IACT
Encyclopedia
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s in the 50 GeV
GEV
GEV or GeV may stand for:*GeV or gigaelectronvolt, a unit of energy equal to billion electron volts*GEV or Grid Enabled Vehicle that is fully or partially powered by the electric grid, see plug-in electric vehicle...
to 50 TeV
TEV
TEV may refer to:* TeV, or teraelectronvolt, a measure of energy* Total Enterprise Value, a financial measure* Total Economic Value, an economic measure* Tobacco etch virus, a plant pathogenic virus of the family Potyviridae....
range can be detected by ground based telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
s. There are currently four major ground based telescopes including CANGAROO III, MAGIC
MAGIC (telescope)
MAGIC is a system of two Imaging Atmospheric Cherenkov telescopes situated at the Roque de los Muchachos Observatory on La Palma, one of the Canary Islands, at about 2200 m above sea level...
, HESS
Hess
Hess or Heß may refer to:* People with the surname Hess * Hess Educational Organization, the largest private provider of English instruction in the Republic of China...
and VERITAS
VERITAS
VERITAS is a major ground-based gamma-ray observatory with an array of four 12m optical reflectors for gamma-ray astronomy in the GeV - TeV energy range. The telescope design is based on the design of the existing 10m gamma-ray telescope of the Fred Lawrence Whipple Observatory...
. Currently under design are the Cherenkov Telescope Array
Cherenkov Telescope Array
The Cherenkov Telescope Array or CTA is a project to build a new generation ground-based gamma-ray instrument in the energy range extending from some tens of GeV to above 100 TeV...
(CTA), and the Advanced Gamma-ray Imaging System (AGIS).
Due to the rapidly falling flux of gamma-ray photons from cosmic
COSMIC
Constellation Observing System for Meteorology, Ionosphere, and Climate is a program designed to provide advances in meteorology, ionospheric research, climatology, and space weather by using GPS satellites in conjunction with low Earth orbiting satellites...
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.
The IACT works by imaging the very brief flash of Cherenkov radiation generated by the cascade of relativistic charged particle
Charged particle
In physics, a charged particle is a particle with an electric charge. It may be either a subatomic particle or an ion. A collection of charged particles, or even a gas containing a proportion of charged particles, is called a plasma, which is called the fourth state of matter because its...
s produced when a very high-energy gamma ray strikes the atmosphere. This shower of charged particles, known as an Extensive Air Shower
Air shower (physics)
An air shower is an extensive cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a primary cosmic ray enters the atmosphere...
(EAS) is initiated at an altitude of 10–20 km. The incoming gamma-ray photon undergoes pair production
Pair production
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...
in the vicinity of the nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
of an atmospheric
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
. The electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
-positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
pair produced are of extremely high energy and immediately undergo Bremsstrahlung
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
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
1 E-9 s
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is 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 brief flash of Cherenkov radiation comprises a large segmented mirror which reflects the Cherenkov light onto an array of photomultiplier
Photomultiplier
Photomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
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
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
s produced by cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
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
Order of magnitude
An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...
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 HESS telescope array which has detected several new sources of very high energy gamma-ray photons in recent years.
The IACT was pioneered by the Whipple collaboration
Fred Lawrence Whipple Observatory
The Fred Lawrence Whipple Observatory is an astronomical observatory owned and operated by the Smithsonian Astrophysical Observatory and is their largest field installation outside of their main site in Cambridge, MA...
and led to the discovery of TeV emission from the Crab Nebula
Crab Nebula
The Crab Nebula is a supernova remnant and pulsar wind nebula in the constellation of Taurus...
in 1989. The Whipple 10m
Fred Lawrence Whipple Observatory
The Fred Lawrence Whipple Observatory is an astronomical observatory owned and operated by the Smithsonian Astrophysical Observatory and is their largest field installation outside of their main site in Cambridge, MA...
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
Active galactic nucleus
An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...
Markarian 421
Markarian 421
Markarian 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 to 434 million light-years from the Earth...
. The HEGRA
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 to build its successor, MAGIC, at the same site.It was...
telescope array was the first system to use multiple telescopes, a technique known as stereoscopy
Stereoscopy
Stereoscopy refers to a technique for creating or enhancing the illusion of depth in an image by presenting two offset images separately to the left and right eye of the viewer. Both of these 2-D offset images are then combined in the brain to give the perception of 3-D depth...
. The largest IACT telescope is the 17 m MAGIC telescope at the Roque de los Muchachos Observatory
Roque de los Muchachos Observatory
Roque de los Muchachos Observatory is an astronomical observatory located in the municipality of Garafía on the island of La Palma in the Canary Islands...
in La Palma
La Palma
La Palma is the most north-westerly of the Canary Islands. La Palma has an area of 706 km2 making it the fifth largest of the seven main Canary Islands...
, to which second 17m telescope was added in 2009. However, , the HESS collaboration are currently planning construction of a 30 m dish in Namibia
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
.