Ultra-high-energy cosmic ray
Encyclopedia
In astroparticle physics
, an ultra-high-energy cosmic ray (UHECR) or extreme-energy cosmic ray (EECR) is a cosmic ray
with an extreme kinetic energy
, far beyond both its rest mass and energies typical of other cosmic rays.
These particles are significant for astrophysics and fundamental physics theory, because they have energies comparable to the Greisen–Zatsepin–Kuzmin limit
, which occurs at about 5 electron volts (8 J
). This limit should be the maximum energy of cosmic rays that have traveled long distances (about 160 million light years), due to the theoretical energy losses of higher-energy rays and to scattering from photons in the cosmic microwave background.
in New Mexico in 1962.
Cosmic rays with even higher energies have since been observed. Among them was the Oh-My-God particle observed on the evening of 15 October 1991 over Dugway Proving Ground
, Utah. Its observation was a shock to astrophysicists
, who estimated its energy to be approximately (50 J)—in other words, a subatomic particle
with kinetic energy equal to that of a baseball
(5 ounces or 142 g) traveling at about 100 kilometres per hour (62.1 mph).
It was most probably a proton
traveling at about times the speed of light
, so close that in a year-long race between light and the cosmic ray, the ray would fall behind only 46 nanometers ( light-years), or 0.15 femtoseconds .
The energy of this particle is some 40 million times that of the highest energy protons that can currently be produced in any terrestrial particle accelerator
. However only a small fraction of this energy would be available for an interaction with a proton or neutron on Earth, with most of the energy remaining in the form of kinetic energy of the products of the interaction. The effective energy available for such a collision is the square root of double the product of the particle's energy and the mass energy of the proton, which for this particle gives , roughly 50 times the collision energy of the Large Hadron Collider
.
Since the first observation, by the University of Utah
's Fly's Eye Cosmic Ray Detector
, at least fifteen similar events have been recorded, confirming the phenomenon. These very high energy cosmic rays are very rare; the energy of most cosmic rays is between 10 MeV and 10 GeV.
cosmic microwave background radiation
limit the distance that these particles can travel before losing energy; this is known as the Greisen–Zatsepin–Kuzmin limit
or GZK limit.
The source of such high energy particles has been a mystery for many years. Recent results from the Pierre Auger Observatory show that ultra-high-energy cosmic ray arrival directions appear to be correlated with extragalactic supermassive black holes at the center of nearby galaxies called active galactic nuclei (AGN)
. However, since the angular correlation scale used is fairly large (3.1 degrees) these results do not unambiguously identify the origins of such cosmic rays. The AGN could merely be closely associated with the actual sources, for example in galaxies or other astrophysical objects that are clumped with matter on large scales within 100 Mpc.
Additional data collection is important for further investigating possible AGN sources for these highest energy particles.
Some of the supermassive black holes in AGN are known to be rotating, as in the Seyfert galaxy
MCG 6-30-15 with time-variability in their inner accretion disks. Black hole spin is a potentially effective agent to drive UHECR production, provided ions are suitably launched to circumvent limiting factors deep within the nucleus, notably curvature radiation and inelastic scattering with radiation from the inner disk. Low-luminosity, intermittent Seyfert galaxies may meet the requirements with the formation of a linear accelerator several light years away from the nucleus, yet within their extended ion tori whose UV radiation ensures a supply of ionic contaminants. The corresponding electric fields are small, on the order of 10 V/cm, whereby the observed UHECRs are indicative for the astronomical size of the source. Improved statistics by the Pierre Auger Observatory will be instrumental in identifying the presently tentative association of UHECRs (from the Local Universe) with Seyferts and LINER
s.
Near an active galactic nucleus, one of these particles can fall into the black hole, while the other escapes, as described by the Penrose process
. Some of the particles that escape will collide with incoming particles creating collisions of very high energy. It is in these collisions, according to Pavlov, that ordinary visible protons can form. These protons would have very high energies. Pavlov claims that evidence of this is present in the form of ultra-high-energy cosmic rays.
Ultra-high energy cosmic rays may also be produced by the decay of super-heavy dark matter "X particles" such as Holeum
s. Such very energetic decay products, carrying a fraction of the mass of the X particle, are believed to be a plausible explanation for the observed ultra-high energy cosmic rays (UHECR).
, HiRes
and Yakutsk Extensive Air Shower Array found the GZK cutoff, while Akeno-AGASA
observed the events above the cutoff (11 events in the past 10 years). The result of the Akeno-AGASA experiment is smooth near the GZK cutoff energy. If one assumes that the Akeno-AGASA result is correct and consider its implication, a possible explanation for the AGASA data on GZK cutoff violation would be a shower caused by dark matter particles. A dark matter particle is not constrained by the GZK cutoff, since it interacts weakly with cosmic background radiation. Recent measurements by the Pierre Auger Project have found a correlation between the direction of high energy cosmic rays and the location of AGN.
, USA) in Mendoza Province
, western Argentina
.
A larger cosmic-ray detector array is also planned for the northern hemisphere as part of the Pierre Auger complex.
The Pierre Auger Observatory, in addition to obtaining directional information from the cluster of water tanks used to observe the cosmic-ray-shower components, also has four telescopes trained on the night sky to observe fluorescence
of the nitrogen
molecules as the shower particles traverse the sky, giving further directional information on the original cosmic ray.
Astroparticle physics
Astroparticle physics, the same as particle astrophysics, is that branch of particle physics that studies elementary particles of astronomical origin, and their relation to astrophysics and cosmology. It is a relatively new field of research emerging at the intersection of particle physics,...
, an ultra-high-energy cosmic ray (UHECR) or extreme-energy cosmic ray (EECR) is a 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...
with an extreme kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
, far beyond both its rest mass and energies typical of other cosmic rays.
These particles are significant for astrophysics and fundamental physics theory, because they have energies comparable to the Greisen–Zatsepin–Kuzmin limit
Greisen–Zatsepin–Kuzmin limit
The Greisen–Zatsepin–Kuzmin limit is a theoretical upper limit on the energy of cosmic rays coming from "distant" sources. The limit is 5×1019 eV, or about 8 joules. The limit is set by slowing-interactions of cosmic ray protons with the microwave background radiation over long distances...
, which occurs at about 5 electron volts (8 J
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
). This limit should be the maximum energy of cosmic rays that have traveled long distances (about 160 million light years), due to the theoretical energy losses of higher-energy rays and to scattering from photons in the cosmic microwave background.
Observational history
The first observation of a cosmic ray with an energy exceeding (16 J) was made by John Linsley and Livio Scarsi at the Volcano Ranch experimentVolcano Ranch experiment
The Volcano Ranch experiment was an array of particle detectors in Volcano Ranch, New Mexico, used to measure ultra-high-energy cosmic rays from 1959 to 1963....
in New Mexico in 1962.
Cosmic rays with even higher energies have since been observed. Among them was the Oh-My-God particle observed on the evening of 15 October 1991 over Dugway Proving Ground
Dugway Proving Ground
Dugway Proving Ground is a US Army facility located approximately 85 miles southwest of Salt Lake City, Utah in southern Tooele County and just north of Juab County...
, Utah. Its observation was a shock to astrophysicists
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
, who estimated its energy to be approximately (50 J)—in other words, a subatomic particle
Subatomic particle
In physics or chemistry, subatomic particles are the smaller particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles...
with kinetic energy equal to that of a baseball
Baseball (object)
A baseball is a ball used primarily in the sport of the same name, baseball. The ball features a rubber or cork center, wrapped in yarn and covered in leather. It is in circumference . The yarn or string used to wrap the baseball can be up to one mile in length...
(5 ounces or 142 g) traveling at about 100 kilometres per hour (62.1 mph).
It was most probably a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
traveling at about times the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, so close that in a year-long race between light and the cosmic ray, the ray would fall behind only 46 nanometers ( light-years), or 0.15 femtoseconds .
The energy of this particle is some 40 million times that of the highest energy protons that can currently be produced in any terrestrial particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
. However only a small fraction of this energy would be available for an interaction with a proton or neutron on Earth, with most of the energy remaining in the form of kinetic energy of the products of the interaction. The effective energy available for such a collision is the square root of double the product of the particle's energy and the mass energy of the proton, which for this particle gives , roughly 50 times the collision energy of the Large Hadron Collider
Large Hadron Collider
The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....
.
Since the first observation, by the University of Utah
University of Utah
The University of Utah, also known as the U or the U of U, is a public, coeducational research university in Salt Lake City, Utah, United States. The university was established in 1850 as the University of Deseret by the General Assembly of the provisional State of Deseret, making it Utah's oldest...
's Fly's Eye Cosmic Ray Detector
High Resolution Fly's Eye Cosmic Ray Detector
The High Resolution Fly's Eye or HiRes detector was an ultra-high-energy cosmic ray observatory that operated in the western Utah desert from May 1997 until April 2006. HiRes utilized the atmospheric fluorescence technique that was pioneered by the Utah group first in tests at the Volcano Ranch...
, at least fifteen similar events have been recorded, confirming the phenomenon. These very high energy cosmic rays are very rare; the energy of most cosmic rays is between 10 MeV and 10 GeV.
Active galactic cores as one possible source of the particles
Interactions with blue-shiftedBlue shift
A blueshift is any decrease in wavelength ; the opposite effect is referred to as redshift. In visible light, this shifts the colour from the red end of the spectrum to the blue end...
cosmic microwave background radiation
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....
limit the distance that these particles can travel before losing energy; this is known as the Greisen–Zatsepin–Kuzmin limit
Greisen–Zatsepin–Kuzmin limit
The Greisen–Zatsepin–Kuzmin limit is a theoretical upper limit on the energy of cosmic rays coming from "distant" sources. The limit is 5×1019 eV, or about 8 joules. The limit is set by slowing-interactions of cosmic ray protons with the microwave background radiation over long distances...
or GZK limit.
The source of such high energy particles has been a mystery for many years. Recent results from the Pierre Auger Observatory show that ultra-high-energy cosmic ray arrival directions appear to be correlated with extragalactic supermassive black holes at the center of nearby galaxies called active galactic nuclei (AGN)
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...
. However, since the angular correlation scale used is fairly large (3.1 degrees) these results do not unambiguously identify the origins of such cosmic rays. The AGN could merely be closely associated with the actual sources, for example in galaxies or other astrophysical objects that are clumped with matter on large scales within 100 Mpc.
Additional data collection is important for further investigating possible AGN sources for these highest energy particles.
Some of the supermassive black holes in AGN are known to be rotating, as in the Seyfert galaxy
Seyfert galaxy
Seyfert galaxies are a class of galaxies with nuclei that produce spectral line emission from highly ionized gas, named after Carl Keenan Seyfert, the astronomer who first identified the class in 1943...
MCG 6-30-15 with time-variability in their inner accretion disks. Black hole spin is a potentially effective agent to drive UHECR production, provided ions are suitably launched to circumvent limiting factors deep within the nucleus, notably curvature radiation and inelastic scattering with radiation from the inner disk. Low-luminosity, intermittent Seyfert galaxies may meet the requirements with the formation of a linear accelerator several light years away from the nucleus, yet within their extended ion tori whose UV radiation ensures a supply of ionic contaminants. The corresponding electric fields are small, on the order of 10 V/cm, whereby the observed UHECRs are indicative for the astronomical size of the source. Improved statistics by the Pierre Auger Observatory will be instrumental in identifying the presently tentative association of UHECRs (from the Local Universe) with Seyferts and LINER
Liner
Liner or LINER may refer to:In line drawing:* Eye liner, a type of makeup* Liner, a sable brush used by coach paintersIn linings:...
s.
Other possible sources of the particles
Other possible sources of the UHECR are:- radio lobes of powerful radio galaxiesRadio galaxyRadio galaxies and their relatives, radio-loud quasars and blazars, are types of active galaxy that are very luminous at radio wavelengths, with luminosities up to 1039 W between 10 MHz and 100 GHz. The radio emission is due to the synchrotron process...
- intergalactic shocks created during the epoch of galaxy formation
- hypernovaHypernovaHypernova , also known as a type 1c Supernova, refers to an incredibly large star that collapses at the end of its lifespan...
e - gamma-ray bursts
- decay products of supermassive particles from topological defectTopological defectIn mathematics and physics, a topological soliton or a topological defect is a solution of a system of partial differential equations or of a quantum field theory homotopically distinct from the vacuum solution; it can be proven to exist because the boundary conditions entail the existence of...
s, left over from phase transitions in the early universe - Particles undergoing the Penrose effectPenrose processThe Penrose process is a process theorised by Roger Penrose wherein energy can be extracted from a rotating black hole...
.
Conversion of dark matter into ultra-high-energy particles
It is hypothesized that active galactic nuclei are capable of converting dark matter into high energy protons. Yuri Pavlov and Andrey Grib at the Alexander Friedmann Laboratory for Theoretical Physics at St. Petersburg hypothesize that dark matter particles are about 15 times heavier than protons, and that they can decay into pairs of particles of a type that interacts with ordinary matter.Near an active galactic nucleus, one of these particles can fall into the black hole, while the other escapes, as described by the Penrose process
Penrose process
The Penrose process is a process theorised by Roger Penrose wherein energy can be extracted from a rotating black hole...
. Some of the particles that escape will collide with incoming particles creating collisions of very high energy. It is in these collisions, according to Pavlov, that ordinary visible protons can form. These protons would have very high energies. Pavlov claims that evidence of this is present in the form of ultra-high-energy cosmic rays.
Ultra-high energy cosmic rays may also be produced by the decay of super-heavy dark matter "X particles" such as Holeum
Holeum
Holeums are stable, quantized gravitational bound states of primordial or micro black holes.-Introduction:Holeums were proposed by L.K. Chavda and Abhijit Chavda in 2002. They have all the properties associated with cold dark matter...
s. Such very energetic decay products, carrying a fraction of the mass of the X particle, are believed to be a plausible explanation for the observed ultra-high energy cosmic rays (UHECR).
Dark matter particles as ultra-high-energy particles
High energy cosmic rays traversing intergalactic space suffer the GZK cutoff above 1020 eV due to interactions with cosmic background radiation if the primary cosmic ray particles are protons or nuclei. The Pierre Auger ProjectPierre Auger Observatory
The Pierre Auger Observatory is an international cosmic ray observatory designed to detect ultra-high-energy cosmic rays: single sub-atomic particles with energies beyond 1020 eV...
, HiRes
High Resolution Fly's Eye Cosmic Ray Detector
The High Resolution Fly's Eye or HiRes detector was an ultra-high-energy cosmic ray observatory that operated in the western Utah desert from May 1997 until April 2006. HiRes utilized the atmospheric fluorescence technique that was pioneered by the Utah group first in tests at the Volcano Ranch...
and Yakutsk Extensive Air Shower Array found the GZK cutoff, while Akeno-AGASA
AGASA
The Akeno Giant Air Shower Array is a very large surface array designed to study the origin of ultra-high-energy cosmic rays. It covers an area of 100 km2 and consists of 111 surface detectors and 27 muon detectors...
observed the events above the cutoff (11 events in the past 10 years). The result of the Akeno-AGASA experiment is smooth near the GZK cutoff energy. If one assumes that the Akeno-AGASA result is correct and consider its implication, a possible explanation for the AGASA data on GZK cutoff violation would be a shower caused by dark matter particles. A dark matter particle is not constrained by the GZK cutoff, since it interacts weakly with cosmic background radiation. Recent measurements by the Pierre Auger Project have found a correlation between the direction of high energy cosmic rays and the location of AGN.
Pierre Auger Observatory
Pierre Auger Observatory is an international cosmic ray observatory designed to detect ultra-high-energy cosmic rays (subatomic particles (protons or other nuclei) with energies beyond 1020 electron-volts). These high-energy particles have an estimated arrival rate of just 1 per square kilometer per century, therefore, in order to record a large number of these events, the Auger Observatory has created a detection area of 3,000 km² (the size of Rhode IslandRhode Island
The state of Rhode Island and Providence Plantations, more commonly referred to as Rhode Island , is a state in the New England region of the United States. It is the smallest U.S. state by area...
, USA) in Mendoza Province
Mendoza Province
The Province of Mendoza is a province of Argentina, located in the western central part of the country in the Cuyo region. It borders to the north with San Juan, the south with La Pampa and Neuquén, the east with San Luis, and to the west with the republic of Chile; the international limit is...
, western Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
.
A larger cosmic-ray detector array is also planned for the northern hemisphere as part of the Pierre Auger complex.
The Pierre Auger Observatory, in addition to obtaining directional information from the cluster of water tanks used to observe the cosmic-ray-shower components, also has four telescopes trained on the night sky to observe fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
of the nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
molecules as the shower particles traverse the sky, giving further directional information on the original cosmic ray.
Ultra-high-energy cosmic ray observatories
- AGASAAGASAThe Akeno Giant Air Shower Array is a very large surface array designed to study the origin of ultra-high-energy cosmic rays. It covers an area of 100 km2 and consists of 111 surface detectors and 27 muon detectors...
- Akeno Giant Air Shower Array in Japan - Antarctic Impulse Transient AntennaANtarctic Impulse Transient AntennaThe AA experiment has been designed to study ultra-high-energy cosmic neutrinos by detecting the radio pulses emitted by their interacting with the Antarctic ice sheet...
(ANITA) detects ultra-high-energy cosmic neutrinoNeutrinoA neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s believed to be caused by ultra-high-energy cosmic rays - Extreme Universe Space ObservatoryExtreme Universe Space ObservatoryThe Extreme Universe Space Observatory is the first Space mission concept devoted to the investigation of cosmic rays and neutrinos of extreme energy...
- GRAPES-3GRAPES-3GRAPES-3 is a project for cosmic ray study with air shower detector array and large area muon detectors at Ooty in southern India...
(Gamma Ray Astronomy PeV EnergieS 3rd establishment) is a project for cosmic ray study with air shower detector array and large area muon detectors at Ooty in southern India. - High Resolution Fly's Eye Cosmic Ray DetectorHigh Resolution Fly's Eye Cosmic Ray DetectorThe High Resolution Fly's Eye or HiRes detector was an ultra-high-energy cosmic ray observatory that operated in the western Utah desert from May 1997 until April 2006. HiRes utilized the atmospheric fluorescence technique that was pioneered by the Utah group first in tests at the Volcano Ranch...
(HiRes) - LOPES (telescope)LOPES (telescope)The LOPES project is a cosmic ray detector array, located in Karlsruhe, Germany, and is operated in coincidence with an existing, well calibrated air shower experiment called KASCADE....
- LOFAR PrototypE Station is located in Karlsruhe, Germany is part of the LOFARLOFARLOFAR is the Low Frequency Array for radio astronomy, built by the Netherlands astronomical foundation ASTRON and operated by ASTRON's radio observatory....
project. - MARIACHIMARIACHIMARIACHI, the Mixed Apparatus for Radar Investigation of Cosmic-rays of High Ionization, is an apparatus for the detection of ultra-high-energy cosmic rays via bi-static radar using VHF transmitters. MARIACHI is also the name of the research project at Brookhaven National Laboratory operating the...
- Mixed Apparatus for Radar Investigation of Cosmic-rays of High Ionization located on Long Island, USA. - Pierre Auger ObservatoryPierre Auger ObservatoryThe Pierre Auger Observatory is an international cosmic ray observatory designed to detect ultra-high-energy cosmic rays: single sub-atomic particles with energies beyond 1020 eV...
- Telescope Array ProjectTelescope Array ProjectThe Telescope Array project is an international collaboration involving research and educational institutions in Japan, Taiwan, China, Russia, South Korea, and the United States. The experiment is designed to observe ultra-high-energy cosmic ray air showers using a combination of ground array and...
- Yakutsk Extensive Air Shower Array
- The COSMICi project at FAMU is developing technology for a distributed network of low-cost detectors for UHECR showers in collaboration with MARIACHIMARIACHIMARIACHI, the Mixed Apparatus for Radar Investigation of Cosmic-rays of High Ionization, is an apparatus for the detection of ultra-high-energy cosmic rays via bi-static radar using VHF transmitters. MARIACHI is also the name of the research project at Brookhaven National Laboratory operating the...
.
External links
- The Highest Energy Particle Ever Recorded The details of the event from the official site of the Fly's Eye detector.
- John WalkerJohn Walker (programmer)John Walker is a computer programmer and a co-founder of the computer-aided design software company Autodesk, and a co-author of early versions of AutoCAD, a product Autodesk originally acquired from programmer Michael Riddle...
's lively analysis of the 1991 event, published in 1994 - Origin of energetic space particles pinpointed, by Mark Peplow for news@nature.com, published January 13, 2005.
- Ultra High Energy Cosmic Rays (1908), Ritzian relativity in action?