Osaka Metropolitan Univeristy/L-Insight, Kyoto University/Ryuunosuke Takeshige
The Earth’s atmosphere has been blasted by the second strongest ultrahigh-energy cosmic ray (UHECR) to ever be observed. While its exact origins remain unknown, astrophysicists are pointing the finger at the Local Void, a fundamentally empty area of the cosmos next to the Milky Way.
The subatomic particle that the ray beamed to Earth has been dubbed the “Amaterasu” particle by scientists in an homage to the mythological Japanese sun goddess. It arrived on May 27, 2021 and has just been described by researchers in the journal Science today. The study was led by researchers from the University of Utah (the U) and the University of Tokyo.
The ray was detected by the Telescope Array, a field of 507 detectors spread out over 700 square km (about 270 square miles) in Utah’s West Desert. The arrays operate at an elevation of about 1,200 m (4,000 ft) in the dry, light-pollution-free air that allows them the exceptional ability to detect particles from cosmic rays….Story continues….
Source: Did the Local Void shoot a monster extreme-energy cosmic ray our way?
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Cosmic rays or astroparticles 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 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.
Cosmic rays were discovered by Victor Hess in 1912 in balloon experiments, for which he was awarded the 1936 Nobel Prize in Physics. Direct measurement of cosmic rays, especially at lower energies, has been possible since the launch of the first satellites in the late 1950s. Particle detectors similar to those used in nuclear and high-energy physics are used on satellites and space probes for research into cosmic rays.
Data from the Fermi Space Telescope (2013) have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernova explosions of stars. Based on observations of neutrinos and gamma rays from blazar TXS 0506+056 in 2018, active galactic nuclei also appear to produce cosmic rays. The term ray (as in optical ray) seems to have arisen from an initial belief, due to their penetrating power, that cosmic rays were mostly electromagnetic radiation.
Nevertheless, following wider recognition of cosmic rays as being various high-energy particles with intrinsic mass, the term “rays” was still consistent with then known particles such as cathode rays, canal rays, alpha rays, and beta rays. Meanwhile “cosmic” ray photons, which are quanta of electromagnetic radiation (and so have no intrinsic mass) are known by their common names, such as gamma rays or X-rays, depending on their photon energy.
Measurements of the energy and arrival directions of the ultra-high-energy primary cosmic rays by the techniques of density sampling and fast timing of extensive air showers were first carried out in 1954 by members of the Rossi Cosmic Ray Group at the Massachusetts Institute of Technology. The experiment employed eleven scintillation detectors arranged within a circle 460 metres in diameter on the grounds of the Agassiz Station of the Harvard College Observatory.
From that work, and from many other experiments carried out all over the world, the energy spectrum of the primary cosmic rays is now known to extend beyond 1020 eV. A huge air shower experiment called the Auger Project is currently operated at a site on the Pampas of Argentina by an international consortium of physicists.
The project was first led by James Cronin, winner of the 1980 Nobel Prize in Physics from the University of Chicago, and Alan Watson of the University of Leeds, and later by scientists of the international Pierre Auger Collaboration. Their aim is to explore the properties and arrival directions of the very highest-energy primary cosmic rays.
The results are expected to have important implications for particle physics and cosmology, due to a theoretical Greisen–Zatsepin–Kuzmin limit to the energies of cosmic rays from long distances (about 160 million light years) which occurs above 1020 eV because of interactions with the remnant photons from the Big Bang origin of the universe.
Currently the Pierre Auger Observatory is undergoing an upgrade to improve its accuracy and find evidence for the yet unconfirmed origin of the most energetic cosmic rays. High-energy gamma rays (>50 MeV photons) were finally discovered in the primary cosmic radiation by an MIT experiment carried on the OSO-3 satellite in 1967.
Components of both galactic and extra-galactic origins were separately identified at intensities much less than 1% of the primary charged particles. Since then, numerous satellite gamma-ray observatories have mapped the gamma-ray sky.
The most recent is the Fermi Observatory, which has produced a map showing a narrow band of gamma ray intensity produced in discrete and diffuse sources in our galaxy, and numerous point-like extra-galactic sources distributed over the celestial sphere.
Cosmic rays can be divided into two types:
- galactic cosmic rays (GCR) and extragalactic cosmic rays, i.e., high-energy particles originating outside the solar system, and
- solar energetic particles, high-energy particles (predominantly protons) emitted by the sun, primarily in solar eruptions.
However, the term “cosmic ray” is often used to refer to only the extrasolar flux. Cosmic rays originate as primary cosmic rays, which are those originally produced in various astrophysical processes. Primary cosmic rays are composed mainly of protons and alpha particles (99%), with a small amount of heavier nuclei (≈1%) and an extremely minute proportion of positrons and antiprotons.
Secondary cosmic rays, caused by a decay of primary cosmic rays as they impact an atmosphere, include photons, hadrons, and leptons, such as electrons, positrons, muons, and pions. The latter three of these were first detected in cosmic rays.
Primary cosmic rays mostly originate from outside the Solar System and sometimes even outside the Milky Way. When they interact with Earth’s atmosphere, they are converted to secondary particles. The mass ratio of helium to hydrogen nuclei, 28%, is similar to the primordial elemental abundance ratio of these elements, 24%.
The remaining fraction is made up of the other heavier nuclei that are typical nucleosynthesis end products, primarily lithium, beryllium, and boron. These nuclei appear in cosmic rays in greater abundance (≈1%) than in the solar atmosphere, where they are only about 10−3 as abundant (by number) as helium. Cosmic rays composed of charged nuclei heavier than helium are called HZE ions.
Due to the high charge and heavy nature of HZE ions, their contribution to an astronaut’s radiation dose in space is significant even though they are relatively scarce. This abundance difference is a result of the way in which secondary cosmic rays are formed. Carbon and oxygen nuclei collide with interstellar matter to form lithium, beryllium, and boron in a process termed cosmic ray spallation.
Spallation is also responsible for the abundances of scandium, titanium, vanadium, and manganese ions in cosmic rays produced by collisions of iron and nickel nuclei with interstellar matter. At high energies the composition changes and heavier nuclei have larger abundances in some energy ranges. Current experiments aim at more accurate measurements of the composition at high energies.
When cosmic rays enter the Earth’s atmosphere, they collide with atoms and molecules, mainly oxygen and nitrogen. The interaction produces a cascade of lighter particles, a so-called air shower secondary radiation that rains down, including x-rays, protons, alpha particles, pions, muons, electrons, neutrinos, and neutrons.
All of the secondary particles produced by the collision continue onward on paths within about one degree of the primary particle’s original path. Typical particles produced in such collisions are neutrons and charged mesons such as positive or negative pions and kaons. Some of these subsequently decay into muons and neutrinos, which are able to reach the surface of the Earth. Some high-energy muons even penetrate for some distance into shallow mines, and most neutrinos traverse the Earth without further interaction.
Others decay into photons, subsequently producing electromagnetic cascades. Hence, next to photons, electrons and positrons usually dominate in air showers. These particles as well as muons can be easily detected by many types of particle detectors, such as cloud chambers, bubble chambers, water-Cherenkov, or scintillation detectors. The observation of a secondary shower of particles in multiple detectors at the same time is an indication that all of the particles came from that event.
Cosmic rays impacting other planetary bodies in the Solar System are detected indirectly by observing high-energy gamma ray emissions by gamma-ray telescope. These are distinguished from radioactive decay processes by their higher energies above about 10 MeV.
Related contents:
Detecting cosmic rays from a galaxy far, far away”. Science Daily. 21 September 2017. Retrieved 26 December 2017.
Cosmic Influences on the Earth”. Earth System: History and Natural Variability. Vol. I. Eolss Publishers. p. 165. ISBN 978-1-84826-104-4.
Evidence shows that cosmic rays come from exploding stars” (Press release). Washington, DC: American Association for the Advancement of Science.
Are cosmic rays electromagnetic radiation?”. NASA. Archived from the original on 31 May 2000. Retrieved 11 December 2012.
Astronomy without a telescope – ‘Oh-my-God’ particles”. Universe Today. Retrieved 17 February 2013.
European Organization for Nuclear Research (CERN). 2021. p. 3. Retrieved 9 October 2022.
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