Atmospheric Cherenkov Imaging Technique Research Articles

Prospective study on observations of gamma-ray emission from active galactic nuclei using the HADAR experiment

The High Altitude Detection of Astronomical Radiation (HADAR) experiment is a refracting terrestrial telescope array based on the atmospheric Cherenkov imaging technique. It is a hybrid array consisting of four water-lens telescopes and a surrounding scintillation detector array for observing Cherenkov light induced by 10 GeV–10 TeV cosmic rays and gamma rays in the atmosphere. The water-lens telescope mainly consists of a hemispherical lens with a diameter of 5 m acting as a Cherenkov light collector, a cylindrical metal tank with a 4 m radius and 7 m height, and an imaging system at the bottom of the tank. The sky region covered by HADAR is much larger than the current generation of Imaging Atmospheric Cherenkov Telescopes, and even the CTA. The field-of-view (FOV) of HADAR can reach up to 60 degrees. The HADAR experiment possesses the advantages of a large field-of-view and low energy threshold, so it can continuously scan wide portions of the sky and easily observe extragalactic gamma-ray sources. The majority of the extragalactic gamma-ray sources detected at very high energy (VHE) energies are active galactic nuclei (AGNs). In this study, we present the potential of using the HADAR experiment for detecting AGN. Based on the AGN catalog sources of the Fermi Large Area Telescope (Fermi-LAT), the observed energy is extrapolated to the VHE range. The VHE gamma rays propagating over cosmological distances can interact with the low-energy of the extragalactic background light (EBL) and produce electron-positron pairs. Therefore, we consider the absorption effects of different EBL models when calculating the expected gamma ray spectra of the AGN sample. We select the sample with redshift measurements and locations inside the FOV of HADAR from 4LAC catalog. In total, there are 375 BL Lacertae objects (BL Lacs) and 289 flat-spectrum radio quasars (FSRQs) satisfying the selection conditions. The integral gamma ray spectra are derived and compared with the sensitivity curve of HADAR, the number of sources with fluxes above the sensitivity of HADAR is counted. Further, we calculate the statistical significance of HADAR for AGN source observation based on the equi-zenith angle sky scanning analysis method. The simulation results reveal that a total of 31 sources of Fermi-LAT AGN can be detected by HADAR with a significance greater than five standard deviations over a one-year survey period, most of which are BL Lacs.

Volcanoes muon imaging using Cherenkov telescopes

A detailed understanding of a volcano inner structure is one of the key-points for the volcanic hazards evaluation. To this aim, in the last decade, geophysical radiography techniques using cosmic muon particles have been proposed. By measuring the differential attenuation of the muon flux as a function of the amount of rock crossed along different directions, it is possible to determine the density distribution of the interior of a volcano. Up to now, a number of experiments have been based on the detection of the muon tracks crossing hodoscopes, made up of scintillators or nuclear emulsion planes.Using telescopes based on the atmospheric Cherenkov imaging technique, we propose a new approach to study the interior of volcanoes detecting of the Cherenkov light produced by relativistic cosmic-ray muons that survive after crossing the volcano. The Cherenkov light produced along the muon path is imaged as a typical annular pattern containing all the essential information to reconstruct particle direction and energy. Our new approach offers the advantage of a negligible background and an improved spatial resolution.To test the feasibility of our new method, we have carried out simulations with a toy-model based on the geometrical parameters of ASTRI SST-2M, i.e. the imaging atmospheric Cherenkov telescope currently under installation onto the Etna volcano. Comparing the results of our simulations with previous experiments based on particle detectors, we gain at least a factor of 10 in sensitivity. The result of this study shows that we resolve an empty cylinder with a radius of about 100m located inside a volcano in less than 4 days, which implies a limit on the magma velocity of 5m/h.

High-energy astrophysics

The development of the atmospheric Cherenkov imaging technique has led to significant advances in gamma-ray detection sensitivity in the energy range from 200 GeV to 50 TeV. The Whipple Observatory 10m reflector has detected the first galactic and extragalactic sources in the Northern Hemisphere; the Crab Nebula has been established as the standard candle for ground-based gamma-ray astronomy. The highly variable Active Galactic Nuclei, Markarian 421 and Markarian 501, have proved to be particularly interesting. A new generation of telescopes with improved sensitivity has the promise of interesting measurements of fundamental phenomena in physics and astrophysics. VERITAS (the Very Energetic Radiation Imaging Telescope Array System) is one such next generation system; it is an array of seven large atmospheric Cherenkov telescopes planned for a site in southern Arizona.

VERITAS: the Very Energetic Radiation Imaging Telescope Array System

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) represents an important step forward in the study of extreme astrophysical processes in the universe. It combines the power of the atmospheric Cherenkov imaging technique using a large optical reflector with the power of stereoscopic observatories using arrays of separated telescopes looking at the same shower. The seven identical telescopes in VERITAS, each of aperture 10 m, will be deployed in a filled hexagonal pattern of side 80 m; each telescope will have a camera consisting of 499 pixels with a field of view of 3.5°. VERITAS will substantially increase the catalog of very high energy ( E>100 GeV) γ-ray sources and greatly improve measurements of established sources.

Particle Astrophysics with High Energy Photons

The development of the atmospheric Cherenkov imaging technique has led to significant advances in γ-ray detection sensitivity in the energy range from 200 GeV to 50 TeV. The Whipple Observatory 10 m reflector has detected the first galactic and extragalactic sources in the Northern Hemisphere; the Crab Nebula has been established as the standard candle for ground-based γ-ray astronomy. The highly variable Active Galactic Nuclei, Markarian 421 and Markarian 501, have proved to be particularly interesting. A new generation of telescopes with improved sensitivity has the promise of interesting measurements of fundamental phenomena in physics and astrophysics. VERITAS (the Very Energetic Radiation Imaging Telescope Array System) is one such next generation system; it is an array of seven large atmospheric Cherenkov telescopes planned for a site in southern Arizona.

Application of a Kohonen network classifier in TeV -ray astronomy

A Kohonen type unsupervised artificial neural network has been used to increase the sensitivity of the atmospheric Cherenkov imaging technique used in ground-based TeV -ray astronomy. The network classifies Cherenkov events as -induced or hadron-induced on the basis of their spatial frequency components. When used in conjunction with the established Supercuts classifier it increases the sensitivity of the technique by .

Measurement of TeV gamma-ray spectra with the Cherenkov imaging technique

In this paper, we seek to establish reliable methods for extracting energy spectra for TeV gamma-ray sources observed using the atmospheric Cherenkov Imaging Technique. Careful attention has been paid to the calculation of the telescope gain, and we obtain good agreement between direct measurements, with a statistical error of about 10%, and an absolute calibration from the background cosmic-ray trigger rate that has an overall error of 18%. Two independent analyses that are based on different Monte Carlo shower simulations, employ different selection criteria in order to retain a large fraction of gamma-ray events, and use different approaches to spectral estimation are presented here. The first is a fairly traditional method that builds on established image selection techniques and calculates the detector collection area and an energy estimation function. The error in measuring the enrgy of a single event is estimated at 36%, and we try to compensate for this poor energy resolution. The second analysis uses more elegant gamma-ray selection criteria and implicity incorporates the properties of the detector into the simulations that are then compared with the data in order to obtain source spectra. The two simulations are compared to each other and to the data, with the aim of establishing that each method is robust and insensitive to simulation details. Finally, we consider the main sources of systematic errors, the largest of which is in the telescope gain calibration, arising from an incomplete knowledge of the relevant factors, and is estimated to be 16%. The effect of possible errors in the simulations is also considered. Both methods have been applied to a part of the Whipple observatory database on the Crab Nebula for the 1988/89 observing season, while the first method has also been applied to data taken in 1995/96. The statistical error in the flux constant is about 8% and that in the spectral index is about 5%, while the corresponding systematic errors are estimated to be 18% and 2%, respectively. The results presented here show good agreement between the two methods as well as between the two seasons. However, a comprehensive consideration of the implications of the derived spectra and a comparison to other work is addressed in another paper.

A Search for TeV Counterparts to BATSE Gamma‐Ray Bursts

Intense effort has gone into the observation of optical, radio, and X-ray gamma-ray burst (GRB) counterparts, either simultaneous to the burst or as quasi-steady lingering remnants. Here we report on a similar study at higher energies of 250 GeV and above using ground-based telescopes. The recent technical advances represented by the atmospheric Cherenkov imaging technique (Cawley & Weekes 1995) have opened up the field of gamma-ray astronomy above 250 GeV and raised the possibility that these techniques can be used with excellent fluence sensitivity in exploring the GRB phenomenon. Observations by the Whipple collaboration of nine BATSE positions, one acquired within 2 minutes of the reported BATSE burst time, using coordinates distributed through the BATSE Coordinates Distribution Network (BACODINE) are reported. No evidence of TeV emission is found, and upper limits to the high-energy delayed or extended emission of observed candidates are calculated.

VHE gamma-ray sources and implications

This paper highlights the status of very high-energy (VHE) gamma-ray astronomy with emphasis on the astrophysical significance of the discovery of TeV γ-radiation from four celestial objects, Crab Nebula, PSR B1706-44, Mrk 421 and Mrk 501, detected at very high confidence levels using the atmospheric Cherenkov imaging technique.

Development of the atmospheric Cherenkov imaging technique

Following a brief history of the atmospheric Cherenkov imaging technique and a summary of the current status of the field, we describe some of the potential developments of the method over the next few years. Due to constraints of space, we shall confine the discussion to single stand-alone telescopes—stereo imaging will not be discussed.

A search for a geomagnetic effect on the sensitivity of the atmospheric Cherenkov imaging technique

We have searched the Whipple 1988-89 canonical Crab Nebula database for evidence of a possible geomagnetic field effect on the sensitivity of the atmospheric Cherenkov imaging technique. At the present sensitivity of the imaging technique, the selection of gamma-ray-like events on the basis of image characteristics appears to be unaffected by the transverse field strength. We are unable to detect any broadening of shower images or any increase in integrated light content.