Cosmic ray astronomy

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Cosmic ray astronomy is a branch of observational astronomy where scientists attempt to identify and study the potential sources of extremely high-energy (ranging from 1 MeV to more than 1 EeV) charged particles called cosmic rays coming from outer space. [1] [2] These particles, which include protons (nucleus of hydrogen), electrons, positrons and atomic nuclei (mostly of helium, but potentially of all chemical elements), travel through space at nearly the speed of light (such as the ultra-high-energy "Oh-My-God particle" [3] ) and provide valuable insights into the most energetic processes in the universe. Unlike other branches of observational astronomy, it uniquely relies on charged particles as carriers of information. [1]

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Detection methods

The Pierre Auger Observatory in Argentina is the world's largest cosmic ray observatory Observatorio pierre auger.jpg
The Pierre Auger Observatory in Argentina is the world's largest cosmic ray observatory

Astronomers use ground-based detectors, high-altitude research balloons, artificial satellites and other methods to detect cosmic rays. Ground-based detectors, often spread over large areas (for example, the Pierre Auger Observatory is an array of detectors spread over 3,000 square kilometers), identify and analyze the secondary particles (electrons, positrons, photons, muons, etc.) produced in a chain reaction of particle interactions triggered by the collision of cosmic rays and Earth's atmosphere. [1] The properties of the original cosmic ray particle, such as arrival direction and energy, are inferred from the measured properties of the extensive air shower, which is the cascade of secondary particles collectively showering down through the atmosphere. There are two kinds of ground-based detectors: Surface detector arrays analyze the air shower at a unique altitude, whereas air fluorescence detectors record the shower development in the atmosphere, based on the interactions of air shower particles with nitrogen molecules in the atmosphere. [4] Modern "hybrid" detectors, such as the Pierre Auger Observatory in Argentina and the Large High Altitude Air Shower Observatory in Sichuan, China, take advantage of the complementary nature of these two. Moreover, scientific balloons (such as the one used in Cosmic Ray Energetics and Mass Experiment [5] ) and satellites (such as China's Dark Matter Particle Explorer or DAMPE telescope) can also be used to observe pure cosmic rays at very high altitudes and in outer space.

Benefits

By studying the energy, direction, and composition of cosmic rays, scientists can uncover the sources and acceleration mechanisms behind these particles, which reveal astrophysical processes such as supernova explosions, black hole accretion, and galactic magnetic fields. Observations of cosmic rays led to the discovery of subatomic particles beyond the proton, neutron, and electron, including the positron and the muon, laying the groundwork for modern particle physics. It reveals the nucleosynthetic processes leading to the origin of the elements. [2] By measuring cosmic rays, scientists discovered the presence of magnetic fields and radiation in the Solar System. Some cosmic rays originate from beyond the Solar System or galaxy, allowing scientists to estimate the amount and composition of matter in the universe, providing crucial information about its makeup. Cosmic rays are generated in extreme astrophysical environments such as exploding stars, black holes, and galactic collisions and provide a rare window into these processes. Energetic cosmic rays can interact with objects traveling through space, altering their isotopic composition. By studying these isotopes in meteorites, scientists can determine when they formed and fell on Earth, providing insights into the history of the Solar System. Cosmic rays have practical applications, including monitoring soil moisture for agriculture and irrigation practices and carbon-14 dating, which helps determine the ages of archaeological artifacts and geological formations. [6]

History

Austrian-American astronomer Victor Hess first discovered cosmic rays in 1912 with airborne ballons. Hess.jpg
Austrian-American astronomer Victor Hess first discovered cosmic rays in 1912 with airborne ballons.

Historical milestones in cosmic ray astronomy inclue Victor Hess's discovery of cosmic rays during balloon flights in 1912; [6] the identification of new subatomic particles like the positron and muon in the 1930s, expanding our understanding of particle physics; [7] Pierre Victor Auger's discovery of extensive particle showers from cosmic ray collisions high in the atmosphere; [8] ground-based detectors measuring cosmic ray flux and energy spectrum in the 1940s-1950s; [9] the establishment of the Volcano Ranch cosmic ray observatory in the 1960s, initiating large-scale experiments; [10] the discovery of cosmic ray anisotropy (the fact that cosmic rays do not arrive uniformly from every region of the sky) in the 1960s, unveiling variations in flux and direction; the emergence of high-energy gamma-ray telescopes in the 1980s-1990s, enabling observations of gamma rays produced by cosmic ray interactions; the advent of space-based detectors like AMS-02 on the International Space Station in the 2000s, providing insights from space; [11] and recent progress in multi-messenger astronomy in the 2010s, integrating cosmic ray observations with other astrophysical signals for a more complete view of cosmic phenomena. [12]

Future

With advancements in technology and the development of more sensitive detection systems, astronomers anticipate making new discoveries about the sources, acceleration mechanisms, and propagation of cosmic rays. These insights will contribute to a deeper understanding of the underlying physics governing the cosmos. Future cosmic ray observatories, such as the Cherenkov Telescope Array, will use advanced techniques to detect gamma rays produced by cosmic ray interactions in Earth's atmosphere. Since these gamma rays will be the most sensitive means to study cosmic rays near their source, these observatories will enable astronomers to study cosmic rays with unprecedented precision. [13] Cosmic ray astronomy faces difficulty in identifying the exact sources of cosmic rays because charged particles are deflected by magnetic fields in space, and as a result tracing the paths of cosmic rays back to their origins require sophisticated modeling techniques and multi-messenger observations to infer their source locations. Moreover, due to the high-energy nature of these rays, the need for full-sky exposure, [14] minimization of deflection by magnetic fields and elimination of background from distant sources present technical challenges.

Related Research Articles

<span class="mw-page-title-main">Cosmic ray</span> High-energy particle, mainly originating outside the Solar system

Cosmic rays or astroparticles are high-energy particles or clusters of particles that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own galaxy, and from distant galaxies. Upon impact with Earth's atmosphere, cosmic rays produce showers of secondary particles, some of which reach the surface, although the bulk are deflected off into space by the magnetosphere or the heliosphere.

The Greisen–Zatsepin–Kuzmin limit (GZK limit or GZK cutoff) is a theoretical upper limit on the energy of cosmic ray protons traveling from other galaxies through the intergalactic medium to our galaxy. The limit is 5×1019 eV (50 EeV), or about 8 joules (the energy of a proton travelling at ≈ 99.99999999999999999998% the speed of light). The limit is set by the slowing effect of interactions of the protons with the microwave background radiation over long distances (≈ 160 million light-years). The limit is at the same order of magnitude as the upper limit for energy at which cosmic rays have experimentally been detected, although indeed some detections appear to have exceeded the limit, as noted below. For example, one extreme-energy cosmic ray, the Oh-My-God Particle, which has been found to possess a record-breaking 3.12×1020 eV (50 joules) of energy (about the same as the kinetic energy of a 95 km/h baseball).

In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray with an energy greater than 1 EeV (1018 electronvolts, approximately 0.16 joules), far beyond both the rest mass and energies typical of other cosmic ray particles.

<span class="mw-page-title-main">HEGRA</span>

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.

<span class="mw-page-title-main">Air shower (physics)</span> Cascade of atmospheric subatomic particles

Air showers are extensive cascades of subatomic particles and ionized nuclei, produced in the atmosphere when a primary cosmic ray enters the atmosphere. When a particle of the cosmic radiation, which could be a proton, a nucleus, an electron, a photon, or (rarely) a positron, interacts with the nucleus of a molecule in the atmosphere, it produces a vast number of secondary particles, which make up the shower. In the first interactions of the cascade especially hadrons are produced and decay rapidly in the air, producing other particles and electromagnetic radiation, which are part of the shower components. Depending on the energy of the cosmic ray, the detectable size of the shower can reach several kilometers in diameter.

The High Resolution Fly's Eye or HiRes detector was an ultra-high-energy cosmic ray observatory that operated in the West Desert of Utah from May 1997 until April 2006. HiRes used the "atmospheric fluorescence" technique that was pioneered by the Utah group first in tests at the Volcano Ranch experiment and then with the original Fly's Eye experiment. The experiment first ran as the HiRes prototype in a tower configuration operating in conjunction with the CASA and MIA. The prototype was later reconfigured to view 360 degrees in azimuth. HiRes-II followed later and was located on a hilltop about 13km away. HiRes-I and HiRes-II operated in stereo.

<span class="mw-page-title-main">Pierre Auger Observatory</span> International cosmic ray observatory in Argentina

The Pierre Auger Observatory is an international cosmic ray observatory in Argentina designed to detect ultra-high-energy cosmic rays: sub-atomic particles traveling nearly at the speed of light and each with energies beyond 1018 eV. In Earth's atmosphere such particles interact with air nuclei and produce various other particles. These effect particles (called an "air shower") can be detected and measured. But since these high energy particles have an estimated arrival rate of just 1 per km2 per century, the Auger Observatory has created a detection area of 3,000 km2 (1,200 sq mi)—the size of Rhode Island, or Luxembourg—in order to record a large number of these events. It is located in the western Mendoza Province, Argentina, near the Andes.

The Akeno Giant Air Shower Array (AGASA) was an array of particle detectors designed to study the origin of ultra-high-energy cosmic rays. It was deployed from 1987 to 1991 and decommissioned in 2004. It consisted of 111 scintillation detectors and 27 muon detectors spread over an area of 100 km2. It was operated by the Institute for Cosmic Ray Research, University of Tokyo at the Akeno Observatory.

<span class="mw-page-title-main">IceCube Neutrino Observatory</span> Neutrino detector at the South Pole

The IceCube Neutrino Observatory is a neutrino observatory developed by the University of Wisconsin–Madison and constructed at the Amundsen–Scott South Pole Station in Antarctica. The project is a recognized CERN experiment (RE10). Its thousands of sensors are located under the Antarctic ice, distributed over a cubic kilometre.

<span class="mw-page-title-main">IACT</span> Device to detect very-high-energy gamma ray photons

IACT stands for imaging atmosphericCherenkov telescope or technique. It is a device or method to detect very-high-energy gamma ray photons in the photon energy range of 50 GeV to 50 TeV.

<span class="mw-page-title-main">Neutrino detector</span> Physics apparatus which is designed to study neutrinos

A neutrino detector is a physics apparatus which is designed to study neutrinos. Because neutrinos only weakly interact with other particles of matter, neutrino detectors must be very large to detect a significant number of neutrinos. Neutrino detectors are often built underground, to isolate the detector from cosmic rays and other background radiation. The field of neutrino astronomy is still very much in its infancy – the only confirmed extraterrestrial sources as of 2018 are the Sun and the supernova 1987A in the nearby Large Magellanic Cloud. Another likely source is the blazar TXS 0506+056 about 3.7 billion light years away. Neutrino observatories will "give astronomers fresh eyes with which to study the universe".

<span class="mw-page-title-main">VERITAS</span> Ground-based gamma-ray observatory

VERITAS is a major ground-based gamma-ray observatory with an array of four 12 meter optical reflectors for gamma-ray astronomy in the GeV – TeV photon energy range. VERITAS uses the Imaging Atmospheric Cherenkov Telescope technique to observe gamma rays that cause particle showers in Earth's atmosphere that are known as extensive air showers. The VERITAS array is located at the Fred Lawrence Whipple Observatory, in southern Arizona, United States. The VERITAS reflector design is similar to the earlier Whipple 10-meter gamma-ray telescope, located at the same site, but is larger in size and has a longer focal length for better control of optical aberrations. VERITAS consists of an array of imaging telescopes deployed to view atmospheric Cherenkov showers from multiple locations to give the highest sensitivity in the 100 GeV – 10 TeV band. This very high energy observatory, completed in 2007, effectively complements the Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope due to its larger collection area as well as coverage in a higher energy band.

Astroparticle physics, also called particle astrophysics, is a branch of particle physics that studies elementary particles of astrophysical origin and their relation to astrophysics and cosmology. It is a relatively new field of research emerging at the intersection of particle physics, astronomy, astrophysics, detector physics, relativity, solid state physics, and cosmology. Partly motivated by the discovery of neutrino oscillation, the field has undergone rapid development, both theoretically and experimentally, since the early 2000s.

<span class="mw-page-title-main">Extragalactic cosmic ray</span>

Extragalactic cosmic rays are very-high-energy particles that flow into the Solar System from beyond the Milky Way galaxy. While at low energies, the majority of cosmic rays originate within the Galaxy (such as from supernova remnants), at high energies the cosmic ray spectrum is dominated by these extragalactic cosmic rays. The exact energy at which the transition from galactic to extragalactic cosmic rays occurs is not clear, but it is in the range 1017 to 1018 eV.

<span class="mw-page-title-main">Cosmic-ray observatory</span> Installation built to detect high-energy-particles coming from space

A cosmic-ray observatory is a scientific installation built to detect high-energy-particles coming from space called cosmic rays. This typically includes photons, electrons, protons, and some heavier nuclei, as well as antimatter particles. About 90% of cosmic rays are protons, 9% are alpha particles, and the remaining ~1% are other particles.

<span class="mw-page-title-main">Gamma-ray astronomy</span> Observational astronomy performed with gamma rays

Gamma-ray astronomy is a subfield of astronomy where scientists observe and study celestial objects and phenomena in outer space which emit cosmic electromagnetic radiation in the form of gamma rays, i.e. photons with the highest energies at the very shortest wavelengths. Radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy.

<span class="mw-page-title-main">Very-high-energy gamma ray</span> Gamma radiation with photon energies between 100GeV and 100TeV

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 meters, or frequencies of 2 × 1025 to 2 × 1028 Hz. Such energy levels have been detected from emissions from astronomical sources such as some binary star systems containing a compact object. For example, radiation emitted from Cygnus X-3 has been measured at ranges from GeV to exaelectronvolt-levels. Other astronomical sources include BL Lacertae, 3C 66A Markarian 421 and Markarian 501. Various other sources exist that are not associated with known bodies. For example, the H.E.S.S. catalog contained 64 sources in November 2011.

<span class="mw-page-title-main">High Altitude Water Cherenkov Experiment</span>

The High Altitude Water Cherenkov Experiment or High Altitude Water Cherenkov Observatory is a gamma-ray and cosmic ray observatory located on the flanks of the Sierra Negra volcano in the Mexican state of Puebla at an altitude of 4100 meters, at 18°59′41″N97°18′30.6″W. HAWC is the successor to the Milagro gamma-ray observatory in New Mexico, which was also a gamma-ray observatory based around the principle of detecting gamma-rays indirectly using the water Cherenkov method.

Francis Louis Halzen is a Belgian particle physicist. He is the Hilldale and Gregory Breit Distinguished Professor at the University of Wisconsin–Madison and Director of its Institute for Elementary Particle Physics. Halzen is the Principal Investigator of the IceCube Neutrino Observatory at the Amundsen–Scott South Pole Station in Antarctica, the world's largest neutrino detector which has been operational since 2010.

The Large High Altitude Air Shower Observatory (LHAASO) is a gamma-ray and cosmic-ray observatory in Daocheng, in the Garzê Tibetan Autonomous Prefecture in Sichuan, China. It is designed to observe air showers triggered by gamma rays and cosmic rays. The observatory is at an altitude of 4,410 metres (14,470 ft) above sea level. Observations started in April 2019.

References

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