Using the photons from the Crab Nebula seen by GLAST to calibrate MAGIC and the imaging air Cherenkov telescopes

In this article we discuss the possibility of using the observations by GLAST of standard gamma sources, as the Crab Nebula, to calibrate imaging air Cherenkov detectors, MAGIC in particular, and optimise their energy resolution. We show that at around 100 GeV the absolute energy calibration uncertainty of Cherenkov telescopes can be reduced to 10% by means of such cross-calibration procedure.     

5@5 – a 5 GeV energy threshold array of imaging atmospheric Cherenkov telescopes at 5 km altitude

We discuss the concept and the performance of a powerful future ground-based astronomical instrument, 5@5 – a 5 GeV energy threshold stereoscopic array of several large imaging atmospheric Cherenkov telescopes (IACTs) installed at a very high mountain elevation of about 5 km a.s.l. – for the study of the γ-ray sky at energies from approximately 5 to 100 GeV, where the capabilities of both the current space-based and ground-based γ-ray projects are quite limited. With its potential to detect the “standard” EGRET γ-ray sources with spectra extending beyond several GeV in exposure times from 1 to 10³ s, such a detector may serve as an ideal “gamma-ray timing explorer” for the study of transient non-thermal phenomena like γ-radiation from AGN jets, synchrotron flares of microquasars, the high energy (GeV) counterparts of gamma ray bursts, etc. 5@5 also would allow detailed γ-ray spectroscopy of persistent nonthermal sources like pulsars, supernova remnants, plerions, radiogalaxies, and others, with unprecedented for γ-ray astronomy photon statistics. The existing technological achievements in the design and construction of multi(1000)-pixel, high resolution imagers, as well as of large, 20 m diameter class multi-mirror dishes with rather modest optical requirements, would allow the construction of such a detector in the foreseeable future, although in the longer terms from the point of view of ongoing projects of 100 GeV threshold IACT arrays like HESS which is in the build-up phase. An ideal site for such an instrument could be a high-altitude, 5 km a.s.l. or more, flat area with a linear scale of about 100 m in a very arid mountain region in the Atacama desert of Northern Chile.     

Brief history of ground-based very high energy gamma-ray astrophysics with atmospheric air Cherenkov telescopes

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 9σ 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.     

The major upgrade of the MAGIC telescopes,Part II: A performance study using observations of the Crab Nebula

MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes located in the Canary island of La Palma, Spain. During summer 2011 and 2012 it underwent a series of upgrades, involving the exchange of the MAGIC-I camera and its trigger system, as well as the upgrade of the readout system of both telescopes. We use observations of the Crab Nebula taken at low and medium zenith angles to assess the key performance parameters of the MAGIC stereo system. For low zenith angle observations, the standard trigger threshold of the MAGIC telescopes is ∼ 50  GeV. The integral sensitivity for point-like sources with Crab Nebula-like spectrum above 220 GeV is (0.66 ± 0.03)% of Crab Nebula flux in 50 h of observations. The angular resolution, defined as the σ of a 2-dimensional Gaussian distribution, at those energies is ≲ 0.07°, while the energy resolution is 16%. We also re-evaluate the effect of the systematic uncertainty on the data taken with the MAGIC telescopes after the upgrade. We estimate that the systematic uncertainties can be divided in the following components: < 15% in energy scale, 11%–18% in flux normalization and ± 0.15 for the energy spectrum power-law slope.     

Impact of aerosols and adverse atmospheric conditions on the data quality for spectral analysis of the H.E.S.S. telescopes

The Earth’s atmosphere is an integral part of the detector in ground-based imaging atmospheric Cherenkov telescope (IACT) experiments and has to be taken into account in the calibration. Atmospheric and hardware-related deviations from simulated conditions can result in the mis-reconstruction of primary particle energies and therefore of source spectra. During the eight years of observations with the High Energy Stereoscopic System (H.E.S.S.) in Namibia, the overall yield in Cherenkov photons has varied strongly with time due to gradual hardware aging, together with adjustments of the hardware components, and natural, as well as anthropogenic, variations of the atmospheric transparency. Here we present robust data selection criteria that minimize these effects over the full data set of the H.E.S.S. experiment and introduce the Cherenkov transparency coefficient as a new atmospheric monitoring quantity. The influence of atmospheric transparency, as quantified by this coefficient, on energy reconstruction and spectral parameters is examined and its correlation with the aerosol optical depth (AOD) of independent MISR satellite measurements and local measurements of atmospheric clarity is investigated.     

A Monte Carlo template based analysis for air-Cherenkov arrays

We present a high-performance event reconstruction algorithm: an Image Pixel-wise fit for Atmospheric Cherenkov Telescopes (ImPACT). The reconstruction algorithm is based around the likelihood fitting of camera pixel amplitudes to an expected image template. A maximum likelihood fit is performed to find the best-fit shower parameters. A related reconstruction algorithm has already been shown to provide significant improvements over traditional reconstruction for both the CAT and H.E.S.S. experiments. We demonstrate a significant improvement to the template generation step of the procedure, by the use of a full Monte Carlo air shower simulation in combination with a ray-tracing optics simulation to more accurately model the expected camera images. This reconstruction step is combined with an MVA-based background rejection.Examples are shown of the performance of the ImPACT analysis on both simulated and measured (from a strong VHE source) gamma-ray data from the H.E.S.S. array, demonstrating an improvement in sensitivity of more than a factor two in observation time over traditional image moments-fitting methods, with comparable performance to previous likelihood fitting analyses. ImPACT is a particularly promising approach for future large arrays such as the Cherenkov Telescope Array (CTA) due to its improved high-energy performance and suitability for arrays of mixed telescope types.     

Gamma–hadron separation methods for the VERITAS array of four imaging atmospheric Cherenkov telescopes

Ground-based arrays of imaging atmospheric Cherenkov telescopes have emerged as the most sensitive γ-ray detectors in the energy range of about 100 GeV and above. The strengths of these arrays are a very large effective collection area on the order of 10⁵ m², combined with excellent single photon angular and energy resolutions. The sensitivity of such detectors is limited by statistical fluctuations in the number of Cosmic-ray initiated air showers that resemble γ-ray air showers in many ways. In this paper, we study the performance of simple event reconstruction methods when applied to simulated data of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) experiment. We review methods for reconstructing the arrival direction and the energy of the primary photons, and examine means to improve on their performance. For a software threshold energy of 300 GeV (100 GeV), the methods achieve point source angular and energy resolutions of σ_63% = 0.1° (0.2°) and σ_68% = 15% (22%), respectively. The main emphasis of the paper is the discussion of γ–hadron separation methods for the VERITAS experiment. We find that the information from several methods can be combined based on a likelihood ratio approach and the resulting algorithm achieves a γ–hadron suppression with a quality factor that is substantially higher than that achieved with the standard methods used so far.     

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.     

Searching for tau neutrinos with Cherenkov telescopes

Cherenkov telescopes have the capability of detecting high energy tau neutrinos in the energy range of 1–1000 PeV by searching for very inclined showers. If a tau lepton, produced by a tau neutrino, escapes from the Earth or a mountain, it will decay and initiate a shower in the air which can be detected by an air shower fluorescence or Cherenkov telescope. In this paper, we present detailed Monte Carlo simulations of corresponding event rates for the VERITAS and two proposed Cherenkov Telescope Array sites: Meteor Crater and Yavapai Ranch, which use representative AGN neutrino flux models and take into account topographic conditions of the detector sites. The calculated neutrino sensitivities depend on the observation time and the shape of the energy spectrum, but in some cases are comparable or even better than corresponding neutrino sensitivities of the IceCube detector. For VERITAS and the considered Cherenkov Telescope Array sites the expected neutrino sensitivities are up to factor 3 higher than for the MAGIC site because of the presence of surrounding mountains.     

The optical reflector system for the CANGAROO-II imaging atmospheric Cherenkov telescope

A new imaging atmospheric Cherenkov telescope with a light-weight reflector has been constructed. Light, robust, and durable mirror facets of containing carbon fiber reinforced plastic laminates were developed for the telescope. The reflector has a parabolic shape (f/1.1) with a 30 m² surface area, which consists of 60 spherical mirror facets. The image size of each mirror facet is 0°.08 (FWHM) on average. The attitude of each facet can be adjusted by stepping motors. After the first in situ adjustment, a point image of about 0°.14 (FWHM) over 3° field of view was obtained. The effect of gravitational load on the optical system was confirmed to be negligible at the focal plane. The telescope has been in operation with an energy threshold for γ-rays of 300 GeV since May 1999.     

A multivariate analysis approach for the imaging atmospheric Cherenkov telescopes system H.E.S.S.

A multivariate particle classification approach is proposed and applied to the analysis of the data from H.E.S.S. (High Energy Stereoscopic System). The combination of results from the three shower reconstruction methods: Hillas, model and 3D-model, leads to a substantial gain in the discrimination power between photons and hadrons. The construction and use of a combined effective estimator improves by several factors the signal-to-background ratio which is extremely important in case of studies of the faint and extended sources. The results of this approach are presented for a typical set of sources. The consequent gain in the sensitivity is shown through a comparison to the H.E.S.S. published results.     

Instrumentation for very high energy gamma-ray astronomy

Considerable progress has been made in the last half-decade in the field of very high energy (VHE) gamma-ray astronomy (photons with energies between 10¹¹ and 10¹³ eV). The high background level due to the isotropic cosmic ray flux which has bedevilled the field since its inception in the early 1960's can now be reduced to such a degree that significant gamma-ray signals from several sources become visible within a few hours of observation. The instrumentation and methodologies which have made this possible are reviewed. A brief historical introduction is followed by a summary of the salient properties of the atmospheric Cherenkov flash associated with VHE gamma-ray events. The major components of a VHE gamma-ray astronomy telescope are then reviewed. This is followed by a discussion of the different methodologies currently being used to discriminate against the cosmic ray background. Properties of several specific installations are then summarized, and possible future developments in VHE instrumentation are briefly discussed.     

On the optimum spacing of stereoscopic imaging atmospheric Cherenkov telescopes

For stereoscopic systems of imaging atmospheric Cherenkov telescopes a key parameter to optimize the sensitivity for VHE γ-ray point sources is the intertelescope spacing. Using pairs of telescopes of the HEGRA IACT system, the sensitivity of two-telescope stereo IACT systems is studied as a function of the telescope spacing, ranging from 70 to 140 m. Data taken during the 1997 outburst of Mrk 501 are used to evaluate both the detection rates before cuts, and the sensitivity for weak signals after cuts to optimize the significance of signals. Detection rates decrease by about 1/3 between the minimum and maximum spacings. The significance of signals is essentially independent of distance in the range investigated.     

Performance of the MAGIC stereo system obtained with Crab Nebula data

MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes located in the Canary island of La Palma. Since autumn 2009 both telescopes have been working together in stereoscopic mode, providing a significant improvement with respect to the previous single-telescope observations. We use observations of the Crab Nebula taken at low zenith angles to assess the performance of the MAGIC stereo system. The trigger threshold of the MAGIC telescopes is 50 − 60 GeV. Advanced stereo analysis techniques allow MAGIC to achieve a sensitivity as good as (0.76 ± 0.03)% of the Crab Nebula flux in 50 h of observations above 290 GeV. The angular resolution at those energies is better than ∼0.07°. We also perform a detailed study of possible systematic effects which may influence the analysis of the data taken with the MAGIC telescopes.     

Upgrading and testing the 3D reconstruction of gamma-ray air showers as observed with an array of Imaging Atmospheric Cherenkov Telescopes

Stereoscopic arrays of Imaging Atmospheric Cherenkov Telescopes allow to reconstruct gamma-ray-induced showers in three dimensions, which offers several advantages: direct access to the shower parameters in space and straightforward calorimetric measurement of the incident energy. In addition, correlations between the different images of the same shower are taken into account. An analysis method based on a simple 3D-model of electromagnetic showers was recently implemented in the framework of the H.E.S.S. experiment. In the present article, the method is completed by an additional quality criterion, which reduces the background contamination by a factor of about 2 in the case of extended sources, while keeping gamma-ray efficiency at a high level. On the other hand, the dramatic flares of the blazar PKS 2155-304 in July 2006, which provided H.E.S.S. data with an almost pure gamma-ray sample, offered the unique opportunity of a precision test of the 3D-reconstruction method as well as of the H.E.S.S. simulations used in its calibration. An agreement at a few percent level is found between data and simulations for the distributions of all 3D shower parameters.     

The Optical Layout Of The HEGRA Cherenkov Telescopes

The raytracing technique was used to derive a suitable design for the HEGRA system of Cherenkov telescopes, which is at present commissioned at La Palma. The reflectors with a diameter of 3.9 m consist of 30 spherical mirrors with focal lengths in the range of 4.88 – 4.94 m. It is shown that 93% of the photons from the Cherenkov light emitted by an extended air shower are contained in the camera pixels, 0.25° in diameter, for the full field of view of = ± 2.5°. The optical performance of the HEGRA design is compared to other layouts.     

The system of imaging atmospheric Cherenkov telescopes: The new prospects for VHE gamma ray astronomy

Using Monte Carlo simulations the possibilities are investigated for registration of VHE gamma radiation by means of systems of imaging air Cherenkov telescopes (IACT). It is shown that even a system of IACT's with moderate properties (three telescopes with the geometrical area of the optical reflector5 m ²and the angular size of the pixel0.41 ⁰ ) could provide the energy resolution 20–25% and achieve the sensitivity (minimum detectable flux) up to 10^-12 photon/cm ²s at the effective energy threshold 1 TeV.     

Multi-component study of extended sources with a likelihood method

The spectral and morphological analysis for gamma-ray sources with multiple emission components remains a major challenge for Cherenkov telescopes due to background emission from diffuse gamma rays. Current methods of background suppression, usually based on the bin-by-bin subtraction of OFF-source data do not allow an analysis of the various background components. As an alternative, we present an approach based on an event-by-event likelihood fit of ON-source data, using a combined spectral model for the source emission as well as for the gamma-like background obtained from fits of the OFF-source data. Multiple emission components are separated by successive fits in different energy regimes and spectral variations inside the extended source is derived. The performance of this approach is evaluated with toy Monte-Carlo studies. For the application to real data, two well-studied H.E.S.S. sources are re-examined: the extragalactic point-source PKS 2155-304 and the extended pulsar wind nebula HESS J1825-137. For the latter, radial variation of the emission spectral index was evaluated, confirming earlier findings by the H.E.S.S. collaboration.