The History of Ground-Based Very High Energy Gamma-Ray Astrophysics with the Atmospheric Air Cherenkov Telescope Technique

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Nuclear Physics B (Proc. Suppl.) 239–240 (2013) 26–34 www.elsevier.com/locate/npbps

The History of Ground-Based Very High Energy Gamma-Ray Astrophysics with the Atmospheric Air Cherenkov Telescope Technique Razmik Mirzoyan Max-Planck-Institute for Physics, Munich, Germany

Abstract In the recent two decades the ground-based technique of imaging atmospheric Cherenkov telescopes has established itself as a powerful new discipline in science. As of today some ∼ 150 sources of gamma rays of very different types, of both galactic and extragalactic origin, have been discovered due to this technique. The study of these sources is providing clues to many basic questions in astrophysics, astro-particle physics, physics of cosmic rays and cosmology. The current generation of telescopes, despite the young age of the technique, offers a solid performance. The technique is still maturing, leading to the next generation large instrument known under the name Cherenkov Telescope Array. The latter’s sensitivity will be an order of magnitude higher than that of the currently best instruments VERITAS, H.E.S.S. and MAGIC. This article is devoted to outlining the milestones in a long history that step-by-step have given shape to this technique and have brought about today’s successful source marathon.

1. Introduction Several very interesting papers, along with the classical book of Jelley [1], have been devoted to the history of Cherenkov emission ([2], [3], [4], [5], [6], [7]) and its use for ground-based very high energy (VHE) gamma astrophysics. It is not the intention of this paper to repeat those comprehensive articles, but rather to give a personal impression about the main developments that have played a key role in evolution of the groundbased VHE gamma astrophysics. The author is involved for three decades in the VHE gamma astrophysics and could in person observe some of the relatively recent important developments. In recent years the Imaging Atmospheric Cherenkov Technique (IACT) has made giant steps in establishing itself as a very successful new branch of astrophysics, see, for example, [8] for a recent review. With the discovery of very high energy (VHE) gamma rays from the Crab nebula with 9σ significance in 1989, the Whipple team, operating the 10m diameter IACT in Arizona, 0920-5632/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysbps.2013.05.004

laid the foundation for the new science [9]. The discovery of the second source of extragalactic nature, this time Mkn-421, followed in 1992, again by the Whipple team, led by Trevor Weekes. This important discovery stopped the speculations that it was a science of a single source only. In the meantime other telescope installations were built, like HEGRA and CANGAROO, who very soon independently confirmed the Crab nebula as a source. It took another few years until MKN-501 was firmly discovered as a third source, in 1996. The designs and parameters of the telescopes were improving. For example, an optimal pixel size of ∼ 0.25◦ , based on better and faster PMTs, started to be used in the imaging camera, at first by the Whipple team, but a few years later also by HEGRA [47]. CANGAROO used PMTs of angular aperture of 0.11◦ [11]. The CAT telescope, put into operation in late autumn 1996 [12], on the same site as the previous ASGAT [13] and THEMISTOCLE [14] instruments, started operation using a pixel size of 0.12◦ . But before going into the details of relatively recent

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important developments that lead us to today’s success, I want to go back in time to the last century for showing how the things have began. 2. Very early days Oliver Heaviside calculated the movement of an electron in a transparent medium with a speed higher than that of light (please note that until the beginning of the 20th century scientists believed that the space was filled by “ether”, transporting the electromagnetic waves). He showed that such movement would be accompanied by a specific conical emission see, for example, [15]. Though he published a series of papers on this issue, these stayed unnoticed; his genius has advanced understanding of the problem by half a century. Also Arnold Sommerfeld calculated the task of a charge moving in vacuum with a speed v ≥ c. The relativistic principles prohibit such a motion in vacuum but in a medium with a given refraction index n his equations gave a valid solution. In his paper in 1904 he calculated this hypothetical task and came to very similar conclusions about the necessity to emit special emission [16]. The first experimental notices about the bluish glow from bottles with liquids containing radioactive radium salts in her dark cellar, is attributed to Madame Curie in 1910, who thought of it as some kind of luminescence [17]. The first systematic study of the effect was performed by the French scientist Mallet, who published three papers in the years 1926-1929. Mallet described the phenomenon and even could measure its continous emission spectrum that, because of the absense of emission lines and bands, contradicted the previously assumed fluorescence explanation. But he failed to reveal the most significant polarisation and anisotropy effects of the unknown emission [18]. In 1932, young Pavel Cherenkov became a PhD student of Sergey Vavilov, who in 1934 became the director of the newly created Lebedev Institute for Physics of the Academy of Sciences of the former Soviet Union. S. Vavilov assigned the task of studying the bluish “luminescent” emission to Cherenkov as a topic for his PhD thesis. Soon afterwards Cherenkov found out that he would spend many hours in the dark, cold cellar. That was necessary for accommodating his eyes, the measuring instrument, to the darkness, for seeing the extremely faint emission. Soon after starting Cherenkov formally complained about the unusual working conditions and the unusual task, but after detailed explanatory work of Valilov he agreed to continue the studies. He started varying the temperature and the pressure of liquids in large margins, but still the emission

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was there. He added special additives to the liquids that should have quenched the luminescence, but that did not happen. Importantly, he failed to find the characteristics of luminescence spectral lines, and instead detected light emission with a continuous spectrum. Cherenkov could measure light emission even from solvent liquids. Based on these results, in 1934 he wrote an article about his studies that S. Vavilov declined to coauthor [19]. Instead Vavilov wrote his own explanatory paper that appeared next to Cherenkov’s paper in the same issue of the journal [20]. Vavilov interpreted that continuous spectrum as bremsstrahlung emission of electrons. It took another three years until Cherenkov could show in a simple, elegant experiment the anisotropic character of the emission; light was emitted only within a certain angular range in the forward direction. He submitted his discovery paper to the journal Nature. For an unknown reason the journal declined publishing it. After that he submitted it to the Physical Review, where finally it was published in 1937 [21]. In the meantime the theoreticians Igor Tamm and Ilya Frank put forward a theoretical explanation of that phenomenon [22]. Let n be the refraction index of a transparent, dielectric medium, and c/n the speed of electromagnetic interactions in it (c is the speed of light in vacuum). When a charged particle moves with a speed higher than c/n, along its path it asymmetrically polarizes the medium, that is a somewhat too slow for following the fast escaping particle. Short afterwards the medium relaxes by emitting anisotropic radiation in the forward direction. Sergei Vavilov became the president of the Academy of Sciences of the Soviet Union in 1946. The work of Cherenkov, Tamm, Frank and Vavilov got the highest recognition in the Soviet Union and they were awarded the then most prestigious Stalin prize. Vavilov was living in permanent fear about his very famous botanist and geneticist brother Nikolai, who was first arrested in 1940, then sentenced to death and subsequently commuted to imprisonment of 20 years; Nikolai died in prison in 1943, presumably from hunger. As a result of multiple heart attacks Sergei Vavilov died in 1951. In 1956, the works of Cherenkov, Tamm and Frank were awarded the Nobel prize. Vavilov was not nominated, only alive people can be awarded the Nobel prize. It is interesting to note that in Russian literature some preference in this discovery is given to Vavilov, in fact the effect is known under the name of Vavilov-Cherenkov emission. Some reflection of this could be the fact that while being one of the very few Nobel prize winners in the former Soviet Union, only very late, in 1964, Cherenkov was accepted as a corresponding member in the Soviet Academy of Sciences.

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Let us now have a look into the next events that have played a key role in the evolution of the IACT technique. 2.0.1. Discovery of Cherenkov emission in atmosphere In 1948, Patrick Blackett, while studying the emission of light from the night sky and aurorae, estimated that there should be a 10−4 part coming from air shower elementary particles producing Cherenkov emission in the atmosphere [24]. During his visit of Harwell in 1952 he met with J.V. Jelley and W. Galbraith and learnt that they were also experimenting with Cherenkov light emission, but in water. Blackett mentioned to them about his estimation of the Cherenkov light contribution in the atmosphere. Very shortly after that Jelley and Porter built a simple setup consisting of a 25cm diameter parabolic signaling mirror of a short focal length, fixed it into a dustbin and put a single 5cm PMT in its focus. They did not had to wait for long until, the telescope was counting pulses, one every two minutes. They had to prove that these were in fact Cherenkov pulses coming from air showers. For this purpose they put the small telescope into coincidence with their large area Geiger counters for measuring air showers and observed coincident events. Within one year they improved the performance of their telescope and succeeded to measure the polarization of Cherenkov light as well found out that, compared to the green part of the spectrum, there was more light in the blue. Galbraith and Jelley published a discovery paper in Nature in 1953 which has marked the beginning of atmospheric Cherenkov technique and measurements. Except for relatively small-scale test measurements not very much happened in the following few years. 2.1. First ideas on astronomy by means of gamma-rays In a seminal paper Philip Morrison suggested to measure gamma rays from possible cosmic sources in 1958 [25]. He considered that measurements in the energy range (0.2 − 400)MeV using detectors on high flying aircrafts or balloon flights below ∼ 25g/cm2 could provide a real clue to cosmic rays and sources. Different type detectors could be efficient for measuring gamma rays in that energy range. He mentioned the Crab Nebula as a source of gamma rays but, interestingly from today’s point of view, at lower energies; the expanding gas shell in the nebula contains radioactive debris from the original explosion. He predicted that a flux of specific nuclear gamma rays, mainly of 226 Ra, should be detectable, with an incoming flux of 10−2 gamma/cm2 ·s. Guiseppe Cocconi suggested to measure gamma ray sources at TeV energies at the International Cosmic Ray

Conference (ICRC) in Moscow in 1959 [26]. He suggested constructing an air shower array at a high mountain altitude with an angular resolution of ∼ 1◦ for measuring ∼ 1012 eV gammas. He made some very optimistic prediction on the possible flux of gamma rays from the Crab Nebula, which should have been about 1000 times higher than the background. He acknowledged the over-optimistic nature of his estimate, mentioning that even if the efficiency in the production of gamma rays were substantially lower, still the Crab Nebula could be detected. Very soon afterwards, Georgy Zatsepin, who was considering Alexander Chudakov as an expert for atmospheric Cherenkov emission, approached him and discussed the possibility of detecting gamma sources. For understanding the development of air showers in the atmosphere Alexander Chudakov and Natalia Nesterova were measuring the lateral distribution of Cherenkov light on Pamir mountain at a height of 3800ma.s.l. in 1953-1955. They were using large size Geiger counters and eight Cherenkov light receivers as detectors[27]. Chudakov became excited by the prospects of the new possible science and started planning systematic study of cosmic gamma ray source candidates in the sky. 3. First generation atmospheric Cherenkov telescopes 3.1. Chudakov’s telescopes in Crimea Chudakov and his colleagues built a system of 4 telescopes in Katsiveli, Crimea, near the shore of the Black Sea in 1960 [23]. One year later, they increased the number of telescopes to 12. These were based on parabolic mirrors of 1.55m diameter and focus of 60cm, previously used by militaries for securing the Black see border. 4.5cm diameter PMTs were set in their foci, with a diaphragm limiting the field of view to 1.75 FWHM. Chudakov calculated a lens with a special form for reducing aberrations and improving timing and placed it in front of the PMTs. Every 3 telescopes were rigidly connected and adjusted to observe the same direction. The 4 independent mounts, with 3 mirrors each, were able to independently rotate and observe sources with a pointing precision of 0.2◦ in elevation and 0.4◦ in azimuth. Four-fold coincidences between these telescopes were required. A rate-stabilizing circuitry was developed for getting rid of count-rate instabilities due to variation of the background light emission. The experiment was continued for ∼ 4 years. By using flashes of Cherenkov light initiated by muons in a

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piece of Plexiglas of a given thickness, the researchers estimated a photon threshold of 280ph/m−2 (for the wavelength range 300 − 600nm). The shower counting rate was slightly more than 3Hz and the estimated energy threshold was ∼ 3.4T eV for gammas. The observations were performed in the drift-scan mode: the telescopes were pointed to a direction through which, after a short while, the targeted source would pass. Then such scans, each of ∼ 7 minutes in duration, were repeated many times over the nights. It is impressive to see the list of sources observed by Chudakov’s crew. One should mention that back in that time nothing was known about the pulsars and their periods, about X-ray sources. The common believe was that radio sources should be good targets as gamma emitters. Together with the Crab Nebula (47 scans), Cygnus A (191 scans), Cassiopea A (20 scans), Virgo A (20 scans) also Perseus A (7 scans) and Saggitarius A (7 scans) were observed. The long observations of Cygnus A were due to initially small indications of a possible signal that the researchers were trying to confirm. Some not very systematic observations were performed for clusters of galaxies like Ursa Majoris II, Corona Borealis, Bootes, Coma Berenices. In the end, they could not observe signal from any of the sources. The signal upper limit from the direction of the Crab nebula for a total of ∼ 5.5 hours of observations was 5 × 10−11 ph/cm2 · s for the threshold of ∼ 4 − 5T eV. Today we know that the integral flux of the Crab nebula above 4TeV is ∼ 2.5 × 10−12 ph/cm2 · s. This means that the upper flux limit set by Chudakov and his colleague was ∼ 20 times higher than the current flux from the source. So, for measuring a significant signal from the Crab nebula they had to measure ∼ 400 times longer and that was of course very improbable. An important consequence of this non-detection of the Crab Nebula was that it turned down the ∼ 1000 times higher flux originally estimated by Cocconi. Also, non-detection of a signal cast strong doubts on the origin of electrons in the Crab nebula as secondaries produced in cosmic proton interactions with the nebula; pp → π → μ → e (the common believe was that gammas were produced due to pp → π0 → 2γ). That meant that in fact the electrons could have been accelerated in the nebula. 3.2. Photographing showers with Image Intensifiers An important measurement was realized by Hill and Porter in 1960 [28]. They coupled an image intensifier to a 25cm diameter Schmidt telescope of a 24◦ field of view and shot the first images of air showers in

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the sky. Because of the small size of the mirror, only images of showers with energies ≥ 0.5PeV could be recorded. The trigger came from a 5 inch PMT in the focus a conventional mirror that was set in close proximity. Bright background stars could be identified on images. The event rate was ∼ 7/hour. Researchers realized that measuring the shape of an air shower could give clues to the true direction of the shower as well as to its impact point on the ground. The estimated high angular resolution of ∼ 0.2◦ was considered as a very important feature that could allow one to largely (100 times!) suppress the background [29]; moreover, one can read there that “the stereoscopic technique with two separate telescopes would greatly enhance the potentialities”. This technique convincingly demonstrated the power of imaging but because of the small mirror size and the related high energies, because of the bulky image intensifier and the relatively long integration time of the phosphor screen (∼ 1μs), and some other disadvantages it turned out to be impractical. 3.3. Monte Carlo simulations and ”stereo” observations Member of the Chudakov’s crew Victor Zatsepin published a remarkable Monte Carlo study paper in 1964 [30]. By simulating on the Soviet “Ural”-type computer (operated by a specially trained staff) he obtained the first equal photon density contours of air shower images produced by gamma rays as well as their angular distributions and radial photon densities. In particular, he mentioned in his paper that ‘”since the maximum intensity of the light from a shower does not coincide with the direction of arrival of the primary particle, in researches in which the determination of the angular coordinates of the primary particle is made by photographing the light flash from the shower one should seek improved accuracy in this determination by photographing the shower simultaneously from several positions”. In a recent discussion he mentioned that almost 50 years ago he was intensively considering if there was any practical means of photographing showers from many directions, or more explicitly, which type of camera could do that [31]. As one can see from the above, already ∼ 50 years ago some researchers clearly understood the potential of the coincidence measurements, better known today as stereoscopy. The experiments with image intensifiers continued for some more years, but no further breakthrough happened. In the following years many experiments as a rule, of smaller scale than the one from Chudakov,

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though, were built and operated. Except for the 10m diameter Whipple telescope, which played an absolutely central role in giving birth to gamma astronomy (this will be discussed in some detail below), no major technical improvements were achieved until 80’s. As a rule, researchers used (0.6 − 1.5)m diameter military searchlight mirrors of parabolic shape of F/0.5 optics that were severely suffering from coma aberration. They used coincidence between a few of such mirror elements, which allowed lowering the energy threshold of the instrument. 3.4. Harwell-Glencullen telescope One should mention the compact telescope in Harwell, UK, which later on was moved to Glencullen valley near Dublin [29]. The telescope, set on a gun mount, consisted of two 92cm diameter back-silvered mirrors, each with a single PMT in its focus, viewing a ∼ 5◦ field of view. The coincident count rate was (0.7 − 1.7)Hz. Observations of quasars, the Crab nebula, Cygnus A, M31, and some other sources, were performed in the winter seasons of 1963, 1964 and 1965. Data collected from 75 useful nights, showed no signal [32]. From today’s point of view it is interesting to note that the data taking was done visually, namely the operator was watching a pair of scalers and every minute writing down the numbers in a log book [5]. In 1967-1968, a new telescope of very fast electronic design was developed and built in Glencullen that was shifted to Harwell for giving it a fast timing system for pulsar studies. Later on it was moved to Malta, where in early 1969 it started observations. This telescope had a very fast performance, namely it was using four F/2 mirrors of 90cm diameter, fast PMTs, fast amplifiers and a 3.5ns coincidence resolving time. No positive detections were made, flux upper limits for certain pulsars in the range of (0.5 − 2.5) × 10−11 ph/cm2 · s were set [33]. 3.5. Double Beam Technique Another interesting development in gamma astronomy is related to two 6.5m diameter reflectors set at a large distance (∼ 120m) for stellar interferometry in Narrabri, Australia. In 1968, the researchers carried out observations of the Crab nebula and two pulsars [34]. Later on, Grindlay and colleagues made a step forward by using the above telescopes for the “double beam” observation technique. Each telescope had 2 PMTs. While the main PMTs were inclined towards one another on ∼ 0.4◦ for observing the shower maximum region from a selected source direction, the other two PMTs were inclined to even lower angles of ∼ 1.3◦ towards each

other for measuring a signal from the “muon core” of the showers. The authors claimed that in this way they could reject ∼ 50% of the hadron background. That was not much but the principle was very interesting; modifications of it will be widely used in future. For giving an impression about the imaginary spirit of some of the observations about 40 years ago, it is interesting to shortly discuss one selected observation made with this instrument [35]. In that article a detection of a ∼ 5σ pulsed signal from the Crab pulsar is claimed on a flux level, for example, of 8 × 10−12 ph/cm2 · s at 1T eV, and this is at the absence of any DC signal from the Crab nebula. Today we know that the cited pulsed flux level makes ∼ 40% of the DC flux from the Crab nebula. Concerning the pulsed flux from the Crab pulsar at 1TeV we do not yet even know if there is flux at such a high energy. But we know that the pulsar flux at ∼ 400GeV is about 100 times less than the DC flux from the nebula [36]. Because of the space limitation of this article the author cannot give a fair description of all the existing telescope installations and their details like, for example, for the Haleakala telescope in Hawaii, the Potchefstrum telescope in South African Republic, the telescopes of the Durham group, the telescope installations in India. These are reflected in the proceedings of the workshop series of “Towards a Major Atmospheric Cherenkov Detector” as well as in proceedings of the international cosmic ray conferences. 4. The second generation telescopes 4.1. The 10m Whipple telescope In 1967, Giovanni Fazio and colleagues began constructing a 10m diameter, F/0.7 telescope on mount Hopkins, at the Whipple observatory at a height of 2300m a.s.l. [37]. The large diameter of the telescope, together with fast PMTs in the focal plane provided a low threshold not achievable earlier. The telescope started operating in 1968, initially with a single 5 inch PMT in the focus that somewhat later was increased to two and afterwards to ten PMTs for simultaneous ON and OFF source observations. Trevor Weekes has joined the Smithsonian project in 1966. In 1968 he coauthored G. Fasio and two other colleagues in an interesting observational paper about13 sources, among which were, for example, Crab nebula (of course!), M87, M82, IC443 [38]. The set flux upper limits above the threshold of 2T eV were modest, but today we know that the above listed sources in fact are gamma-ray emitters. In 1977 Weekes and Turver suggested to use two telescopes, each equipped with a 37-pixel imaging camera, at a separation of 100m. They expected that such a

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stereoscopic imaging system shall strongly suppress the background [39]. A 37-pixel imaging camera, covering a field of view of 3.5◦ in the sky, was built and installed on the 10m telescope in ∼ 1983. The next real milestone was the image parameterisation suggested by Michael Hillas in 1985 at the La Jolla conference [40]. A few years later that has helped the Whipple team to measure the famous signal from the Crab nebula (see below). Since then this set of parameters are known under the classical name “Hillas” parameters and are widely used by all IACTs. Though in the following years the Whipple team could measure marginal signals from Crab on a significance level of ∼ (3 − 4)σ, the real breakthrough happened in 1988 with the observation of the 9σ signal! This measurement [41] has marked the beginning of a new era and is considered as the birthday of the groundbased gamma astronomy. The persistent work of Trevor Weekes for over 20 years finally paid off, a new branch of science started its marathon! In the meantime other telescope installations were built, like HEGRA and CANGAROO, which very soon independently confirmed the Crab nebula as a source. 4.2. GT-48 in Crimea Since the late 1960’s the group in Crimean Astrophysical Laboratory (CrAO) led by Arnold Stepanian, used two parabolic searchlight mirrors of 1.5m diameter in coincidence for studying gamma sources (this must not be mixed up with the group of Chudakov from Lebedev’s institute in Moscow, who for several years was experimenting in Crimea). They reported detections of Cassiopea and Cyg X3 in the beginning of the 1970’s. Especially the latter has made a big resonance. In the 1980’s, the group started constructing a set of two large telescopes, separated by some distance, named GT-48. Large gun mounts from a Canadian ship were used for these telescopes. On each mount they have built six telescopes, three of the imaging type with 37 pixels and another three operating a single UV-sensitive, solar blind PMT. Every telescope had 4 mirrors of 1.2m diameter and 5m focus. The goal of the Crimean group was to profit from the stereo observations see, for example, [42]. Because they did not want to sacrifice neither the threshold nor the coincidence rate, they put the telescopes at 20m distance from each other. Their relatively small mirrors and low altitude of the location provided a threshold of ≥ 900GeV. The proximity of the telescopes did not allow them to fully exploit the differences in image parameters otherwise seen from largely separated detectors. In 1989, this installation was put into opera-

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tion and in subsequent years it measured quite a number of sources. 4.3. HEGRA The first telescope of HEGRA [43] was designed in 1990, as a somewhat modified version of the Yerevan Physics Institute (YerPhI) first Cherenkov telescope. The latter was reported at the first dedicated international workshop on VHE gamma astronomy organized by Arnold Stepanian in Crimea in 1989 [44]. It was the prototype of the planned five telescope “stereo”array (the experimental proposal was submitted in February 1985), installed at 100m distance from each other [45]. Each telescope was planned to have a 3m diameter tessellated mirror of 5m2 area and to be equipped with a 37-pixel imaging camera in a 5m focal plane. The first telescope was built at Nor Amberd cosmic ray station (2000m a.s.l.) on mount Aragats in 1989. The commissioning of the telescope showed a shower count rate of ∼ (0.3 − 0.5)Hz near zenith. The pixels used UV-transparent light guides of a conical form (focons), made of Plexiglas and subtending an angular aperture of 0.41◦ in the sky. The imaging camera was based on the most advanced Soviet FEU-130 type PMTs with a GaP first dynode of very high amplitude resolution. The equatorial mechanical mount of the first telescope was re-designed with the help of the YerPhI group engineer and built in the workshop of the Max-Planck-Institute for Physics in Munich. A 37-pixel imaging camera, almost a clone of what was built in Nor Amberd, was rebuilt in the collaborating institution in Kiel, Germany. The very high quality glass mirrors were produced in YerPhI. In mid 1991 the main parts for the planned five telescopes, including the mirrors, PMTs, shaft encoders, etc., were sent from Armenia via Munich to Canary island La Palma. The first telescope was installed on the Roque de los Muchachos observatory in La Palma in late fall 1991. A ∼ 5σ hint of the first signal from Crab appeared after two months of data taking, in late fall 1992. In the following year the second telescope with the same pixel size but with one more ring in the camera (61 pixels) and a larger reflector of 4.2m was built and put into operation at 100m distance from the first one. The stereo observations, the power of which has been predicted in a dedicated Monte-Carlo study paper in 1993 [46], could start. In the following years four more telescope of the same size as the second telescope but with 271 pixels of size of 0.25◦ were added. In the end, the second telescope, too, was given a 271-pixel camera and the array was completed in 1997. The last upgrade in the same year was the increase of the mirror area of the first telescope to 10.3m2 . HEGRA, operated

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till 2002. It has convincingly demonstrated the longawaited power of stereo observations [47]. These second generation imaging telescopes provided only a handful of sources, but it became clear that still there was a big potential in this technique that was just waiting to be explored. 4.4. 7-Telescope Array The Japanese 7-Telescope Array was originally planned as a detector including two arrays, each of 127 imaging telescopes, operating in coincidence [48]. Each telescope had 3m diameter mirror and a 256 pixel camera. In 1996-1997 three out of seven such telescopes were built and installed in Dugway, Utah, USA, the rest four were planned to be installed within one year. The telescopes started taking data on several interesting objects as, for example, measuring the flaring MKN-501. Unfortunately, by a wrong turn of a 20 foot long unarmed missile that hit the two data taking containers, the operation of this array was discontinued short before the end of 1997.

the Crab nebula down to ∼ 60GeV [51]. The comparison with today’s more precise measurements show that their reported flux was ∼ 2.5 times too low. With the operation of the MAGIC telescope it became obvious that the “classical” imaging method can provide much higher efficiency than the solar power plant detectors, so several years ago they ceased their operation. It is interesting to note that the MAGIC-I telescope, that had only 236m2 mirror area, could perform successful measurements also in the sub-100 GeV energy range. This is in striking contrast with the above assumption about the threshold dependency on the mirror area. The author believes that the above mentioned dependence of the threshold on the mirror area, that even now is circulating in publications, is not correct. The lower threshold of a telescope is simply proportional to the used mirror area. This can be explained by the fact that for an imaging telescope it is not the light of the night sky that sets the lower threshold, but the higherlevel requirement that for analyzing an image one needs some minimum amount of charge, something of the order of ∼ 100 photo electrons. To the knowledge of the author in the first time this was recognized in [52].

5. Solar power plants as gamma-ray telescopes It was recognized rather early that mirror-based solar power plants can offer large mirror area of several thousand m2 that could be used for collecting scarce photons from sub-100 GeV gamma showers. In the beginning of 90’s the threshold energy of the 10m Whipple telescope of ∼ 75m2 reflecting area was estimated to be (300 − 400)GeV. The common believe was that for lowering the threshold energy of a telescope by a factor of n one needs to increase its mirror area by n2 times. So, for example, for lowering the threshold energy of ∼ 1T eV of the ∼ 10m2 HEGRA telescope by a factor of 20 down to ∼ 50GeV one needed to increase its mirror area by 400 times! i.e. one needs a mirror area of 4000m2 ! Not a single telescope can offer such a huge mirror area. But the solar power plants with distributed mirror area can do that. Several solar power plants were rendered into gamma-ray detectors. The technique of doing that was quite different for different research teams. For example, while the GRAAL instrument [49] was attempting to collect Cherenkov photons from heliostats in the field into a ∼ 1m-size Winston cone, the other detectors were trying to organize a kind-of imaging in the central light collection tower, directing light from given heliostats to their own PMT channels. For a review of converted solar power plants please see [50]. Some interesting measurements were performed by using these arrays. The French CELESTE instrument tried to measure flux from

6. The 3rd generation telescopes

The third generation telescopes were designed well before the potential of the second generation telescopes was fully exhausted. Already in 1995 the first presentations on the concrete concept of 17m diameter MAGIC were made [53]. These were followed by the VERITAS proposal in fall 1996 and one year later by H.E.S.S.. While both of these arrays were following the goal of doing astrophysics with a stereo system of 10m diameter telescopes, which were well-known thanks to the Whipple telescope and the experience of HEGRA, the MAGIC design instead was aiming to go towards the sub-100GeV energy range, down to 3040GeV, into “terra incognita”. Obviously this task was more demanding and challenging, several novel techniques and technologies were necessary for making it possible. When HEGRA stopped operating in 2002, the collaboration split into two parts. One part together with the scientists from France, largely people from the CAT experiment, created the core of the H.E.S.S. collaboration and built their instrument in Namibia. The other part stayed in La Palma, in the original site of HEGRA, and together with scientists from Spain and Italy created the core of the MAGIC collaboration.

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6.1. H.E.S.S. The H.E.S.S. collaboration could get supported by the German and French financial agencies, while the VERITAS team had to wait for financial support for several more years. The H.E.S.S. telescopes were built and operated in Namibia in 2002-2004. Already in the beginning the H.E.S.S. team has performed a scan along the galactic plane and made a very reach harvest of galactic sources. This array has turned out to be a very efficient instrument, making a very high number of important discoveries and measurements above the energies (160-200)GeV. Recently a very large telescope of 28m diameter has been set in the center of H.E.S.S. that shall allow them to perform observations also in the very low energy range of few tens of GeV. 6.2. VERITAS The VERITAS telescopes, unlike H.E.S.S., which are operating imaging cameras of ∼ 5◦ aperture, are using cameras of 3.5◦ field of view. Otherwise both instruments are similar and both have increased the originally planned 10m diameter of their telescopes up to 12m. VERITAS was built in Arizona next to the administrative building of the Harvard-Smithsonian center and inaugurated in 2007. Not surprisingly, also VERITAS turned out to be a very successful instrument that in the recent several years has made a high number of important discoveries and measurements. 6.2.1. CANGAROO CANGAROO was a collaboration between several universities from Japan and the university of Adelaide. The collaboration started operating a 3.8m size single piece telescope of parabolic shape that was used earlier for lunar ranging.It started operating in 1992 at a threshold of a few TeV and in the following years has discovered several new sources of gamma rays. Ten years later four telescopes of 10m size were built. These had some differences in the design. Along with technical problems, mostly related to the chosen type of mirrors, there were also technical and organizational problems related to the data analysis. In few occasions detections of new sources could not be confirmed by the H.E.S.S. telescope [54]. A couple of years ago this array has terminated its operation. 6.3. MAGIC Initially, mostly because of financial reasons, MAGIC was planned as a single telescope. Several innovations were necessary for operating a single telescope also in the very low energy range ≤ 100GeV.

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It was necessary to provide a very fast response time for the telescope, so a reflector of parabolic design was chosen. Along with this, very fast hemispherical PMTs were developed for the needs of MAGIC by ElectronT ubes from England. In combination with the light guides and a mat lacquer coating, these provided an enhanced quantum efficiency. The PMT analog signals were converted into light and by using optical fibers transported to the electronic room where they were converted back into electrical pulses practically in the absence of any degradation of time features. The MAGICI telescope was built and put into operation in 20032004. The fast signals were initially read out by using 300MS ample/s custom-built FADCs that starting 2007 were exchanged against 2GS ample/s fast multiplexed FACDs [55]. The latter allowed MAGIC to suppress further down the hadron-induced background by a factor of two for energies ≥ 100GeV and by a factor of three for energies ≤ 100GeV [56]. In fact, MAGIC could do observations of some selected sources as, for example, the Crab Nebula, at energies as low as ∼ (50 − 60)GeV. By developing a special trigger configuration, the so-called SUM-trigger, the researchers could operate the telescope even at a very low threshold of ≥ 25GeV. This allowed them to discover a pulsed signal component from the Crab pulsar. This has made a strong impact on the pulsar theory models. The next serious improvement of MAGIC’s sensitivity was due to the construction and operation of the second telescope at 85m distance from the first one. This has essentially doubled the sensitivity of the first telescope.

7. CTA: the 4th generation major instrument 7.1. CTA A few years ago the researchers understood that one needed to unify the efforts of different collaborations and move towards one major instrument. Finally the series of “Towards a Major Atmospheric Cherenkov Detector” workshops, taking place between 1989 and 2005 (the last one), served its purpose. A few years ago, a new collaboration was formed for building the Cherenkov Telescope Array [57]. This collaboration includes practically all the researchers worldwide working with the atmospheric Cherenkov technique and many newer groups who are interested in exploring the sky in gamma rays with unprecedented sensitivity. Now the CTA collaboration is moving from the prototyping into the construction phase. Large number of different size telescopes are planned to be built in southern

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