Ground-based gamma-ray astronomy

Summary This report covers developments in the field of gamma-ray astronomy, essentially in the energy range 300 GeV to 300 TeV, reported at the XXIV International Cosmic-Ray Conference in Rome in 1995. Highlights which receive the main attention are the failure of several experiments to detect TeV photons from several supernova remnants at the level predicted on current models of shock acceleration of cosmic-ray protons, and the detection of a rapidly variable flaring output of TeV photons from two BL Lac objects (Markarian 421 and Markarian 501). Although techniques are now maturing to the extent that different measurements of the flux and spectrum from the Crab Nebula are now converging, unresolved differences remain over the existence of TeV pulses from certain pulsars. However, one well-supported instance of very intense pulsation for an hour from AE Aquarii was presented. The Vela Pulsar was reported as a steady source. Rapporteur talk given at the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

The future of gamma-ray astronomy

The field of gamma-ray astronomy has experienced impressive progress over the last decade. Thanks to the advent of a new generation of imaging air Cherenkov telescopes (H.E.S.S., MAGIC, VERITAS) and thanks to the launch of the Fermi-LAT satellite, several thousand gamma-ray sources are known today, revealing an unexpected ubiquity of particle acceleration processes in the Universe. Major scientific challenges are still ahead, such as the identification of the nature of Dark Matter, the discovery and understanding of the sources of cosmic rays, or the comprehension of the particle acceleration processes that are at work in the various objects. This paper presents some of the instruments and mission concepts that will address these challenges over the next decades.     

Ground-based gamma-ray astronomy: General remarks

Summary The ground-based gamma-ray astronomy has its coming of age currently with 4 sources detected at high significance levels, after experiencing about 30 years history. The energetic gamma rays provide direct means for uncovering higher-energy non-thermal phenomena at various celestial objects. We need more data with improved sensitivities, and multitudes of such efforts are now under way. Paper presented at the Special Session on ground-based gamma-ray astronomy of the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

Large-area atmospheric Cherenkov detectors for high-energy gamma-ray astronomy

Summary This paper describes the development of new ground-based gammaray detectors to explore the energy region between 20 and 200 GeV. This region in energy is interesting because it is currently unexplored by any experiment. The proposed detectors use the atmospheric Cherenkov technique, in which Cherenkov radiation produced in the gamma-ray air showers is detected using mirrors and light-sensitive devices. The important feature of the proposed experiments is the use of large mirror collection areas, which should allow for a significant improvement (i.e. reduction) in energy threshold over existing experiments. Large mirror areas are available for relatively low cost at central tower solar power plants, and there are two groups developing gamma-ray experiments using solar heliostat arrays. This paper summarizes the progress in the design of experiments using this novel approach. Paper presented at the Special Session on ground-based gamma-ray astronomy of the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

Application of solar arrays in high-energy gamma-ray astronomy

Summary The previously suggested use of large solar-energy arrays for additional applications in astronomy and astrophysics is herein analysed for the case of high-energy gamma-ray astronomy. We have studied by Monte Carlo techniques the response of an array of 800 m² reflective area to the Čerenkov light produced by relativistic electrons of high-energy cascades, initiated by primary cosmic rays (gamma-rays and protons) in the Earth's atmosphere. The calculation shows that night-time use of these devices may give information on the study of gamma-ray sources which is consistent with the results obtained from specific apparatuses, from the point of view of both energy threshold and detectable fluxes. This result confirms the possibility of designing multiscope arrays operating daily as solar-energy collectors and at night-time as cosmic gamma-ray detectors, with obvious advantages from the financial point of view.

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Riassunto Si analizza la possibile utilizzazione dei campi eliostatici per applicazioni di tipo astrofisico nel caso dell'astronomia gamma di alta energia. Le prestazioni di un apparato con superficie riflettente di 800 m² come raccoglitore della luce Čerenkov emessa dagli elettroni relativistici di sciami estesi atmosferici (EAS) sono state studiate con un metodo di Montecarlo. I calcoli mostrano che l'uso notturno di questo sistema per la ricerca di sorgenti gamma può fornire risultati confrontabili con quelli ottenuti da apparati specifici, sia per quanto riguarda la soglia di rivelazione (E _s<100 GeV) che i flussi rivelabili. Dati gli ovvi vantaggi di tipo economico, questo risultato suggerisce la possibilità di progettare gli apparati in modo multiuso, cioè tali da poter operare come collettori solari nelle ore diurne e come rivelatori di EAS in quelle notturne.

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Резюме В этой работе анализируется применение больших солнечных антенных решеток для случая гамма-астрономии высоких энергий. Мы исследуем с помощью метода Монте-Карло отклик антенной решетки 800 м² на черенковское излучение, образованное релятивистскими электронами в широких атмосферных ливнях. Вычисления показывают, что использование таких приборов в ночное время может дать информацию об источниках гамма-лучей, которая согласуется с результатами, полученными с помощью специальной аппаратуры, с точки зрения энергетического порогаE _s<100 ГэВ и с точки зрения детектируемых потоков. Этот результат подтверждает возможность многоцелевого использования антенных решеток; в дневное время как коллекторов солнечной энергии, а в ночное время как детекторов космических гамма-лучей.

     

Antimatter in the Universe: constraints from gamma-ray astronomy

We review gamma-ray observations that constrain antimatter – both baryonic and leptonic - in the Universe. Antimatter is probed through ordinary matter, with the resulting annihilation gamma-rays providing indirect evidence for its presence. Although it is generally accepted that equal amounts of matter and antimatter have been produced in the Big Bang, gamma-rays have so far failed to detect substantial amounts of baryonic antimatter in the Universe. Conversely, positrons are abundantly observed through their annihilation in the central regions of our Galaxy and, although a wealth of astrophysical sources are plausible, their very origin is still unknown. As both antimatter questions – the source of the Galactic positrons and the baryon asymmetry in the Universe - can be investigated through the low energy gamma-ray channel, the mission concept of a dedicated space telescope is sketched out.     

STAR CLUSTER: a tracking array for gamma-ray astronomy

Summary A new type of extensive air shower array-telescope is being constructed, close to the GREX scintillator array at Haverah Park. The design of the new instrument is based on identification and measurement of individual shower particles by means of pattern recognition techniques applied to data from LST chambers above and below a thin sheet of high-Z material. Air showers will be detected in the range from 10¹⁴ eV to 10¹⁸ eV. The directions of the shower primaries will be reconstructed with an accuracy of a few tenths of a degree using triangulation between tracks of detected shower particles: >1 GeV electrons, photons and muons. Particle densities measured at the detector locations will be used to calculate shower sizes and core locations. Information about the height of production of the high-energy secondaries will be obtained for each shower. This information will be taken into account in calculating the primary energies. It will be used together with the observed muon content (ratio of muons to all particles), to draw conclusions about the nature of the primary particles. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

An innovative silicon photomultiplier digitizing camera for gamma-ray astronomy

The single-mirror small-size telescope (SST-1M) is one of the three proposed designs for the small-size telescopes (SSTs) of the Cherenkov Telescope Array (CTA) project. The SST-1M will be equipped with a 4 m-diameter segmented reflector dish and an innovative fully digital camera based on silicon photo-multipliers. Since the SST sub-array will consist of up to 70 telescopes, the challenge is not only to build telescopes with excellent performance, but also to design them so that their components can be commissioned, assembled and tested by industry. In this paper we review the basic steps that led to the design concepts for the SST-1M camera and the ongoing realization of the first prototype, with focus on the innovative solutions adopted for the photodetector plane and the readout and trigger parts of the camera. In addition, we report on results of laboratory measurements on real scale elements that validate the camera design and show that it is capable of matching the CTA requirements of operating up to high moonlight background conditions.     

Angular resolution of an array of tracking-chamber detectors for use in high-energy gamma-ray astronomy

Summary We describe a method of analysis for the reconstruction of the arrival direction of a high-energy photon impacting on the top of the atmosphere by using the directions of the particles on the shower front as measured by a ground-based array of tracking chambers uniformly distributed on a square surface; as an example, an array with enclosed surface of (120×120) m² has been considered. Estimates on the accuracy of reconstruction are given, after application of the method to computer-simulated shower directions as a function of the number of ?smapling? detectors and of their accuracy in the measurement of the ?local? shower direction. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

EAS-TOP: Results of the gamma-ray astronomy at 1014 eV

Summary The EAS-TOP Extensive Air Shower array, on top of the under-ground Gran Sasso Laboratory, is fully operating as a γ-ray astronomy observatory since the beginning of 1989. After showing the measured angular resolution of the detector, we present the results obtained from the analysis of 280 days of measurements in 1989–1990, with the purpose of investigating possible emission (d.c. and sporadic) from the candidate sources in the Northern hemisphere at an energy, >200 TeV. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Investigation on the effectiveness of VHE gamma-ray astronomy techniques based on imaging of ?erenkov light flashes

Summary A high-velocity Monte Carlo method is applied to very-high-energy gamma-ray astronomy techniques. The discrimination effectiveness against the cosmic-ray background is investigated for the technique based on the two-dimensional imaging of the Čerenkov light flashes created by the extensive air showers in the atmosphere. It is shown that previously proposed discrimination methods using the difference in the Čerenkov light flash shape of gamma-rays—and background-induced showers have low effectiveness. Two new discrimination methods are proposed, based on the difference in the orientation of Čerenkov light spot and fluctuations of the light density. The combination of these methods allows us to increase the ?signal/noise? ratio by 4–5 times.

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Riassunto Si applica un metodo di simulazione Monte Carlo veloce alle tecniche in uso in astronomia gamma di altissima energia. Si studia l'efficienza di discriminazione contro la radiazione di fondo cosmica per il metodo basato sulle immagini bidimensionali degli impulsi di luce Čerenkov creati dai grandi sciami nell'atmosfera. Si dimostra che i metodi di discriminazione precedentemente proposti, basati sulla differenza di forma tra gl'impulsi di luce Čerenkov prodotti da sciami iniziati da fotoni gamma o da radiazioni di fondo, hanno una bassa efficienza. Si propongono, quindi, due nuovi metodi di discriminazione basati sulla differenza di orientazione della macchia luminosa Čerenkov e sulle fluttuazioni della densità luminosa. L'uso di entrambi i metodi permette di aumentare il rapporto segnale-rumore di un fattore 4–5.

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Резюме В технике гамма-астрономии очень высоких энергий используется метод Монте-Карло. Исследуется эффективность дискриминации космических лучей для техики, основанной на двумерной визуализации вспышек черенковского излучения, образованных широкими атмосферными ливнями. Показывается, что ранее предложенные методы дискриминации, используюжие различии в форме вспышек черенковского излучения для гамма-лугей и фоновыми ливнями, имеют низкую эффективность. Предлагаются два новых метода дискриминации, которые основаны на различиии в ориентации пятна черенковского излучения и флуктуаций интенсивности света. Комбинация этих методов позволяет нам увеличить отношение сигнала к шуму в 4–5 раз.

     

TeV astronomy

With the successful realization of the current-generation of ground-based detectors, TeV Astronomy has entered into a new era. We review recent advances in VHE astronomy, focusing on the potential of Imaging Atmospheric Cherenkov Telescopes （IACTs）, and highlight astrophysical implications of the results obtained within recent years.     

Cosmic X-ray astronomy

The results obtained from observations of the celestial x-radiation over the past nine years are reviewed. Emphasis is placed on the study of the discrete sources, particularly those which have been identified with astronomical objects known from optical and radio studies. The isotropic background x-radiation is dealt with briefly, and the theories which account for x-ray production on the astronomical scale are mentioned where applicable. Finally, the observations needed to further advance this exciting subject are discussed.     

Gravitational wave astronomy

We are entering a new era of gravitational-wave astronomy. The ground-based interferometers have reached their initial design sensitivity in the audio band. Several upper limits have been set for anticipated astrophysical sources from the science data. The advanced detectors in the US and in Europe are expected to be operational around 2015. New advanced detectors are also planned in Japan and in India. The first direct detections of gravitational waves are expected within this decade. In the meanwhile, three pulsar timing array projects are forming an international collaboration to detect gravitational waves directly in the nanoHertz range using timing data from millisecond pulsars. The first direct detection of nanoHertz gravitational waves are also expected within this decade. In this paper, we review the status of current gravitational-wave detectors, possible types of sources, observational upper limits achieved, and future prospects for direct detection of gravitational waves     

Superconducting detectors in astronomy

Radiation detectors based on superconducting phenomena are becoming increasingly important for observational astronomy. Recent developments in this important field, together with relevant background, are described here. After a general introduction to superconductivity and the field of superconductor-based radiation sensors, the main detector types are examined with regard to their physical form, operating principles and principal advantages. All major forms of superconducting detectors used in contemporary research such as tunnelling detectors, mixers, hot-electron bolometers and transition edge sensitive devices are discussed with an emphasis on how more recent developments are overcoming the shortcomings of the previous device generations. Also, discussed are new ideas in superconducting detector technology that may find applications in the coming years.