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IACT stands for Imaging Atmospheric (or Air) Cherenkov Telescope or Technique. It is a device or method to detect
very-high-energy gamma ray Very-high-energy gamma ray (VHEGR) denotes gamma radiation with photon energies of 100 GeV (gigaelectronvolt) to 100 TeV (teraelectronvolt), i.e., 1011 to 1014 electronvolts. This is approximately equal to wavelengths between 10−17 and 10−20 ...
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
s in the
photon energy Photon energy is the energy carried by a single photon. The amount of energy is directly proportional to the photon's electromagnetic frequency and thus, equivalently, is inversely proportional to the wavelength. The higher the photon's frequency, ...
range of 50 
GeV GEV may refer to: * ''G.E.V.'' (board game), a tabletop game by Steve Jackson Games * Ashe County Airport, in North Carolina, United States * Gällivare Lapland Airport, in Sweden * Generalized extreme value distribution * Gev Sella, Israeli-Sou ...
to 50 
TeV TEV may refer to: * Transient Earth Voltage: a term for voltages appearing on the metal work of switchgear due to internal partial discharges * TeV, or teraelectronvolt or trillion electron volt, a measure of energy * Total Enterprise Value, a ...
. There are four operating IACT systems:
High Energy Stereoscopic System High Energy Stereoscopic System (H.E.S.S.) is a system of imaging atmospheric Cherenkov telescopes (IACTs) for the investigation of cosmic gamma rays in the photon energy range of 0.03 to 100 TeV. The acronym was chosen in honour of Victor ...
(H.E.S.S.), Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC), First G-APD Cherenkov Telescope (FACT), and Very Energetic Radiation Imaging Telescope Array System (VERITAS). The
Major Atmospheric Cerenkov Experiment Telescope Major Atmospheric Cerenkov Experiment Telescope (MACE) is an Imaging Atmospheric Cerenkov telescope (IACT) located near Hanle (village), Hanle, Ladakh, India. It is the highest (in altitude) and second largest Cerenkov telescope in the world. It ...
(MACE) is under construction in Hanle, Ladakh, India and is set to be the highest and second-largest IACT. The
Cherenkov Telescope Array The Cherenkov Telescope Array or CTA is a multinational, worldwide project to build a new generation of ground-based gamma-ray instrument in the energy range extending from some tens of GeV to about 300 TeV. It is proposed as an open observatory a ...
(CTA) is a multinational project to build next-generation IACTs and is scheduled to begin data collection in 2022.


Background

Due to the rapidly falling flux of gamma-ray photons from cosmic sources in this energy regime, space-based detectors become ineffective due to their small collection areas which are often limited to some tens or hundreds of square centimeters. In the case of the IACT, the Earth's atmosphere is used as the detection medium, implying a collection area of many hundreds of square meters. This enables IACT instruments to detect gamma-ray photons in an energy regime inaccessible to space-based instruments.


Technique

The IACT works by imaging the very short flash of Cherenkov radiation generated by the cascade of relativistic
charged particle In physics, a charged particle is a particle with an electric charge. It may be an ion, such as a molecule or atom with a surplus or deficit of electrons relative to protons. It can also be an electron or a proton, or another elementary particle, ...
s produced when a very-high-energy gamma ray strikes the atmosphere. This shower of charged particles, known as an
Extensive Air Shower An air shower is an extensive (many kilometres wide) cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a ''primary'' cosmic ray (i.e. one of extraterrestrial origin) enters the atmosphere. When a partic ...
(EAS), is initiated at an altitude of 10–20 km. The incoming gamma-ray photon undergoes
pair production Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. Pair production often refers specific ...
in the vicinity of the
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucle ...
of an
atmospheric An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
. The
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...
-
positron The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...
pair produced are of extremely high energy and immediately undergo
Bremsstrahlung ''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
or "Braking Radiation". This radiation produced is itself extremely energetic, with many of the photons undergoing further pair production. A cascade of charged particles ensues which, due to its extreme energy, produces a flash of Cherenkov radiation lasting between 5 and 20  ns. The total area on the ground illuminated by this flash corresponds to many hundreds of square meters, which is why the effective area of IACT telescopes is so large. The instrument used to detect the Cherenkov radiation usually comprises a large segmented mirror which reflects the Cherenkov light onto an array of
photomultiplier A photomultiplier is a device that converts incident photons into an electrical signal. Kinds of photomultiplier include: * Photomultiplier tube, a vacuum tube converting incident photons into an electric signal. Photomultiplier tubes (PMTs for sho ...
tubes. The tubes are coupled to fast electronics which amplify, digitise and record the pattern or image of the shower. The most effective mode of operation is to use an array of such telescopes, which can be typically located 70 to 120 meters apart. The primary advantage of this mode of operation is that the energy threshold (the peak sensitivity) of the telescope can be lowered as local
muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...
s produced by
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
induced showers can be eliminated. This is because the narrow Cherenkov light cone produced by local muons will only be recorded by a single telescope. The shower reconstruction and background rejection offered by an array of telescopes provides an
order of magnitude An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic dis ...
increase in sensitivity and improved angular and energy resolution as compared to a single telescope. This advantage has been used to great effect by the H.E.S.S. telescope array which has detected several new sources of very high energy gamma-ray photons in recent years.


History

The IACT was pioneered by the Whipple collaboration and led to the discovery of TeV emission from the
Crab Nebula The Crab Nebula (catalogue designations Messier object, M1, New General Catalogue, NGC 1952, Taurus (constellation), Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus (constellation), Taurus. The common name ...
in 1989. The Whipple 10m telescope also discovered the first extra-galactic source of TeV emission with the detection of very-high-energy gamma-ray emission from the
active galaxy An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not prod ...
Markarian 421 Markarian 421 (Mrk 421, Mkn 421) is a blazar located in the constellation Ursa Major. The object is an active galaxy and a BL Lacertae object, and is a strong source of gamma rays. It is about 397 million light-years (redshift: z=0.0308 eq. 122M ...
. The
HEGRA HEGRA, which stands for ''High-Energy-Gamma-Ray Astronomy'', was an atmospheric Cherenkov telescope for Gamma-ray astronomy. With its various types of detectors, HEGRA took data between 1987 and 2002, at which point it was dismantled in order ...
telescope array was the first system to use multiple telescopes, a technique known as
stereoscopy Stereoscopy (also called stereoscopics, or stereo imaging) is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. The word ''stereoscopy'' derives . Any stereoscopic image is ...
. The MAGIC system at the
Roque de los Muchachos Observatory Roque de los Muchachos Observatory ( es, Observatorio del Roque de los Muchachos, ORM) is an astronomical observatory located in the municipality of Garafía on the island of La Palma in the Canary Islands, Spain. The observatory site is operated ...
in
La Palma La Palma (, ), also known as ''La isla bonita'' () and officially San Miguel de La Palma, is the most north-westerly island of the Canary Islands, Spain. La Palma has an area of making it the fifth largest of the eight main Canary Islands. The ...
began with one 17 m telescope in 2004, to which a second 17 m telescope was added in 2009. The First G-APD Cherenkov Telescope (FACT) at the Roque de los Muchachos Observatory became operational in 2011.FACT: A novel camera for Cherenkov telescopes for ground-based gamma-ray astronomy
/ref> The H.E.S.S. collaboration has added to their system in
Namibia Namibia (, ), officially the Republic of Namibia, is a country in Southern Africa. Its western border is the Atlantic Ocean. It shares land borders with Zambia and Angola to the north, Botswana to the east and South Africa to the south and ea ...
a new 28 m IACT, currently the largest IACT telescope, which has been operational since July 2012.


See also

*
List of telescope types The following are lists of devices categorized as types of telescopes or devices associated with telescopes. They are broken into major classifications with many variations due to professional, amateur, and commercial sub-types. Telescopes can be ...
*
Pavel Cherenkov Pavel Alekseyevich Cherenkov (russian: Па́вел Алексе́евич Черенко́в ; July 28, 1904 – January 6, 1990) was a Soviet physicist who shared the Nobel Prize in physics in 1958 with Ilya Frank and Igor Tamm for the discove ...


References


External links


Aspera European network portalAstroparticle physics news on TwitterOfficial web page of CTA
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