- Email: [email protected]
Brief history of ground-based very high energy gamma-ray astrophysics with atmospheric air Cherenkov telescopes
Astroparticle Physics 53 (2014) 91–99
Contents lists available at ScienceDirect
Astroparticle Physics journal homepage: www.elsevier.com/locate/astropart
Brief history of ground-based very high energy gamma-ray astrophysics with atmospheric air Cherenkov telescopes Razmik Mirzoyan Max-Planck-Institute for Physics, Munich, Germany
a r t i c l e
i n f o
Article history: Available online 27 November 2013 Keywords: Very high energy gamma ray astronomy Very high energy gamma ray astrophysics Atmospheric Cherenkov telescope Air Cherenkov telescope Cherenkov light emission
a b s t r a c t The discovery of the Crab Nebula as the first source of TeV gamma rays in 1989, using the technique of ground-based imaging air Cherenkov telescope, has marked the birthday of observational gamma astronomy in very high energy range. The team led by Trevor Weekes, after twenty years of trial and error, success and misfortune, step-by-step improvements in both the technique and understanding of gamma shower discrimination methods, used the 10 m diameter telescope on Mount Hopkins in Arizona, and succeeded measuring a 9r signal from the direction of Crab Nebula. As of today over 160 sources of gamma rays of very different types, of both galactic and extra-galactic origin, have been discovered due to this technique. This is a really fast evolving branch in science, rapidly improving our understanding of the most violent and energetic sources and processes in the sky. The study of these sources provides clues to many basic questions in astrophysics, astro-particle physics, physics of cosmic rays and cosmology. Today’s telescopes, despite the young age of the technique, offer a solid performance. The technique is still maturing, leading to the next generation large instrument. This article is devoted to outlining the milestones in a long history that step-by-step have made this technique emerge and have brought about today’s successful source hunting. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Several very comprehensive papers, along with the classical book of Jelley [1], have been devoted to the history of Cherenkov emission [2–7] and its use for ground-based very high energy (VHE) gamma astrophysics. The author will not try to retell these articles but rather to give personal impressions about the main developments that have played a key role in the evolution of ground-based VHE gamma astrophysics. Also, in the author’s impression some of the important aspects of early developments, especially those in the former USSR, are not so well known. The author has been involved in VHE gamma astrophysics for three decades and could in person observe the relatively recent key developments. In recent years, the Imaging Atmospheric Cherenkov Technique (IACT) has made giant steps, establishing itself as a powerful 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 a 9r significance in 1989, the Whipple team, operating the 10 m diameter IACT in Arizona, laid the foundation for the new science [9]. It took another three years until the second source, this time the extra-galactic Mkn-421, was discovered in 1992, again by the Whipple team, led by Trevor Weekes. This important discovery stopped the wide-spread speculations that
E-mail address: [email protected] 0927-6505/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.astropartphys.2013.11.004
this was a science of a single source only. In the meantime, other telescope installations were built, like HEGRA [10] and CANGAROO [11], which very soon and 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 fast PMTs, started to be used in the imaging camera, at first by the Whipple team, but a few years later also by HEGRA. CANGAROO used PMTs of angular aperture of 0.11°. The CAT telescope, put into operation in late 1996 [12], on the same Themis site in French Pyrénées as the previous non-imaging ASGAT [13] and THEMISTOCLE [14] instruments, used a pixel size of 0.12°. But before going into the details of relatively recent important developments that led us to today’s success, let us go back in time to the last century for showing how things began. 2. The early days Oliver Heaviside calculated (in 1889) 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]. Although he published a series of papers on this issue, these stayed unnoticed; nevertheless, his genius advanced the understanding of the problem by half a century. Also Arnold Sommerfeld
92
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
calculated the task of a charge moving in vacuum with a speed v P 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 similar conclusions as Heaviside about the emission of a special radiation [16]. The first experimental report about the bluish glow from bottles with liquids containing radioactive radium salts in a dark cellar, is attributed to Madame Curie in 1910. She thought of it as some kind of luminescence [17]. Because she was very busy with similar tasks she did not pay the proper attention to that effect. The first systematic studies of the effect were performed by the French researcher Mallet, who published three papers in the years 1926–1929. Mallet described the external features of the phenomenon; moreover, he found out that it had a continuous emission spectrum. The latter, because of the absence of emission lines and bands, contradicted the previously assumed fluorescence explanation. Mallet did not succeed however, to reveal the polarization and anisotropy features of the unknown emission [18], which were the key to its understanding.
In 1932, young Pavel Cherenkov became a PhD student of Sergei Vavilov, who in 1934 became the director of the newly founded in Moscow Lebedev Institute for Physics of the Academy of Sciences of the former Soviet Union. Vavilov gave the task of studying the bluish ‘‘luminescent’’ emission to Cherenkov as the topic for his PhD thesis. Soon afterwards Cherenkov found out that he would spend many hours in a dark, cold cellar. That was necessary for accommodating his eyes, in fact the measuring instrument, to the darkness, for seeing the extremely faint emission. Soon after starting Cherenkov formally complained to the institute’s administration about the unusual working conditions and the unusual task, but after detailed explanatory work of Valilov he agreed to continue these studies. He started varying the temperature and the pressure of liquids by large margins, but still the emission 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 spectral lines characteristic for luminescence, and instead detected light emission with a continuous spectrum. Soon Cherenkov noticed that he could measure bluish light emission even from solvent liquids. Based on these results, he wrote an article in 1934 about his studies that his supervisor Vavilov declined to coauthor [19]. Instead, Vavilov wrote his own paper that appeared next to Cherenkov’s paper in the same issue of the journal [20]. Vavilov wrongly 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 unclear reason the journal declined publishing it. After that
he submitted his discovery as a short letter to Physical Review, where it was published in 1937 [21]. In the meantime the theoreticians Igor Tamm and Ilya Frank had 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 the 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 somewhat too slow to follow the fast escaping particle. Short after passage, the medium relaxes by emitting anisotropic radiation in the forward direction. Sergei Vavilov became the president of the Academy of Sciences of Soviet Union in 1946. The work of Cherenkov, Tamm, Frank and Vavilov got the highest recognition and they were awarded the Stalin prize, then the most prestigious prize in the Soviet Union. Vavilov was in a permanent worry about his famous botanist and geneticist brother Nikolai who, because of his scientific beliefs was slandered, then arrested in 1940. Afterwards he was sentenced to death that subsequently commuted to imprisonment of 20 years. Nikolai died in prison in 1943, presumably from hunger. Despite his very distinguished position in the science hierarchy of the big country, living in the atmosphere of permanent fear and moral sufferings, Vavilov got about ten heart attacks and in the end 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 until now in Russia somewhat higher credit 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 the corresponding member in the Soviet Academy of Sciences.
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
Let us now have a look into the next events that have played a key role in the evolution of the IACT technique.
3. Discovery of Cherenkov emission in the 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 stemming from air shower elementary particles producing Cherenkov emission in the atmosphere [23]. During his visit of Harwell in 1952 he met with Jelley and 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 25 cm diameter parabolic signaling mirror of a short focal length, fixed it into a dustbin and put a single 2 inch PMT in its focus. The telescope started counting pulses, one every two minutes. They had to confirm that in fact these were Cherenkov pulses coming from air showers. For that purpose they operated the small telescope in coincidence with a large-area Geiger counters for measuring air showers and in fact observed coincident events. Within one year they improved the performance of their telescope and succeeded measuring polarization of the detected light. Also, they found that compared to the green part of the spectrum, there was more light in the blue. These two facts proved that in fact they were measuring Cherenkov pulses from air showers. Galbraith and Jelley published a discovery paper in Nature in 1953, which has marked the beginning of atmospheric air Cherenkov technique [24].
93
Guiseppe Cocconi, the at Cornell University, suggested to measure gamma ray sources at TeV energies at the International Cosmic Ray Conference (ICRC) in Moscow in 1959 [26]. He proposed constructing an air shower array at a high mountain altitude with an angular resolution of 1° for measuring 1012 eV gammas. He made one very optimistic prediction: the possible flux of gamma rays from the Crab Nebula could be 1000 times higher than the background. He admitted that his estimate could be over-optimistic, 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 TeV gamma sources. For understanding the development of air showers in the atmosphere Alexander Chudakov and Natasha Nesterova were measuring the lateral distribution of Cherenkov light on the Pamir mountains at an altitude of 3800 m a.s.l. in 1953–1955. They used large-size Geiger counters and eight Cherenkov light receivers as detectors [27]. Chudakov became excited by the prospects of the possible new science and started planning a systematic study of cosmic gamma ray source candidates in the sky.
5. First generation atmospheric Cherenkov telescopes 5.1. 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 [28]. One year later, they increased the number of telescopes to 12. These were based on parabolic searchlight mirrors of 1.55 m diameter and focus of 60 cm, previously used by the military for securing the Black sea border. 4.5 cm diameter PMTs were set in their foci, with a diaphragm limiting the field of view to 1.75° FWHM. Chudakov calculated a lens of a special form for reducing the aberrations and improving the timing of the mirrors and set them in front of the PMTs. Telescopes were rigidly connected in groups of three pointing in 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. The trigger of the telescopes was based on four-fold coincidence. A rate-stabilizing circuitry was developed for suppressing the count-rate instabilities due to intensity variations of the light of night sky.
Except for relatively small-scale test measurements not very much happened in the following several years.
4. First ideas on astronomy by means of gamma rays In 1958 Philip Morrison suggested to measure gamma rays from possible cosmic sources [25]. He considered that measurements in the energy range 0.2–400 MeV, using detectors on high flying aircrafts or on balloon flights could provide a real clue to the cosmic rays and their sources. He considered that different types of detector could be efficiently used for measuring gamma rays in the above energy range. Morrison mentioned the Crab Nebula as a possible source of gamma rays but, interestingly from today’s point of view, at much lower energies. He thought that the expanding gas shell in the nebula shall contain plenty of radioactive debris from the original explosion, which one can measure. He predicted that a flux of specific nuclear gamma rays, mainly from 226Ra, should be detectable, with an incoming flux of 10 2 gamma/cm2 s.
The experiment was continued for 4 years. By using flashes of Cherenkov light initiated by muons in a piece of Plexiglas of a given thickness, the researchers estimated a photon threshold of 280 ph/ m2 (for the wavelength range 300–600 nm) for their detectors. The shower count rate was slightly more than 3 Hz and the estimated energy threshold was 3.4 TeV for gammas.
94
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
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 min in duration, were repeated many times over the observational 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, or about X-ray sources. The common belief 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 Sagittarius A (7 scans) were observed. The long observations of Cygnus A were due to an initial small indication of a possible signal that the researchers were unsuccessfully 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 a signal from any of the above targets. The signal upper limit from the direction of the Crab Nebula for a total of 5.5 h of observation was 5 10 11 ph/cm2 s for the threshold of 4–5 TeV. It is an interesting question if Chudakov and his colleagues could have discovered a gamma signal from the Crab Nebula back in the 1960’s. Today we know that the integral flux of the Crab Nebula above 4 TeV is 2.5 10 12 ph/cm2 s. This means that the flux limit set by Chudakov and his colleague was 20 times higher than the flux from the source. Consequently, for measuring a significant signal from the Crab Nebula, they had to measure 400 times longer, i.e. for 2200 h; of course that was very improbable to happen. An important consequence of this non-detection of the Crab Nebula was that it invalidated the 1000 times higher flux originally estimated by Cocconi. Also, non-detection of the signal cast strong doubts on the origin of electrons in the Crab Nebula as secondary particles produced in cosmic proton interactions with the nebula; pp ? p ? l ? e (the common belief was that gammas were produced due to pp ? p0 ? 2c). That meant that in fact the electrons could have been accelerated in the nebula.
5.2. First images of air showers An important measurement was realized by Hill and Porter in 1960 [29]. They coupled an image intensifier to a 25 cm diameter Schmidt telescope of a 24° field of view and shot the first images of air showers in the night sky. Because of the small size of the mirror, only images of showers with energies P0.5 PeV were recorded. The trigger came from a 5 inch PMT in the focus of a conventional mirror that was set in close proximity. It was possible to identify bright background stars on the images. The detection rate was 7 events per 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 [30]; 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. 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 (1 ls), and some other disadvantages, it turned out to be impractical.
5.3. Monte Carlo simulations of air showers and the ‘‘stereo’’ observations Victor Zatsepin, member of Chudakov’s crew, published a remarkable Monte Carlo study paper in 1964 [31]. By simulating on the Soviet ‘‘Ural’’-type computer (operated by specially trained staff) he obtained the first contours of equal photon density in 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 to the author that about 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 imaging cameras he could use for that purpose [32]. As one can see from the above, already 50 years ago some researchers clearly understood the potential of coincidence measurements of shower images, 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 were built and operated, typically of smaller scale than the one from Chudakov. Except for the 10 m 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 the 1980s. 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 a coincidence between several such mirror elements, which allowed lowering the energy threshold of the instrument. 5.4. Harwell–Glencullen telescope One should mention the compact telescope in Harwell, UK, which later on was moved to Glencullen valley near Dublin [30]. The telescope, set on a gun mount, consisted of two 92 cm diameter back-silvered mirrors, each with a single PMT in the focus, viewing a 5° field of view. The coincident count rate was 0.7– 1.7 Hz. Observations of quasars, 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 [33]. From today’s point of view it is interesting to note
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
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 opto-electronic design was developed and built in Glencullen, and then installed in 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 in that it used four F/2 mirrors of 90 cm diameter, fast PMTs, fast amplifiers and a 3.5 ns 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 [34].
95
as well as in proceedings of the international cosmic ray conferences. 6. 2nd Generation telescopes 6.1. The 10 m Whipple telescope In 1967, Giovanni Fazio and colleagues began constructing a 10 m diameter, F/0.7 telescope on Mount Hopkins, at the Whipple observatory, at a height of 2300 m a.s.l. [38]. 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.
5.5. Double beam technique Another interesting development in gamma astronomy is related to two 6.5 m diameter reflectors set at a large distance (120 m) for stellar interferometry in Narrabri, Australia. In 1968, the researchers carried out observations of the Crab Nebula and two pulsars, no signal was measured [35]. 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 have been widely used subsequently. For giving an impression about the pioneering and creative spirit of some of the observations about 30–40 years ago, it is interesting to shortly discuss one publication from Grindlay [36]. The author claimed in that paper a detection of a 5r pulsed signal from the Crab pulsar on the flux level of 8 10 12 ph/cm2 s at 1 TeV but in the absence of any DC signal from the Crab Nebula. Today we know that the cited pulsed flux, on the level of 40% of the DC flux from the Crab Nebula, is impossible. Concerning the pulsed flux level from the Crab pulsar at 1 TeV we do not even know yet if the pulsar emits gammas at such a high energy. But we know that the pulsar flux, for example, at 100 GeV is about two orders of magnitude less than the DC flux from the Crab Nebula [37]. 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, of the Haleakala telescope in Hawaii, the Potchefstroom telescope in the South African Republic, the telescopes of the Durham group, the TACTIC, PACT and HAGAR telescopes in India, the SHALON and other former telescopes in Russia. These are largely reflected in the proceedings of the workshop series of ‘‘Towards a Major Atmospheric Cherenkov Detector’’
Trevor Weekes joined the Smithsonian project in 1966. In 1968 he co-authored with Fazio and two other colleagues an interesting observational paper about 13 sources, among which were, for example, the Crab Nebula (of course!), M87, M82, IC443 [39]. Their flux upper limits above the threshold of 2 TeV were modest, but today we know that the above listed sources in fact all are gammaray emitters. In 1977, Weekes and Turver suggested to use two telescopes, each equipped with a 37-pixel imaging camera, at a distance of 100 m. They expected that such a stereoscopic imaging system should strongly suppress the background [40]. In fact, a 37-pixel imaging camera, covering a field of view of 3.5° in the sky, was built and installed on the 10 m telescope in 1983. In the following years the Whipple team could measure marginal signals from Crab on a significance level of slightly below 5r.
96
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
In 1985 Michael Hillas suggested a special parameterization of the measured images [41]. That turned out to be a key milestone in the history of ground-based imaging air Cherenkov telescopes.1 The real breakthrough happened 3 years later; this parameterization helped the Whipple team to measure the famous 9r signal from the Crab Nebula [42]. That measurement marked the beginning of a new era and is commonly considered as the birthday of the ground-based VHE gamma astronomy. The persistent work of Trevor Weekes for over 20 years finally paid off, a new branch of science started its marathon! 6.2. GT-48 in Crimea Since the late 1960’s the group in the Crimean Astrophysical Laboratory (CrAO, located in a small village ‘‘Nauchniij’’) led by Arnold Stepanian used two parabolic searchlight mirrors of 1.5 m 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 also for several years was experimenting in Crimea but at a different location, near the Black sea shore). They reported detections of Cassiopea and Cyg X3 in the beginning of the 1970’s, especially the latter 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 battleship were used for these telescopes. On each mount they built six telescopes, three of the imaging type with 37 pixels in the focus and another three operating a single UV-sensitive, solar blind PMT. Every telescope had 4 mirrors of 1.2 m diameter and 5 m focus. The goal of the Crimean group was to take advantage of stereo observations; see, for example, [43]. They aimed to reach the lowest possible threshold and the highest coincidence rate. Therefore they put the telescopes at 20 m distance from each other. Their relatively small mirror area and the low altitude of the location of 600 m a.s.l. provided a threshold of P900 GeV. The proximity of the telescopes did not allow them to fully exploit the differences in image parameters otherwise seen from largely separated telescopes. In 1989, this installation was put into operation and in subsequent years it measured quite a number of sources.
dedicated international workshop on VHE gamma-ray astronomy organized by Arnold Stepanian in Crimea in 1989 [45].2 It was the prototype of the planned five telescope ‘‘stereo’’ array (the experimental proposal was submitted by Felix Aharonian, by the author and two other colleagues in February 1985), installed at 100 m distance from each other [46]. Each telescope was planned to have a 3 m diameter tessellated mirror of 5 m2 area and to be equipped with a 37-pixel imaging camera in the focal plane at 5 m. The first telescope was built at Nor Amberd cosmic ray station (2000 m a.s.l.) on Mount Aragats in Armenia in 1989. The commissioning of the telescope showed a shower count rate of (0.3–0.5) Hz near zenith. The pixels used light guides of a conical form (focons), made of UV transparent Plexiglas. The imaging camera was based on the most advanced Soviet FEU-130 type PMTs with a GaP first dynode, providing extremely high single photo electron amplitude resolution. The equatorial mechanical mount of the first telescope was re-designed with the help of the YerPhI group engineer Rouben Kankanian and built in the workshops of the Max-Planck-Institute for Physics in Munich in 1991. A 37-pixel imaging camera, a clone of what was built in Nor Amberd, was re-built in the collaborating institution in Kiel, Germany. The high quality glass mirrors were produced in YerPhI. In mid 1991 the main parts for the planned five telescopes, including the mirrors, PMTs, absolute shaft encoders, etc., were sent from Armenia via MaxPlanck-Institute (MPI) for Physics in Munich to the Canary Island La Palma. Eckart Lorenz, the driving force behind the HEGRA array, and a team from MPI Munich installed the first telescope on the Roque de los Muchachos observatory in La Palma in late fall 1991. A 5r signal from the Crab Nebula appeared after two months of data taking, in late fall 1992. In the following year the second telescope with the same type PMTs and pixel size but with one more ring in the camera (61 pixels) and a larger reflector of 4.2 m was built and put into operation at 100 m distance from the first one. The stereo observations, the power of which had been predicted in a dedicated Monte-Carlo study paper [46], started in 1993. Werner Hofmann and Heinz Völk from the MPI for Nuclear Physics in Heidelberg became involved and strongly supported the scientific operation of the HEGRA telescopes. In the following years, four more telescopes of the same size as the second one but with cameras of 271 pixels of size of 0.25° were added. In the end, the second telescope, too, was given a 271-pixel camera and the full array was completed in 1997. The last upgrade in the same year was the increase of the mirror area of the first telescope to 10.3 m2. HEGRA operated till 2002. It convincingly demonstrated the long-awaited power of stereo observations [47]. Although the second generation imaging telescopes found only a handful of sources, it became obvious that there was a big potential in this technique, just waiting to be explored. 6.4. 7-Telescope Array
Still, in a few cases their results remained controversial. 6.3. Hegra The first telescope of HEGRA [10,44] 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 1 Since then this set of parameters, meanwhile simply referred to as ‘‘Hillas’’ parameters are widely used by all IACT teams.
The Japanese 7-Telescope Array was originally planned as a detector including two arrays, each of 127 imaging telescopes, operating in coincidence for measuring either fluorescence or Cherenkov light from air showers [48]. Each telescope had a 3 m 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 remaining four were planned to be installed within one year. The telescopes started taking data on several interesting objects as, for example, the flaring Mkn-501. Unfortunately, soon this experiment suffered a weird misfortune – a 20 foot long unarmed missile of the US army hit and destroyed the two data taking containers shortly before the end of 1997. Thus the experiment was discontinued. 2 The series of international workshops entitled ‘‘Towards a Major Atmospheric Cherenkov Detector’’ was launched in Palaiseau, France, in June 1992. Although the workshop in Crimea happened 3 years earlier, historically the workshop in Palaiseau is referred to as the first one in the series.
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
6.5. The CAT telescope CAT was an imaging telescope designed in 1993 and built and put into operation in 1996 at the Themis solar site in southern France. It had a 17.7 m2 reflector of Davies–Cotton design and operated 600 PMT imaging camera in the focal plane. The reflector consisted of 90 spherical mirrors of 50 cm diameter and 6 m focal length. The camera was based on 546 PMTs of 11 mm size in the central part and 54 PMTs of 28 mm size in the outer two guard rings, both from Hamamatsu. CAT was the first telescope to accommodate the trigger electronics in the camera. The ADC system was fixed on the counterweights. Initially, because of the very fast performance of the PMTs (1.4 ns FWHM), of the electronics and of the telescope, the trigger efficiency on gamma rays turned out to be low. After slowing down the electrical pulses towards 2.5 ns the performance has improved. Also, unlike HEGRA, who were using conical focons as light guides in their imaging cameras, CAT started using Winston cones [12]. In spite of the relatively small size of the reflector, CAT claimed an energy threshold comparable to that of the 10 m Whipple telescope. CAT became one of the productive imaging telescopes and in the subsequent years delivered a wealth of scientific data.
7. Gamma-ray telescopes based on solar power plants It was recognized rather early that mirror-based solar power plants can offer large mirror areas of several thousand m2 that could be used for collecting scarce photons from sub-100 GeV gamma showers. In the beginning of 1990s the achieved threshold energy of the 10 m Whipple telescope of 75 m2 reflecting area was estimated to be 300–400 GeV. The common belief was that for lowering the threshold energy of a telescope by a factor of n one need to increase its mirror area by n2 times. So, for example, for lowering the threshold energy of 1 TeV of the 10 m2 HEGRA CT1 telescope by a factor of 20 down to 50 GeV one needed to increase its mirror area to 4000 m2! A single IACT cannot offer such a huge mirror area. Solar power plants with distributed mirror area, however, can do that, and, some such arrays were readily available for astrophysicists to experiment. Several solar power plants thus were transformed into gammaray detectors. Individual research teams used quite different, ingenious techniques for that. For example, while the GRAAL team [49] was attempting to collect Cherenkov photons from multiple heliostats in the field into a 1 m-size single Winston cone, the STACEE collaboration was trying to organize a kind of imaging in the central light collection tower, directing light from individual heliostats to specific PMT channels. For a review of converted solar power plants please see [50] and references therein. Some interesting measurements were performed with these arrays. The French CELESTE instrument tried to measure the flux from the Crab Nebula down to 60 GeV [51]. The comparison with today’s more precise measurements of MAGIC shows that their reported flux was 2.5 times too low. With the start of operation of the MAGIC telescope in 2004 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 236 m2 of mirror area, could successfully perform measurements also in the sub 100 GeV energy range, down to 50 GeV. This is in striking contrast to the above assumption about the energy threshold dependency on the inverse square root of the mirror
97
area, although this relation can still be found in recent publications. Unlike the non-imaging detectors, the lower threshold of an imaging telescope is simply inversely proportional to the used mirror area. Not the fluctuations of the light of the night sky, but the higher-level requirement of minimum amount of charge for meaningful parameterization of images,3 set the lower threshold for an imaging telescope. Well parameterized images allow one to efficiently select gammas from the vast background of hadrons. Early understanding of this relation played a key role for designing of the MAGIC telescope. To the knowledge of the author, for the first time this was mentioned in [52].
8. 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 17 m diameter MAGIC were made [53]. These were followed by the VERITAS letter of intent in fall 1996 and in the next year by H.E.S.S. While both VERITAS and H.E.S.S. were following the goal of doing astrophysics with a stereo system of 10 m 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-100 GeV energy range, down to 30–40 GeV, into the ‘‘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, at the original site of HEGRA, and together with scientists from Spain, Italy and Switzerland created the core of the MAGIC collaboration. 8.1. H.E.S.S. The H.E.S.S. collaboration was supported by the German and French financial agencies, while the VERITAS team had to wait for several more years for its financing. The H.E.S.S. telescopes were built and started operation in Namibia in 2002–2004. Already in the beginning the H.E.S.S. team performed a scan along the galactic plane and made a very rich harvest of galactic sources. This array turned out to be a very efficient instrument, which made a really high number of important discoveries and measurements above the energies 160–200 GeV, see, for example, [54]. Recently a very large telescope of 28 m diameter has been installed in the center of H.E.S.S. that shall allow performing observations also in the very low energy range of a few tens of GeV [55]. 8.2. Veritas The VERITAS telescopes, unlike H.E.S.S., which are operating imaging cameras of 5° geometrical aperture, are using cameras of 3.5° field of view. Otherwise both instruments are similar and both have increased the originally planned 10 m diameter of their telescopes to 12 m. VERITAS was built in Arizona next to the administrative building of the Harvard–Smithsonian center and inaugurated in mid 2007. Not surprisingly, also VERITAS turned out to be a very successful instrument that in recent years has real3 The practice shows that 100 photo electrons per image are enough for its reliable parameterization.
98
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
ized a high number of important discoveries and measurements, see, for example, [56,57]. 8.3. Cangaroo CANGAROO was collaboration between several universities from Japan and the University of Adelaide. In 1992 the collaboration started operating a 3.8 m size single telescope of parabolic shape that had been used for lunar ranging in the past. In the following years this collaboration discovered several new sources of gamma rays above the threshold of few TeV. Ten years later, additional four telescopes of 10 m size were built and operated. 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 a few occasions sources detected by CANGAROO could not be confirmed by the H.E.S.S. telescope [58]. A couple of years ago this array terminated its operation. 8.4. Magic Initially, mostly because of financial reasons, the 17 m diameter MAGIC was proposed as a single telescope. A 17 m diameter telescope of innovative design of the German Space Agency (DLR) for utilizing the solar energy has served as the prototype for MAGIC. Several innovations were necessary for operating the single telescope also in the very low energy range 6100 GeV, where a strong background from local muons was expected. MAGIC researchers were hoping to strongly suppress the different backgrounds by using an ultra-fast opto-electronic design of the telescope. For this purpose a reflector of parabolic shape was chosen. Along with this, very fast hemispherical PMTs were co-developed for the needs of MAGIC by Electron Tubes from England. The PMT analog signals were converted into light pulses and, by using 162 m long optical fibers, transported to the electronic room, where they were converted back into electrical signals practically with no degradation of time parameters. The MAGIC-I telescope was built and put into operation in 2003–2004. Starting 2007 MAGIC-I used 2 GSample/s fast multiplexed FACDs for the readout [59]. The measurements showed a full bandwidth of 240 MHz for signal channels. The latter allowed MAGIC to suppress further down the contribution from the light of the night sky as well as the hadron-induced background by a factor of 2–3 [60]. In fact, MAGIC could do observations of some selected sources as, for example, the Crab Nebula and its pulsar, at energies as low as 50–60 GeV [61]. By developing a special socalled SUM-trigger configuration, the researchers could operate the telescope even at a very low threshold of P25 GeV, the traditional energy range of satellite born experiments. This allowed MAGIC to discover a pulsed signal from the Crab pulsar. The next serious improvement of MAGIC’s sensitivity was due to adding of the second, clone telescope at 85 m distance from the first one in 2009. Compared to the first telescope, this has essentially doubled the sensitivity; see, for example, [61,62]. 9. CTA: the 4th generation major instrument The series of ‘‘Towards a Major Atmospheric Cherenkov Detector’’ workshops, taking place between 1992 and 2005 (the last one), served its purpose; the researchers decided to unify their efforts and move towards one major instrument. A few years ago, a new collaboration was formed for building the Cherenkov Telescope Array (CTA) [63]. This collaboration includes practically all the researchers worldwide working with the atmospheric air
Cherenkov telescopes and many newer groups who are interested in exploring the sky in VHE gamma rays with 10 times higher sensitivity than any of the existing instruments. Now the CTA collaboration is moving from the prototyping into the construction phase. Over 100 telescopes of 23 m, 12 m and 4–7 m size are planned to be built in southern and northern observatories [63,64]. CTA is going to be the major ground-based instrument for doing astrophysics by means of VHE gamma rays for the next 10–15 years [65].
References [1] J.V. Jelley, Cherenkov Radiation and its Applications, Pergamon Press, 1958; J.V. Jelley, in: Proceedings of the International Workshop on Very High Energy Gamma Ray Astronomy, Ootacamund, India, 1982, p. 64. [2] A.S. Lidvansky, arXiv:astro-ph/0504269. [3] A. Watson, arXiv:1101.4535. [4] T.C. Weekes, arXiv:astro-ph/0508253. [5] D. Fegan, arXiv:1104.2403v1. [6] E. Lorenz, R. Wagner, arXiv:1207.6003v1. [7] A.M. Hillas, Astrop. Phys. (2012), in press. http://dx.doi.org/10.1016/ j.astropartphys.2012.06.002. [8] S. Funk, arXiv:1204.4529. [9] T.C. Weekes et al., Astrophys. J. 342 (1989) 370. [10] R. Mirzoyan, R. Kankanian, F. Krennrich, et al., Nucl. Instrum. Methods A 351 (1994) 513. [11] T. Hara et al., Nucl. Instrum. Methods A 332 (1993) 300; R. Enomoto et al., ApJ 638 (2006) 397. [12] A. Barrau, R. Bazer-Bachi, E. Beyer, et al., Nucl. Instrum. Methods A 416 (1998) 278. [13] P. Goret, T. Palfrey, A. Tabary, et al., A&A 270 (1993) 401. [14] P. Baillon et al., Astropart. Phys. 1 (1993) 341. [15] O. Heaviside, Philos. Mag. S 5 (27) (1889) 324. [16] A. Sommerfeld, Goettingen Nachr. 99 (1904) 363; 201 (1905). [17] Eve Curie ‘‘Madame Curie’’, London, 1941. [18] L. Malet, C. R. Acad. Sci. (Paris) 183 (1926) 274; , C. R. Acad. Sci. (Paris) 187 (1928) 222; , C. R. Acad. Sci. (Paris) 188 (1929) 445. [19] P.A. Cherenkov, Dokl. Akad. Nauk SSSR 2 (1934) 451. [20] S.I. Vavilov, Dokl. Akad. Nauk SSSR 2 (1934) 457. [21] P.A. Cherenkov, Phys. Rev. 52 (1937) 378. [22] I.E. Tamm, I.M. Frank, Dokl. Akad. Nauk SSSR 14 (1937) 109. [23] P.M.S. Blackett, Rep. Conf. Gassiot Comm. Phys. Soc., Emission spectra of the night sky and aurorae, 34, Phys. Soc., London, 1948. [24] W. Galbraith, J.V. Jelley, Nature 171 (1953) 349. [25] P. Morrison, Il Nuovo Cimento 7 (1958) 858. [26] G. Cocconi, in: Proceedings of the International Cosmic Ray Conference, vol. 2, Moscow, 1960, p. 309. [27] A.E. Chudakov, N.M. Nesterova, Sov. Phys. JETF 28 (1955) 384. [28] A.E. Chudakov, V.L. Dadykin, V.I. Zatsepin, N.M. Nesterova, Search of photons of energy 1013 eV from local sources of cosmic radio emission, Proc. P.N. Lebedev Phys. Inst. 26 (1964) 118–141. Transl. Consultants Bureau 26 (1965) 99. [29] D.A. Hill, N.A. Porter, Nature 191 (1960) 690. [30] J.V. Jelley, N.A. Porter, J.R. Quart, Astron. Soc. 4 (1963) 275. [31] V.I. Zatsepin, Sov. Phys. JETF 47 (1964) 689; , (Eng.:) Sov. Phys. JETF 20 (2) (1965) 459. [32] V.I. Zatsepin, Private Communication, 2012. [33] C.D. Long, B. McBreen, E.P. O’Mongain, N.A. Porter, in: Proceedings of 9th ICRC, London, vol. 1, 1965, 318 (1966). [34] D.M. Jennings, G. White, N.A. Porter, E.P. O’Mongain, D.J. Fegan, J. White, Il Nuovo Cimento 20 (1) (1974) 71. [35] R. Hanbury Brown, J. Davis, L.R. Allen, Mon. Not. R. Astron. Soc. 146 (1969) 399. [36] J.E. Grindlay, Astrophys. J. 174 (1972) L9–L17. [37] J. Aleksic et al., MAGIC Coll., Astropart. Phys. 540 (2012) 69. [38] G.G. Fazio, H.F. Helmken, G.H. Rieke, T.C. Weekes, Can. J. Phys. 46 (1968) 451. [39] G.G. Fazio, H.F. Helmken, G.H. Rieke, T.C. Weekes, Astrophys. J. 154 (1968) L83. [40] T.C. Weekes, K.E. Turver, in: R.D. Wills, B. Battrick (Eds.), Proceedings of 12th ESLAB Symposium (Frascati), ESA SP124, vol. 279, 1977. [41] A.M. Hillas, in: Proceedings of 18th ICRC, vol. 3, 1985, p. 445. [42] T.C. Weekes, M.F. Cawley, D.J. Fegan, et al., Astrophys. J. 342 (1989) 379. [43] A.P. Kornienko, A.A. Stepanian, Yu.L. Zyskin, et al., Exp. Astron. 4 (1993) 77. [44] R. Mirzoyan, (HEGRA Collab.) in: P. Fleury, G. Vacanti (Eds.), Towards a Major Atmospheric Cherenkov Detector, Palaiseau, France, June 11–12, 1992, p. 239. [45] F. Aharonian, et al., in: A.A. Stepanian, D.J. Fegan, M.F. Cawley (Eds.), Proceedings of the International Workshop VHE Gamma-Ray Astronomy, Crimea, USSR, April 17–21, vol. 36, 1989. [46] F.A. Aharonian, A.A. Chilingaryan, R.G. Mirzoyan, A.K. Konopelko, A.V. Plyasheshnikov, Exp. Astron. 2 (1993) 331. [47] G. Puehlhofer et al., HEGRA Collab., Astropart. Phys. 20 (2003) 267. [48] M. Teshima, G. Dion, N. Hayashida, et al., in: P. Fleury, G. Vacanti (Eds.), Proceedings of Towards a Major Atmospheric Cherenkov Detector, Palaiseau, France, June 11–12, 1992, 255.
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99 [49] R. Plaga, in: Proceedings of 17th ECRC, vol. 178, 2000; F. Arqueros, et al., Astropart. Phys. 3 (2002) 293. [50] D.A. Smith, arXiv:astro-ph/0608251v1, 2006. [51] M. De Naurois, J. Holder, R. Bazer-Bachi, et al., CELESTE Collab., Astrophys. J. 566 (2002) 343. [52] R. Mirzoyan, in: De Jager (Ed.), Towards a Major Atmospheric Cherenkov Detector-V, Krueger Nat. Park, South Africa, August 8–11, 1997, p. 298. [53] S.M. Bradbury, et al., in: Cresti (Ed.), Proceedings of the International Workshop Towards a Major Atmospheric Cherenkov Detector-IV, Padova, Italy, September 11–13, 1995, p. 277; E. Lorenz, in: Proceedings of TAUP 95, Toledo, Spain, 1995. [54] M. de Naurois, H.E.S.S. collab., Adv. Space Res. 51 (2013) 258. [55] P. Hofverberg, Nucl. Instrum. Methods A 639 (2011) 23. [56] F. Krennrich et al., Nucl. Instrum. Methods A 630 (2011) 16.
99
[57] K. Ragan, Nucl. Instrum. Methods A 692 (2012) 24. [58] M. Mori, in: Shibara, Sakaki, (Eds.), International Workshop ‘‘Cosmic-rays and High Energy Universe’’, Aoyama Gakuin University, Shibuya, Tokyo, Japan, March 5–6, 2007, University of Academy Press, Tokyo, Japan, 93 2007. [59] H. Bartko, F. Goebel, R. Mirzoyan, et al., Nucl. Instrum. Methods A 548 (2005) 464. [60] E. Aliu et al., MAGIC Collab., Astropart. Phys. 30 (2009) 293. [61] J. Aleksic et al., MAGIC collab., Astropart. Phys. 35 (2012) 435. [62] A. De Angelis, Adv. Space Res. 51 (2013) 280. [63] CTA homepage: www.cta-observatory.org. [64] M. Actis, et al., CTA Consortium, arXiV:1008.3703v3; also in Exp. Astron. 32 (2011) 193. [65] B.S. Acharya et al., CTA Consortium, Astrop. Phys. 43 (2013) 3.
Contents lists available at ScienceDirect
Astroparticle Physics journal homepage: www.elsevier.com/locate/astropart
Brief history of ground-based very high energy gamma-ray astrophysics with atmospheric air Cherenkov telescopes Razmik Mirzoyan Max-Planck-Institute for Physics, Munich, Germany
a r t i c l e
i n f o
Article history: Available online 27 November 2013 Keywords: Very high energy gamma ray astronomy Very high energy gamma ray astrophysics Atmospheric Cherenkov telescope Air Cherenkov telescope Cherenkov light emission
a b s t r a c t The discovery of the Crab Nebula as the first source of TeV gamma rays in 1989, using the technique of ground-based imaging air Cherenkov telescope, has marked the birthday of observational gamma astronomy in very high energy range. The team led by Trevor Weekes, after twenty years of trial and error, success and misfortune, step-by-step improvements in both the technique and understanding of gamma shower discrimination methods, used the 10 m diameter telescope on Mount Hopkins in Arizona, and succeeded measuring a 9r signal from the direction of Crab Nebula. As of today over 160 sources of gamma rays of very different types, of both galactic and extra-galactic origin, have been discovered due to this technique. This is a really fast evolving branch in science, rapidly improving our understanding of the most violent and energetic sources and processes in the sky. The study of these sources provides clues to many basic questions in astrophysics, astro-particle physics, physics of cosmic rays and cosmology. Today’s telescopes, despite the young age of the technique, offer a solid performance. The technique is still maturing, leading to the next generation large instrument. This article is devoted to outlining the milestones in a long history that step-by-step have made this technique emerge and have brought about today’s successful source hunting. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Several very comprehensive papers, along with the classical book of Jelley [1], have been devoted to the history of Cherenkov emission [2–7] and its use for ground-based very high energy (VHE) gamma astrophysics. The author will not try to retell these articles but rather to give personal impressions about the main developments that have played a key role in the evolution of ground-based VHE gamma astrophysics. Also, in the author’s impression some of the important aspects of early developments, especially those in the former USSR, are not so well known. The author has been involved in VHE gamma astrophysics for three decades and could in person observe the relatively recent key developments. In recent years, the Imaging Atmospheric Cherenkov Technique (IACT) has made giant steps, establishing itself as a powerful 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 a 9r significance in 1989, the Whipple team, operating the 10 m diameter IACT in Arizona, laid the foundation for the new science [9]. It took another three years until the second source, this time the extra-galactic Mkn-421, was discovered in 1992, again by the Whipple team, led by Trevor Weekes. This important discovery stopped the wide-spread speculations that
E-mail address: [email protected] 0927-6505/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.astropartphys.2013.11.004
this was a science of a single source only. In the meantime, other telescope installations were built, like HEGRA [10] and CANGAROO [11], which very soon and 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 fast PMTs, started to be used in the imaging camera, at first by the Whipple team, but a few years later also by HEGRA. CANGAROO used PMTs of angular aperture of 0.11°. The CAT telescope, put into operation in late 1996 [12], on the same Themis site in French Pyrénées as the previous non-imaging ASGAT [13] and THEMISTOCLE [14] instruments, used a pixel size of 0.12°. But before going into the details of relatively recent important developments that led us to today’s success, let us go back in time to the last century for showing how things began. 2. The early days Oliver Heaviside calculated (in 1889) 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]. Although he published a series of papers on this issue, these stayed unnoticed; nevertheless, his genius advanced the understanding of the problem by half a century. Also Arnold Sommerfeld
92
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
calculated the task of a charge moving in vacuum with a speed v P 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 similar conclusions as Heaviside about the emission of a special radiation [16]. The first experimental report about the bluish glow from bottles with liquids containing radioactive radium salts in a dark cellar, is attributed to Madame Curie in 1910. She thought of it as some kind of luminescence [17]. Because she was very busy with similar tasks she did not pay the proper attention to that effect. The first systematic studies of the effect were performed by the French researcher Mallet, who published three papers in the years 1926–1929. Mallet described the external features of the phenomenon; moreover, he found out that it had a continuous emission spectrum. The latter, because of the absence of emission lines and bands, contradicted the previously assumed fluorescence explanation. Mallet did not succeed however, to reveal the polarization and anisotropy features of the unknown emission [18], which were the key to its understanding.
In 1932, young Pavel Cherenkov became a PhD student of Sergei Vavilov, who in 1934 became the director of the newly founded in Moscow Lebedev Institute for Physics of the Academy of Sciences of the former Soviet Union. Vavilov gave the task of studying the bluish ‘‘luminescent’’ emission to Cherenkov as the topic for his PhD thesis. Soon afterwards Cherenkov found out that he would spend many hours in a dark, cold cellar. That was necessary for accommodating his eyes, in fact the measuring instrument, to the darkness, for seeing the extremely faint emission. Soon after starting Cherenkov formally complained to the institute’s administration about the unusual working conditions and the unusual task, but after detailed explanatory work of Valilov he agreed to continue these studies. He started varying the temperature and the pressure of liquids by large margins, but still the emission 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 spectral lines characteristic for luminescence, and instead detected light emission with a continuous spectrum. Soon Cherenkov noticed that he could measure bluish light emission even from solvent liquids. Based on these results, he wrote an article in 1934 about his studies that his supervisor Vavilov declined to coauthor [19]. Instead, Vavilov wrote his own paper that appeared next to Cherenkov’s paper in the same issue of the journal [20]. Vavilov wrongly 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 unclear reason the journal declined publishing it. After that
he submitted his discovery as a short letter to Physical Review, where it was published in 1937 [21]. In the meantime the theoreticians Igor Tamm and Ilya Frank had 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 the 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 somewhat too slow to follow the fast escaping particle. Short after passage, the medium relaxes by emitting anisotropic radiation in the forward direction. Sergei Vavilov became the president of the Academy of Sciences of Soviet Union in 1946. The work of Cherenkov, Tamm, Frank and Vavilov got the highest recognition and they were awarded the Stalin prize, then the most prestigious prize in the Soviet Union. Vavilov was in a permanent worry about his famous botanist and geneticist brother Nikolai who, because of his scientific beliefs was slandered, then arrested in 1940. Afterwards he was sentenced to death that subsequently commuted to imprisonment of 20 years. Nikolai died in prison in 1943, presumably from hunger. Despite his very distinguished position in the science hierarchy of the big country, living in the atmosphere of permanent fear and moral sufferings, Vavilov got about ten heart attacks and in the end 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 until now in Russia somewhat higher credit 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 the corresponding member in the Soviet Academy of Sciences.
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
Let us now have a look into the next events that have played a key role in the evolution of the IACT technique.
3. Discovery of Cherenkov emission in the 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 stemming from air shower elementary particles producing Cherenkov emission in the atmosphere [23]. During his visit of Harwell in 1952 he met with Jelley and 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 25 cm diameter parabolic signaling mirror of a short focal length, fixed it into a dustbin and put a single 2 inch PMT in its focus. The telescope started counting pulses, one every two minutes. They had to confirm that in fact these were Cherenkov pulses coming from air showers. For that purpose they operated the small telescope in coincidence with a large-area Geiger counters for measuring air showers and in fact observed coincident events. Within one year they improved the performance of their telescope and succeeded measuring polarization of the detected light. Also, they found that compared to the green part of the spectrum, there was more light in the blue. These two facts proved that in fact they were measuring Cherenkov pulses from air showers. Galbraith and Jelley published a discovery paper in Nature in 1953, which has marked the beginning of atmospheric air Cherenkov technique [24].
93
Guiseppe Cocconi, the at Cornell University, suggested to measure gamma ray sources at TeV energies at the International Cosmic Ray Conference (ICRC) in Moscow in 1959 [26]. He proposed constructing an air shower array at a high mountain altitude with an angular resolution of 1° for measuring 1012 eV gammas. He made one very optimistic prediction: the possible flux of gamma rays from the Crab Nebula could be 1000 times higher than the background. He admitted that his estimate could be over-optimistic, 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 TeV gamma sources. For understanding the development of air showers in the atmosphere Alexander Chudakov and Natasha Nesterova were measuring the lateral distribution of Cherenkov light on the Pamir mountains at an altitude of 3800 m a.s.l. in 1953–1955. They used large-size Geiger counters and eight Cherenkov light receivers as detectors [27]. Chudakov became excited by the prospects of the possible new science and started planning a systematic study of cosmic gamma ray source candidates in the sky.
5. First generation atmospheric Cherenkov telescopes 5.1. 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 [28]. One year later, they increased the number of telescopes to 12. These were based on parabolic searchlight mirrors of 1.55 m diameter and focus of 60 cm, previously used by the military for securing the Black sea border. 4.5 cm diameter PMTs were set in their foci, with a diaphragm limiting the field of view to 1.75° FWHM. Chudakov calculated a lens of a special form for reducing the aberrations and improving the timing of the mirrors and set them in front of the PMTs. Telescopes were rigidly connected in groups of three pointing in 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. The trigger of the telescopes was based on four-fold coincidence. A rate-stabilizing circuitry was developed for suppressing the count-rate instabilities due to intensity variations of the light of night sky.
Except for relatively small-scale test measurements not very much happened in the following several years.
4. First ideas on astronomy by means of gamma rays In 1958 Philip Morrison suggested to measure gamma rays from possible cosmic sources [25]. He considered that measurements in the energy range 0.2–400 MeV, using detectors on high flying aircrafts or on balloon flights could provide a real clue to the cosmic rays and their sources. He considered that different types of detector could be efficiently used for measuring gamma rays in the above energy range. Morrison mentioned the Crab Nebula as a possible source of gamma rays but, interestingly from today’s point of view, at much lower energies. He thought that the expanding gas shell in the nebula shall contain plenty of radioactive debris from the original explosion, which one can measure. He predicted that a flux of specific nuclear gamma rays, mainly from 226Ra, should be detectable, with an incoming flux of 10 2 gamma/cm2 s.
The experiment was continued for 4 years. By using flashes of Cherenkov light initiated by muons in a piece of Plexiglas of a given thickness, the researchers estimated a photon threshold of 280 ph/ m2 (for the wavelength range 300–600 nm) for their detectors. The shower count rate was slightly more than 3 Hz and the estimated energy threshold was 3.4 TeV for gammas.
94
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
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 min in duration, were repeated many times over the observational 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, or about X-ray sources. The common belief 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 Sagittarius A (7 scans) were observed. The long observations of Cygnus A were due to an initial small indication of a possible signal that the researchers were unsuccessfully 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 a signal from any of the above targets. The signal upper limit from the direction of the Crab Nebula for a total of 5.5 h of observation was 5 10 11 ph/cm2 s for the threshold of 4–5 TeV. It is an interesting question if Chudakov and his colleagues could have discovered a gamma signal from the Crab Nebula back in the 1960’s. Today we know that the integral flux of the Crab Nebula above 4 TeV is 2.5 10 12 ph/cm2 s. This means that the flux limit set by Chudakov and his colleague was 20 times higher than the flux from the source. Consequently, for measuring a significant signal from the Crab Nebula, they had to measure 400 times longer, i.e. for 2200 h; of course that was very improbable to happen. An important consequence of this non-detection of the Crab Nebula was that it invalidated the 1000 times higher flux originally estimated by Cocconi. Also, non-detection of the signal cast strong doubts on the origin of electrons in the Crab Nebula as secondary particles produced in cosmic proton interactions with the nebula; pp ? p ? l ? e (the common belief was that gammas were produced due to pp ? p0 ? 2c). That meant that in fact the electrons could have been accelerated in the nebula.
5.2. First images of air showers An important measurement was realized by Hill and Porter in 1960 [29]. They coupled an image intensifier to a 25 cm diameter Schmidt telescope of a 24° field of view and shot the first images of air showers in the night sky. Because of the small size of the mirror, only images of showers with energies P0.5 PeV were recorded. The trigger came from a 5 inch PMT in the focus of a conventional mirror that was set in close proximity. It was possible to identify bright background stars on the images. The detection rate was 7 events per 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 [30]; 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. 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 (1 ls), and some other disadvantages, it turned out to be impractical.
5.3. Monte Carlo simulations of air showers and the ‘‘stereo’’ observations Victor Zatsepin, member of Chudakov’s crew, published a remarkable Monte Carlo study paper in 1964 [31]. By simulating on the Soviet ‘‘Ural’’-type computer (operated by specially trained staff) he obtained the first contours of equal photon density in 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 to the author that about 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 imaging cameras he could use for that purpose [32]. As one can see from the above, already 50 years ago some researchers clearly understood the potential of coincidence measurements of shower images, 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 were built and operated, typically of smaller scale than the one from Chudakov. Except for the 10 m 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 the 1980s. 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 a coincidence between several such mirror elements, which allowed lowering the energy threshold of the instrument. 5.4. Harwell–Glencullen telescope One should mention the compact telescope in Harwell, UK, which later on was moved to Glencullen valley near Dublin [30]. The telescope, set on a gun mount, consisted of two 92 cm diameter back-silvered mirrors, each with a single PMT in the focus, viewing a 5° field of view. The coincident count rate was 0.7– 1.7 Hz. Observations of quasars, 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 [33]. From today’s point of view it is interesting to note
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
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 opto-electronic design was developed and built in Glencullen, and then installed in 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 in that it used four F/2 mirrors of 90 cm diameter, fast PMTs, fast amplifiers and a 3.5 ns 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 [34].
95
as well as in proceedings of the international cosmic ray conferences. 6. 2nd Generation telescopes 6.1. The 10 m Whipple telescope In 1967, Giovanni Fazio and colleagues began constructing a 10 m diameter, F/0.7 telescope on Mount Hopkins, at the Whipple observatory, at a height of 2300 m a.s.l. [38]. 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.
5.5. Double beam technique Another interesting development in gamma astronomy is related to two 6.5 m diameter reflectors set at a large distance (120 m) for stellar interferometry in Narrabri, Australia. In 1968, the researchers carried out observations of the Crab Nebula and two pulsars, no signal was measured [35]. 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 have been widely used subsequently. For giving an impression about the pioneering and creative spirit of some of the observations about 30–40 years ago, it is interesting to shortly discuss one publication from Grindlay [36]. The author claimed in that paper a detection of a 5r pulsed signal from the Crab pulsar on the flux level of 8 10 12 ph/cm2 s at 1 TeV but in the absence of any DC signal from the Crab Nebula. Today we know that the cited pulsed flux, on the level of 40% of the DC flux from the Crab Nebula, is impossible. Concerning the pulsed flux level from the Crab pulsar at 1 TeV we do not even know yet if the pulsar emits gammas at such a high energy. But we know that the pulsar flux, for example, at 100 GeV is about two orders of magnitude less than the DC flux from the Crab Nebula [37]. 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, of the Haleakala telescope in Hawaii, the Potchefstroom telescope in the South African Republic, the telescopes of the Durham group, the TACTIC, PACT and HAGAR telescopes in India, the SHALON and other former telescopes in Russia. These are largely reflected in the proceedings of the workshop series of ‘‘Towards a Major Atmospheric Cherenkov Detector’’
Trevor Weekes joined the Smithsonian project in 1966. In 1968 he co-authored with Fazio and two other colleagues an interesting observational paper about 13 sources, among which were, for example, the Crab Nebula (of course!), M87, M82, IC443 [39]. Their flux upper limits above the threshold of 2 TeV were modest, but today we know that the above listed sources in fact all are gammaray emitters. In 1977, Weekes and Turver suggested to use two telescopes, each equipped with a 37-pixel imaging camera, at a distance of 100 m. They expected that such a stereoscopic imaging system should strongly suppress the background [40]. In fact, a 37-pixel imaging camera, covering a field of view of 3.5° in the sky, was built and installed on the 10 m telescope in 1983. In the following years the Whipple team could measure marginal signals from Crab on a significance level of slightly below 5r.
96
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
In 1985 Michael Hillas suggested a special parameterization of the measured images [41]. That turned out to be a key milestone in the history of ground-based imaging air Cherenkov telescopes.1 The real breakthrough happened 3 years later; this parameterization helped the Whipple team to measure the famous 9r signal from the Crab Nebula [42]. That measurement marked the beginning of a new era and is commonly considered as the birthday of the ground-based VHE gamma astronomy. The persistent work of Trevor Weekes for over 20 years finally paid off, a new branch of science started its marathon! 6.2. GT-48 in Crimea Since the late 1960’s the group in the Crimean Astrophysical Laboratory (CrAO, located in a small village ‘‘Nauchniij’’) led by Arnold Stepanian used two parabolic searchlight mirrors of 1.5 m 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 also for several years was experimenting in Crimea but at a different location, near the Black sea shore). They reported detections of Cassiopea and Cyg X3 in the beginning of the 1970’s, especially the latter 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 battleship were used for these telescopes. On each mount they built six telescopes, three of the imaging type with 37 pixels in the focus and another three operating a single UV-sensitive, solar blind PMT. Every telescope had 4 mirrors of 1.2 m diameter and 5 m focus. The goal of the Crimean group was to take advantage of stereo observations; see, for example, [43]. They aimed to reach the lowest possible threshold and the highest coincidence rate. Therefore they put the telescopes at 20 m distance from each other. Their relatively small mirror area and the low altitude of the location of 600 m a.s.l. provided a threshold of P900 GeV. The proximity of the telescopes did not allow them to fully exploit the differences in image parameters otherwise seen from largely separated telescopes. In 1989, this installation was put into operation and in subsequent years it measured quite a number of sources.
dedicated international workshop on VHE gamma-ray astronomy organized by Arnold Stepanian in Crimea in 1989 [45].2 It was the prototype of the planned five telescope ‘‘stereo’’ array (the experimental proposal was submitted by Felix Aharonian, by the author and two other colleagues in February 1985), installed at 100 m distance from each other [46]. Each telescope was planned to have a 3 m diameter tessellated mirror of 5 m2 area and to be equipped with a 37-pixel imaging camera in the focal plane at 5 m. The first telescope was built at Nor Amberd cosmic ray station (2000 m a.s.l.) on Mount Aragats in Armenia in 1989. The commissioning of the telescope showed a shower count rate of (0.3–0.5) Hz near zenith. The pixels used light guides of a conical form (focons), made of UV transparent Plexiglas. The imaging camera was based on the most advanced Soviet FEU-130 type PMTs with a GaP first dynode, providing extremely high single photo electron amplitude resolution. The equatorial mechanical mount of the first telescope was re-designed with the help of the YerPhI group engineer Rouben Kankanian and built in the workshops of the Max-Planck-Institute for Physics in Munich in 1991. A 37-pixel imaging camera, a clone of what was built in Nor Amberd, was re-built in the collaborating institution in Kiel, Germany. The high quality glass mirrors were produced in YerPhI. In mid 1991 the main parts for the planned five telescopes, including the mirrors, PMTs, absolute shaft encoders, etc., were sent from Armenia via MaxPlanck-Institute (MPI) for Physics in Munich to the Canary Island La Palma. Eckart Lorenz, the driving force behind the HEGRA array, and a team from MPI Munich installed the first telescope on the Roque de los Muchachos observatory in La Palma in late fall 1991. A 5r signal from the Crab Nebula appeared after two months of data taking, in late fall 1992. In the following year the second telescope with the same type PMTs and pixel size but with one more ring in the camera (61 pixels) and a larger reflector of 4.2 m was built and put into operation at 100 m distance from the first one. The stereo observations, the power of which had been predicted in a dedicated Monte-Carlo study paper [46], started in 1993. Werner Hofmann and Heinz Völk from the MPI for Nuclear Physics in Heidelberg became involved and strongly supported the scientific operation of the HEGRA telescopes. In the following years, four more telescopes of the same size as the second one but with cameras of 271 pixels of size of 0.25° were added. In the end, the second telescope, too, was given a 271-pixel camera and the full array was completed in 1997. The last upgrade in the same year was the increase of the mirror area of the first telescope to 10.3 m2. HEGRA operated till 2002. It convincingly demonstrated the long-awaited power of stereo observations [47]. Although the second generation imaging telescopes found only a handful of sources, it became obvious that there was a big potential in this technique, just waiting to be explored. 6.4. 7-Telescope Array
Still, in a few cases their results remained controversial. 6.3. Hegra The first telescope of HEGRA [10,44] 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 1 Since then this set of parameters, meanwhile simply referred to as ‘‘Hillas’’ parameters are widely used by all IACT teams.
The Japanese 7-Telescope Array was originally planned as a detector including two arrays, each of 127 imaging telescopes, operating in coincidence for measuring either fluorescence or Cherenkov light from air showers [48]. Each telescope had a 3 m 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 remaining four were planned to be installed within one year. The telescopes started taking data on several interesting objects as, for example, the flaring Mkn-501. Unfortunately, soon this experiment suffered a weird misfortune – a 20 foot long unarmed missile of the US army hit and destroyed the two data taking containers shortly before the end of 1997. Thus the experiment was discontinued. 2 The series of international workshops entitled ‘‘Towards a Major Atmospheric Cherenkov Detector’’ was launched in Palaiseau, France, in June 1992. Although the workshop in Crimea happened 3 years earlier, historically the workshop in Palaiseau is referred to as the first one in the series.
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
6.5. The CAT telescope CAT was an imaging telescope designed in 1993 and built and put into operation in 1996 at the Themis solar site in southern France. It had a 17.7 m2 reflector of Davies–Cotton design and operated 600 PMT imaging camera in the focal plane. The reflector consisted of 90 spherical mirrors of 50 cm diameter and 6 m focal length. The camera was based on 546 PMTs of 11 mm size in the central part and 54 PMTs of 28 mm size in the outer two guard rings, both from Hamamatsu. CAT was the first telescope to accommodate the trigger electronics in the camera. The ADC system was fixed on the counterweights. Initially, because of the very fast performance of the PMTs (1.4 ns FWHM), of the electronics and of the telescope, the trigger efficiency on gamma rays turned out to be low. After slowing down the electrical pulses towards 2.5 ns the performance has improved. Also, unlike HEGRA, who were using conical focons as light guides in their imaging cameras, CAT started using Winston cones [12]. In spite of the relatively small size of the reflector, CAT claimed an energy threshold comparable to that of the 10 m Whipple telescope. CAT became one of the productive imaging telescopes and in the subsequent years delivered a wealth of scientific data.
7. Gamma-ray telescopes based on solar power plants It was recognized rather early that mirror-based solar power plants can offer large mirror areas of several thousand m2 that could be used for collecting scarce photons from sub-100 GeV gamma showers. In the beginning of 1990s the achieved threshold energy of the 10 m Whipple telescope of 75 m2 reflecting area was estimated to be 300–400 GeV. The common belief was that for lowering the threshold energy of a telescope by a factor of n one need to increase its mirror area by n2 times. So, for example, for lowering the threshold energy of 1 TeV of the 10 m2 HEGRA CT1 telescope by a factor of 20 down to 50 GeV one needed to increase its mirror area to 4000 m2! A single IACT cannot offer such a huge mirror area. Solar power plants with distributed mirror area, however, can do that, and, some such arrays were readily available for astrophysicists to experiment. Several solar power plants thus were transformed into gammaray detectors. Individual research teams used quite different, ingenious techniques for that. For example, while the GRAAL team [49] was attempting to collect Cherenkov photons from multiple heliostats in the field into a 1 m-size single Winston cone, the STACEE collaboration was trying to organize a kind of imaging in the central light collection tower, directing light from individual heliostats to specific PMT channels. For a review of converted solar power plants please see [50] and references therein. Some interesting measurements were performed with these arrays. The French CELESTE instrument tried to measure the flux from the Crab Nebula down to 60 GeV [51]. The comparison with today’s more precise measurements of MAGIC shows that their reported flux was 2.5 times too low. With the start of operation of the MAGIC telescope in 2004 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 236 m2 of mirror area, could successfully perform measurements also in the sub 100 GeV energy range, down to 50 GeV. This is in striking contrast to the above assumption about the energy threshold dependency on the inverse square root of the mirror
97
area, although this relation can still be found in recent publications. Unlike the non-imaging detectors, the lower threshold of an imaging telescope is simply inversely proportional to the used mirror area. Not the fluctuations of the light of the night sky, but the higher-level requirement of minimum amount of charge for meaningful parameterization of images,3 set the lower threshold for an imaging telescope. Well parameterized images allow one to efficiently select gammas from the vast background of hadrons. Early understanding of this relation played a key role for designing of the MAGIC telescope. To the knowledge of the author, for the first time this was mentioned in [52].
8. 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 17 m diameter MAGIC were made [53]. These were followed by the VERITAS letter of intent in fall 1996 and in the next year by H.E.S.S. While both VERITAS and H.E.S.S. were following the goal of doing astrophysics with a stereo system of 10 m 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-100 GeV energy range, down to 30–40 GeV, into the ‘‘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, at the original site of HEGRA, and together with scientists from Spain, Italy and Switzerland created the core of the MAGIC collaboration. 8.1. H.E.S.S. The H.E.S.S. collaboration was supported by the German and French financial agencies, while the VERITAS team had to wait for several more years for its financing. The H.E.S.S. telescopes were built and started operation in Namibia in 2002–2004. Already in the beginning the H.E.S.S. team performed a scan along the galactic plane and made a very rich harvest of galactic sources. This array turned out to be a very efficient instrument, which made a really high number of important discoveries and measurements above the energies 160–200 GeV, see, for example, [54]. Recently a very large telescope of 28 m diameter has been installed in the center of H.E.S.S. that shall allow performing observations also in the very low energy range of a few tens of GeV [55]. 8.2. Veritas The VERITAS telescopes, unlike H.E.S.S., which are operating imaging cameras of 5° geometrical aperture, are using cameras of 3.5° field of view. Otherwise both instruments are similar and both have increased the originally planned 10 m diameter of their telescopes to 12 m. VERITAS was built in Arizona next to the administrative building of the Harvard–Smithsonian center and inaugurated in mid 2007. Not surprisingly, also VERITAS turned out to be a very successful instrument that in recent years has real3 The practice shows that 100 photo electrons per image are enough for its reliable parameterization.
98
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99
ized a high number of important discoveries and measurements, see, for example, [56,57]. 8.3. Cangaroo CANGAROO was collaboration between several universities from Japan and the University of Adelaide. In 1992 the collaboration started operating a 3.8 m size single telescope of parabolic shape that had been used for lunar ranging in the past. In the following years this collaboration discovered several new sources of gamma rays above the threshold of few TeV. Ten years later, additional four telescopes of 10 m size were built and operated. 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 a few occasions sources detected by CANGAROO could not be confirmed by the H.E.S.S. telescope [58]. A couple of years ago this array terminated its operation. 8.4. Magic Initially, mostly because of financial reasons, the 17 m diameter MAGIC was proposed as a single telescope. A 17 m diameter telescope of innovative design of the German Space Agency (DLR) for utilizing the solar energy has served as the prototype for MAGIC. Several innovations were necessary for operating the single telescope also in the very low energy range 6100 GeV, where a strong background from local muons was expected. MAGIC researchers were hoping to strongly suppress the different backgrounds by using an ultra-fast opto-electronic design of the telescope. For this purpose a reflector of parabolic shape was chosen. Along with this, very fast hemispherical PMTs were co-developed for the needs of MAGIC by Electron Tubes from England. The PMT analog signals were converted into light pulses and, by using 162 m long optical fibers, transported to the electronic room, where they were converted back into electrical signals practically with no degradation of time parameters. The MAGIC-I telescope was built and put into operation in 2003–2004. Starting 2007 MAGIC-I used 2 GSample/s fast multiplexed FACDs for the readout [59]. The measurements showed a full bandwidth of 240 MHz for signal channels. The latter allowed MAGIC to suppress further down the contribution from the light of the night sky as well as the hadron-induced background by a factor of 2–3 [60]. In fact, MAGIC could do observations of some selected sources as, for example, the Crab Nebula and its pulsar, at energies as low as 50–60 GeV [61]. By developing a special socalled SUM-trigger configuration, the researchers could operate the telescope even at a very low threshold of P25 GeV, the traditional energy range of satellite born experiments. This allowed MAGIC to discover a pulsed signal from the Crab pulsar. The next serious improvement of MAGIC’s sensitivity was due to adding of the second, clone telescope at 85 m distance from the first one in 2009. Compared to the first telescope, this has essentially doubled the sensitivity; see, for example, [61,62]. 9. CTA: the 4th generation major instrument The series of ‘‘Towards a Major Atmospheric Cherenkov Detector’’ workshops, taking place between 1992 and 2005 (the last one), served its purpose; the researchers decided to unify their efforts and move towards one major instrument. A few years ago, a new collaboration was formed for building the Cherenkov Telescope Array (CTA) [63]. This collaboration includes practically all the researchers worldwide working with the atmospheric air
Cherenkov telescopes and many newer groups who are interested in exploring the sky in VHE gamma rays with 10 times higher sensitivity than any of the existing instruments. Now the CTA collaboration is moving from the prototyping into the construction phase. Over 100 telescopes of 23 m, 12 m and 4–7 m size are planned to be built in southern and northern observatories [63,64]. CTA is going to be the major ground-based instrument for doing astrophysics by means of VHE gamma rays for the next 10–15 years [65].
References [1] J.V. Jelley, Cherenkov Radiation and its Applications, Pergamon Press, 1958; J.V. Jelley, in: Proceedings of the International Workshop on Very High Energy Gamma Ray Astronomy, Ootacamund, India, 1982, p. 64. [2] A.S. Lidvansky, arXiv:astro-ph/0504269. [3] A. Watson, arXiv:1101.4535. [4] T.C. Weekes, arXiv:astro-ph/0508253. [5] D. Fegan, arXiv:1104.2403v1. [6] E. Lorenz, R. Wagner, arXiv:1207.6003v1. [7] A.M. Hillas, Astrop. Phys. (2012), in press. http://dx.doi.org/10.1016/ j.astropartphys.2012.06.002. [8] S. Funk, arXiv:1204.4529. [9] T.C. Weekes et al., Astrophys. J. 342 (1989) 370. [10] R. Mirzoyan, R. Kankanian, F. Krennrich, et al., Nucl. Instrum. Methods A 351 (1994) 513. [11] T. Hara et al., Nucl. Instrum. Methods A 332 (1993) 300; R. Enomoto et al., ApJ 638 (2006) 397. [12] A. Barrau, R. Bazer-Bachi, E. Beyer, et al., Nucl. Instrum. Methods A 416 (1998) 278. [13] P. Goret, T. Palfrey, A. Tabary, et al., A&A 270 (1993) 401. [14] P. Baillon et al., Astropart. Phys. 1 (1993) 341. [15] O. Heaviside, Philos. Mag. S 5 (27) (1889) 324. [16] A. Sommerfeld, Goettingen Nachr. 99 (1904) 363; 201 (1905). [17] Eve Curie ‘‘Madame Curie’’, London, 1941. [18] L. Malet, C. R. Acad. Sci. (Paris) 183 (1926) 274; , C. R. Acad. Sci. (Paris) 187 (1928) 222; , C. R. Acad. Sci. (Paris) 188 (1929) 445. [19] P.A. Cherenkov, Dokl. Akad. Nauk SSSR 2 (1934) 451. [20] S.I. Vavilov, Dokl. Akad. Nauk SSSR 2 (1934) 457. [21] P.A. Cherenkov, Phys. Rev. 52 (1937) 378. [22] I.E. Tamm, I.M. Frank, Dokl. Akad. Nauk SSSR 14 (1937) 109. [23] P.M.S. Blackett, Rep. Conf. Gassiot Comm. Phys. Soc., Emission spectra of the night sky and aurorae, 34, Phys. Soc., London, 1948. [24] W. Galbraith, J.V. Jelley, Nature 171 (1953) 349. [25] P. Morrison, Il Nuovo Cimento 7 (1958) 858. [26] G. Cocconi, in: Proceedings of the International Cosmic Ray Conference, vol. 2, Moscow, 1960, p. 309. [27] A.E. Chudakov, N.M. Nesterova, Sov. Phys. JETF 28 (1955) 384. [28] A.E. Chudakov, V.L. Dadykin, V.I. Zatsepin, N.M. Nesterova, Search of photons of energy 1013 eV from local sources of cosmic radio emission, Proc. P.N. Lebedev Phys. Inst. 26 (1964) 118–141. Transl. Consultants Bureau 26 (1965) 99. [29] D.A. Hill, N.A. Porter, Nature 191 (1960) 690. [30] J.V. Jelley, N.A. Porter, J.R. Quart, Astron. Soc. 4 (1963) 275. [31] V.I. Zatsepin, Sov. Phys. JETF 47 (1964) 689; , (Eng.:) Sov. Phys. JETF 20 (2) (1965) 459. [32] V.I. Zatsepin, Private Communication, 2012. [33] C.D. Long, B. McBreen, E.P. O’Mongain, N.A. Porter, in: Proceedings of 9th ICRC, London, vol. 1, 1965, 318 (1966). [34] D.M. Jennings, G. White, N.A. Porter, E.P. O’Mongain, D.J. Fegan, J. White, Il Nuovo Cimento 20 (1) (1974) 71. [35] R. Hanbury Brown, J. Davis, L.R. Allen, Mon. Not. R. Astron. Soc. 146 (1969) 399. [36] J.E. Grindlay, Astrophys. J. 174 (1972) L9–L17. [37] J. Aleksic et al., MAGIC Coll., Astropart. Phys. 540 (2012) 69. [38] G.G. Fazio, H.F. Helmken, G.H. Rieke, T.C. Weekes, Can. J. Phys. 46 (1968) 451. [39] G.G. Fazio, H.F. Helmken, G.H. Rieke, T.C. Weekes, Astrophys. J. 154 (1968) L83. [40] T.C. Weekes, K.E. Turver, in: R.D. Wills, B. Battrick (Eds.), Proceedings of 12th ESLAB Symposium (Frascati), ESA SP124, vol. 279, 1977. [41] A.M. Hillas, in: Proceedings of 18th ICRC, vol. 3, 1985, p. 445. [42] T.C. Weekes, M.F. Cawley, D.J. Fegan, et al., Astrophys. J. 342 (1989) 379. [43] A.P. Kornienko, A.A. Stepanian, Yu.L. Zyskin, et al., Exp. Astron. 4 (1993) 77. [44] R. Mirzoyan, (HEGRA Collab.) in: P. Fleury, G. Vacanti (Eds.), Towards a Major Atmospheric Cherenkov Detector, Palaiseau, France, June 11–12, 1992, p. 239. [45] F. Aharonian, et al., in: A.A. Stepanian, D.J. Fegan, M.F. Cawley (Eds.), Proceedings of the International Workshop VHE Gamma-Ray Astronomy, Crimea, USSR, April 17–21, vol. 36, 1989. [46] F.A. Aharonian, A.A. Chilingaryan, R.G. Mirzoyan, A.K. Konopelko, A.V. Plyasheshnikov, Exp. Astron. 2 (1993) 331. [47] G. Puehlhofer et al., HEGRA Collab., Astropart. Phys. 20 (2003) 267. [48] M. Teshima, G. Dion, N. Hayashida, et al., in: P. Fleury, G. Vacanti (Eds.), Proceedings of Towards a Major Atmospheric Cherenkov Detector, Palaiseau, France, June 11–12, 1992, 255.
R. Mirzoyan / Astroparticle Physics 53 (2014) 91–99 [49] R. Plaga, in: Proceedings of 17th ECRC, vol. 178, 2000; F. Arqueros, et al., Astropart. Phys. 3 (2002) 293. [50] D.A. Smith, arXiv:astro-ph/0608251v1, 2006. [51] M. De Naurois, J. Holder, R. Bazer-Bachi, et al., CELESTE Collab., Astrophys. J. 566 (2002) 343. [52] R. Mirzoyan, in: De Jager (Ed.), Towards a Major Atmospheric Cherenkov Detector-V, Krueger Nat. Park, South Africa, August 8–11, 1997, p. 298. [53] S.M. Bradbury, et al., in: Cresti (Ed.), Proceedings of the International Workshop Towards a Major Atmospheric Cherenkov Detector-IV, Padova, Italy, September 11–13, 1995, p. 277; E. Lorenz, in: Proceedings of TAUP 95, Toledo, Spain, 1995. [54] M. de Naurois, H.E.S.S. collab., Adv. Space Res. 51 (2013) 258. [55] P. Hofverberg, Nucl. Instrum. Methods A 639 (2011) 23. [56] F. Krennrich et al., Nucl. Instrum. Methods A 630 (2011) 16.
99
[57] K. Ragan, Nucl. Instrum. Methods A 692 (2012) 24. [58] M. Mori, in: Shibara, Sakaki, (Eds.), International Workshop ‘‘Cosmic-rays and High Energy Universe’’, Aoyama Gakuin University, Shibuya, Tokyo, Japan, March 5–6, 2007, University of Academy Press, Tokyo, Japan, 93 2007. [59] H. Bartko, F. Goebel, R. Mirzoyan, et al., Nucl. Instrum. Methods A 548 (2005) 464. [60] E. Aliu et al., MAGIC Collab., Astropart. Phys. 30 (2009) 293. [61] J. Aleksic et al., MAGIC collab., Astropart. Phys. 35 (2012) 435. [62] A. De Angelis, Adv. Space Res. 51 (2013) 280. [63] CTA homepage: www.cta-observatory.org. [64] M. Actis, et al., CTA Consortium, arXiV:1008.3703v3; also in Exp. Astron. 32 (2011) 193. [65] B.S. Acharya et al., CTA Consortium, Astrop. Phys. 43 (2013) 3.