features, a site in the southern hemisphere is attractive. On the other hand, a complementary northern site has to be considered for the study of AGNs and the cosmological evolution of galaxies and star formation.

An ensemble of northern and southern sites will provide full sky coverage and rich physics results.

Data from Cherenkov telescopes of the latest generation have revealed a sky rich in features above 100 GeV energies;

in the last years the number of sources has quadrupled. First sky maps show the band of the Milky Way lined with cosmic accelerators, with complex and resolved morphology.

Extragalactic sources at unprecedented distances of up to 3 billion light years have been detected; the shape of their gamma-ray spectra relates to the density of radiation in the space between galaxies, and thus to the hotly debated history od cosmological structure formation. Gamma rays from distant galaxies probe effects of quantum gravity. A vivid interplay between high energy instruments and other domains of astronomy from radio to X-rays has developed, with common observation campaigns and exchange of data. While the results achieved with current instruments are already very impressive, the detailed understanding of processes in cosmic particle accelerators as well as their use for cosmological applications requires wider energy coverage, improved resolution and higher detection rates. The performance in this domain can be improved dramatically by a much larger deployment based on now well established techniques and observation strategies. The opportunity for discoveries and growth similar to the one experienced in high energy gamma ray astronomy from satellites is now clearly perceived in the adjunt domain of very high energy astronomy. The proposed facility should result in breakthroughs in several fields of modern astronomy such as cosmic rays origin, the environment of compact objects, the physics of pulsars and black holes, and possibly the long standing question of the nature of dark matter.

The performance and scientific potential of arrays of Cherenkov telescopes has been studied in significant detail, what remains to be decided is the exact layout of the telecope array. Ample experience exists in constructing and operating telescopes of the 10-12 m class (H.E.S.S. and VERITAS) and 17 m class (MAGIC). MAGIC II and H.E.S.S. II [29] will also serve as prototypes for CTA developments. Photon detectors with improved quantum efficiency are already under advanced development and testing, and will be available when the array is constructed. After a phase of detailed design (2006-2009), implementation could start around 2010, allowing a significant overlap with the GLAST satellite instruments to be launched at the end of 2007, which covers the energy range below some tens of GeVs and which serves as an all-sky monitor, triggering pointed observations at higher energies.

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IV. C ^ONCLUSION

The MAGIC telescope is operational and taking data since 2004. The first observation year was completed with success and the second is in progress. A second telescope is under construction and will become operational by the end of 2007.

The addition of more telescopes opens the possibility to observe more than one source at a time and to carry out more precise measurements in the stereo mode.

ACKNOWLEDGMENT

I would like to thank my colleagues from the MAGIC collaboration who, with his continous effort, make possible the operation of the telescope and its excellent Physics results, and in particular I want to thank Masahiro Teshima for his help preparing this manuscript.

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