Atmospheric Cherenkov Technique Research Articles

On the potential of the imaging atmospheric Cherenkov technique for study of the mass composition of primary cosmic radiation in the energy region above 30 TeV

The technique of imaging atmospheric Cherenkov telescopes (IACTs) has proved to be an effective tool to register cosmic γ-rays in the very high energy region. The high detection rate of the IACT technique and its capability to reconstruct accurately the air shower parameters make it attractive to use this technique for the study of the mass composition of cosmic rays (CRs). In this article we suggest a new approach to study the CR mass composition in the energy region from 30 TeV/nucleus up to the “knee” region, i.e. up to a few PeV/nucleus, using an array of imaging telescopes of a special architecture. This array consists of telescopes with a relatively small mirror size (∼10 m 2) separated from each other by large distances (∼500 m) and equipped by multi-channel cameras with a modest pixel size (0.3–0.5°) and a sufficiently large viewing angle (6–7°). Compared to traditional IACT systems (like HEGRA, HESS or VERITAS) the IACT array considered in this study could provide a very large detection area (several km 2 or more). At the same time, it allows an accurate measurement of the energy of CR-induced air showers (the energy resolution ranges within 25–35%) and an effective separation of air showers created by different nuclei. Particularly, it is possible to enrich air showers belonging to the nucleus group assigned for selection up to ∼90% purity at a detection efficiency of 15–20% of such showers.

On the potential of the imaging atmospheric Cherenkov technique for study of cosmic gamma rays in the energy range 10-100 TeV

We suggest a new approach for the study of cosmic -rays in the energy region from several TeV to 100 TeV using an array consisting of imaging atmospheric Cherenkov telescopes (IACTs) of relatively small mirror size of about 5-10 m2, but equipped with large, up to 8° cameras with modest pixel size of about 0.3 - 0.5°. The large field of view of these telescopes would allow the detection of air showers initiated by primary 10 TeV -rays from distances up to 500 m or more. The recommended distance between the telescopes varies from 300 to 500 m, and thus an array consisting of several such IACTs could provide huge, up to 1 km2, detection area for ultrahigh-energy -rays. Detailed Monte Carlo simulations performed for a system consisting of 4 IACTs, which could be considered as a cell for a future powerful, 10 TeV threshold, multi-telescope array, show that such detector could provide an effective /hadron separation, a superior angular resolution of about a few arcminutes, and a good energy resolution of about ~20%.

Impact of atmospheric parameters on the atmospheric Cherenkov technique

Atmospheric density profiles as well as several light absorption and scattering processes depend on geographic position and are generally time-variable. Their impact on the atmospheric Cherenkov technique in general (imaging or non-imaging) is investigated. Different density profiles lead to differences in Cherenkov light density of up to 60%. Seasonal variations at mid-latitude sites are of the order of 15–20%. The quest for improved energy calibration of Cherenkov experiments also shows the need for improved transmission calculations, taking all relevant processes into account and using realistic profiles of absorbers. Simulations including the scattering mechanisms also reveal the relevance of Rayleigh and Mie scattering for atmospheric Cherenkov experiments. Refraction and the differences between treating the atmosphere in plane-parallel or spherical geometry are also investigated.

On the parameterization of patterns in the imaging atmospheric Cherenkov technique

A new method for analytical description of images (patterns) of the air showers detected by imaging atmospheric Cherenkov telescopes is proposed. It is shown that the generally accepted procedure constructs the image as a fourth-order plane curve (Booth's lemniscate). The method proposed for.m.s. a uniformly illuminated elliptical image preserving the same first- and second-order moments of the registered light disribution. Such a representation describes the angular size of image correctly and allows us to conclude that pixel size less than 0.2° is not particularly advantageous. Unified formulae for the calculation of Hillas' as well as the new parameters are presented. A comparative analysis of the approaches for separation of gamma- and proton-induced air showers is made. A new parameter is proposed which is more effective than AZWIDTH.

New generation atmospheric Cherenkov detectors

High energy γ-ray astronomy has been established during the last decade through the launch of the Compton Gamma Ray Observatory (CGRO) and the success of its ground-based counterpart, the imaging atmospheric Cherenkov technique. In the aftermath of their important and surprising scientific results a worldwide effort developing and designing new generation atmospheric Cherenkov detectors is underway. These novel instruments will have higher sensitivity at E > 250 GeV, but most importantly, will be able to close the unexplored energy gap between 20 GeV and 250 GeV. Several ground-based detectors are proposed or under construction. Aspects of the techniques used and sensitivity are discussed in this overview paper. The instruments cover largely complementary energy ranges and together are expected to explore the γ-ray sky between 20 GeV and 100 TeV with unprecedented sensitivity.

Cosmic ray proton spectrum determined with the imaging atmospheric Cherenkov technique

The HEGRA system of 4 imaging atmospheric Cherenkov telescopes ~IACTs! has been used to determine the flux and the spectrum of cosmic ray protons over a limited energy range around 1.5 TeV. Although the IACT system is designed for the detection of g-rays with energies above 500 GeV, it has also a large detection area of .10 6 m 2 33 msr for primary protons of energies above 1 TeV and the capability to reconstruct the primary proton energy with a reasonable accuracy DE/E of 50% near this threshold. Furthermore, the principle of stereoscopic detection of air showers permits the effective suppression of air showers induced by heavier primaries already on the trigger level, and in addition on the software level by analysis of the stereoscopic images. The combination of both capabilities permits a determination of the proton spectrum almost independently of the cosmic ray chemical composition. The accuracy of our estimate of the spectral index at 1.5 TeV is limited by systematic uncertainties and is comparable to the accuracy achieved with recent balloon and space borne experiments. In this paper we describe in detail the analysis tools, namely the detailed Monte Carlo simulation, the analysis procedure and the results. We determine the local ~i.e., in the range of 1.5‐3 TeV! differential spectral index to be g p52.7260.02stat60.15syst and obtain an integral flux above 1.5 TeV of F (.1.5 TeV)53.160.6 stat61.2syst310 22 /s sr m 2 . @S0556-2821~99!04107-7#

The CAT imaging telescope for very-high-energy gamma-ray astronomy

The CAT (Cherenkov Array at Thémis) imaging telescope, equipped with a very-high-definition camera (546 fast phototubes with 0.12° spacing surrounded by 54 larger tubes in two guard rings) started operation in Autumn 1996 on the site of the former solar plant Thémis (France). Using the atmospheric Cherenkov technique, it detects and identifies very high energy γ-rays in the range 250 GeV to a few tens of TeV. The instrument, which has detected three sources (Crab nebula, Markarian 421 and Markarian 501), is described in detail.

Study of mass composition and energy spectrum of primary cosmic radiation by the imaging atmospheric Cherenkov technique

Imaging atmospheric Cherenkov telescopes (IACT) have proven to be, up to now, the most effective instruments for the observation of primary TeV -radiation from astrophysical objects. The high detection rate as well as the capability of accurate reconstruction of air-shower parameters by the IACT technique also allow the use of the imaging technique for the study of the energy spectrum and mass composition of cosmic rays. In this paper we discuss the potential of this technique for the separation of different components of cosmic rays in the energy region 1-. We show that the imaging technique allows adequate separation of showers produced by protons from the contributions of all other nuclei, including -particles, and thus allows a measurement of the energy spectrum of the proton component at TeV energies. Such measurements can currently be performed by operating single IACTs designed originally for -ray astronomical purposes. The effective separation of different nucleus groups of the cosmic radiation is a more difficult problem. It benefits from the stereoscopic reconstruction of the shower development in the Earth's atmosphere. Our calculations show that simultaneous detection of air showers by two IACTs located at a distance of about 100 m from each other allows an effective separation of the proton, -particle, medium (O), heavy (Si), and very heavy (Fe) groups of cosmic-ray nuclei.

Strategies for selection of small γ-ray showers using the Imaging Atmospheric Cherenkov Technique

Abstract Methods of selecting small candidate γ-ray events obtained using the Imaging Atmospheric Cherenkov Technique are presented. Three approaches are described: a variant of the standard ‘Supercuts’ analysis technique, a parameter weights approach, and a neural network approach. A cut on the parameter combination Length/Size is developed and shown to be effective at rejecting background due to muons. The techniques are applied to observations of Markarian 421 taken during 1995 and 1996 with the Whipple 10 m reflector. The efficiency and stability of the three approaches are compared, and the Supercuts variant is shown to be the most robust. A signal is detected in the energy range 170 GeV to 300 GeV at a significance level of 6.5 σ, in 7 hours of observation (the standard analysis gives 15.2 σ for E > 300 GeV). This represents the first significant detection of a source at an energy threshold below 200 GeV using the Whipple 10 m system.

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

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.

The use of an ultra-violet camera in the atmospheric Cherenkov technique

The atmospheric Cherenkov technique for gamma ray astronomy is shown to be feasible using an ultraviolet (UV) camera on moon-lit-nights. The Whipple telescope has been used as the reflector and the Crab Nebula as the standard gamma ray beam. Our energy threshold is 1.2 TeV which is compared to 0.4 TeV in the visible.

The atmospheric Cherenkov technique in Very High Energy Gamma-Ray Astronomy

The role of the atmospheric Cherenkov technique in the development of Very High Energy Gamma Ray Astronomy is described; it is shown that the Crab Nebula has played a critical role in this development. The ability of a TeV telescope to see a 5 sigma signal from a source in an hour of observation is now a reality. Sources as weak as 1/20th the strength of the Crab are detectable in principle now at TeV energies.

TeV gamma ray astronomy

The atmospheric Cherenkov technique has been used to study TeV gamma-ray sources for over 30 years; it is the only viable technique in the energy band from 0.1 to 10 TeV and at least three sources have been detected.

Computer simulation methods for investigating the detection characteristics of TeV air Cherenkov telescopes

Determination of the energy thresholds and detection sensitivities of VHE (10 GeV to 20 TeV) gamma ray astronomy telescopes utilizing the atmospheric Cherenkov technique (ACT) requires the use of computer simulations of gamma ray and hadron induced air showers and their associated Cherenkov light production. We present in this paper methods to determine these parameters that utilize the DePauw-Purdue KASCADE shower simulation program for a variety of existing and proposed ACT telescopes. The KASCADE system is a detailed, three-dimensional computer simulation developed by us that generates the Cherenkov photons produced by VHE gamma ray and hadronic (usually proton) air showers. The KASCADE system has been specifically designed to be of general use as a tool in investigating a variety of ACT telescope designs with the goal of maximizing their gamma ray detection sensitivity. Specific methods for achieving these goals are presented. Also presented are detailed discussions of the various algorithms used by the KASCADE program to simulate air showers. Further, as a demonstration of the utility of these methods and the KASCADE programs we present results of a case study of the detection characteristics of the Haleakala Gamma Ray Observatory telescope.

A current compensation system for an atmospheric Cherenkov detector

A simple circuit is developed to maintain a constant anode current (< ± 1%) from a photomultiplier viewing Cherenkov light against wide fluctuations in the night sky background. This has enabled us to maintain a constant photon density threshold for the Cherenkov receivers while observing sources of gamma rays using the atmospheric Cherenkov technique. The circuit has a high figure of merit (= 625) and is fast enough to respond to atmospheric stellar scintillations in the frequency range of dc to 5 kHz.

The atmospheric Cherenkov technique in y-ray astronomy: the early days

At this meeting papers were given by Turver &amp; Weekes and Sreekantan about the current status in the detection of ultra-high-energy y-rays in the energy range 10 11 —10 13 eV, by means of the atmospheric Cherenkov technique. There are two objectives of this short contribution. The first is to describe briefly the early history of the subject, and the second to outline the basic physics involved, which will reveal how the technique is essentially quite different from those used in the other energy bands in the y-ray spectrum.

The atmospheric Cherenkov technique in searches for exploding primordial black holes

The Cherenkov technique has been used with a number of detectors, ranging from 1.5 m 2 mirrors to the Central Receiver Test Facility of 8400 m 2 . Limits have been set to the flux of primordial black holes for various models of the evaporation process.

Observations of Cygnus X-3 at energies above 1000 GeV

The Crimean Observatory Group have reported, in several publications, the detection of high energy y-rays from the X -ray source Cygnus X-3 (Vladimirsky et al. 1973; Vladimirsky et al. 1975, Stepanian et al. 1977, Neshpor et al. 1979, Mukanov et al. 1979). The energy was in the region of 1000 GeV, and observations were made by means of the atmospheric Cherenkov technique. Measurements at lower energies have shown conflicting results. Satellite observations in the 10 8 eV region have shown positive effects from Cygnus X-3 in the case of SAS-2 (Lamb et al. 1977), but not in COS-B (Bennett et al. 1977). Similarly, y-rays have been seen in some balloon flights (Galper et al. 1977), but not in others (McKechnie et al. 1976, White et al. 1980). A Cherenkov study at 300 GeV (Helmken &amp; Weekes 1979) showed no effect, but this has been shown by Neshpor et al. (1979) to be compatible with the Crimean results.

Search for High Energy Gamma-rays from CP 1133

Several proposed models for pulsars, involving shock waves and high magnetic fields, suggest the possibility that high energy γ-rays may be emitted by such objects. We have therefore attempted to detect high energy (~ 10^(14) eV), neutral cosmic rays from CP 1133, using the atmospheric Cherenkov technique.