Ultra-high energy cosmic ray

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.

An extreme-energy cosmic ray (EECR) is an UHECR with energy exceeding (about 8 joule, or the energy of a proton traveling at ≈ % the speed of light), the so-called Greisen–Zatsepin–Kuzmin limit (GZK limit). This limit should be the maximum energy of cosmic ray protons that have traveled long distances (about 160 million light years), since higher-energy protons would have lost energy over that distance due to scattering from photons in the cosmic microwave background (CMB). It follows that EECR could not be survivors from the early universe, but are cosmologically "young", emitted somewhere in the Local Supercluster by some unknown physical process. If an EECR is not a proton, but a nucleus with ''A'' nucleons, then the GZK limit applies to its nucleons, which carry only a fraction of the total energy of the nucleus. For an iron nucleus, the corresponding limit would be . However, nuclear physics processes lead to limits for iron nuclei similar to that of protons. Other abundant nuclei should have even lower limits.

These particles are extremely rare; between 2004 and 2007, the initial runs of the Pierre Auger Observatory (PAO) detected 27 events with estimated arrival energies above , that is, about one such event every four weeks in the 3000 km² area surveyed by the observatory.

There is evidence that these highest-energy cosmic rays might be iron nuclei, rather than the protons that make up most cosmic rays.

The postulated (hypothetical) sources of EECR are known as Zevatrons, named in analogy to Lawrence Berkeley National Laboratory's Bevatron and Fermilab's Tevatron, and therefore capable of accelerating particles to 1 ZeV (10²¹ eV, zetta-electronvolt). In 2004 there was a consideration of the possibility of galactic jets acting as Zevatrons, due to diffusive acceleration of particles caused by shock waves inside the jets. In particular, models suggested that shock waves from the nearby M87 galactic jet could accelerate an iron nucleus to ZeV ranges. In 2007, the Pierre Auger Observatory observed a correlation of EECR with extragalactic supermassive black holes at the center of nearby galaxies called active galactic nuclei (AGN).
However, the strength of the correlation became weaker with continuing observations. Extremely high energies might be explained also by the centrifugal mechanism of acceleration in the magnetospheres of AGN, although newer results indicate that fewer than 40% of these cosmic rays seemed to be coming from the AGN, a much weaker correlation than previously reported. A more speculative suggestion by Grib and Pavlov (2007, 2008) envisages the decay of superheavy dark matter by means of the Penrose process.

Observational history

The first observation of a cosmic ray particle with an energy exceeding (16 J) was made by Dr John D Linsley and Livio Scarsi at the Volcano Ranch experiment in New Mexico in 1962.

Cosmic ray particles with even higher energies have since been observed. Among them was the Oh-My-God particle observed by the University of Utah's Fly's Eye experiment 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, an atomic nucleus with kinetic energy equal to that of a baseball () traveling at about .

The energy of this particle is some 40 million times that of the highest energy protons that have been 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 (see Collider#Explanation). 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 ray particles are very rare; the energy of most cosmic ray particles is between 10 MeV and 10 GeV.

Ultra-high-energy cosmic ray observatories

* AGASA – Akeno Giant Air Shower Array in Japan
* Antarctic Impulse Transient Antenna (ANITA) detects ultra-high-energy cosmic neutrinos believed to be caused by ultra-high-energy cosmic ray particles
* Extreme Universe Space Observatory
* GRAPES-3 (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 Detector (HiRes)
* MARIACHI – Mixed Apparatus for Radar Investigation of Cosmic-rays of High Ionization located on Long Island, USA.
* Pierre Auger Observatory
* Telescope Array Project
* Yakutsk Extensive Air Shower Array
* Tunka experiment
* The COSMICi project at Florida A&M University is developing technology for a distributed network of low-cost detectors for UHECR showers in collaboration with MARIACHI.
* Cosmic-Ray Extremely Distributed Observatory (CREDO)

Pierre Auger Observatory

Pierre Auger Observatory is an international cosmic ray observatory designed to detect ultra-high-energy cosmic ray particles (with energies beyond 10²⁰ eV). 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 Island) in Mendoza Province, western Argentina. 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 particle.

In September 2017, data from 12 years of observations from PAO supported an extragalactic source (outside of Earth's galaxy) for the origin of extremely high energy cosmic rays.

Suggested explanations

Neutron stars

One suggested source of UHECR particles is their origination from neutron stars. In young neutron stars with spin periods of <10 ms, the magnetohydrodynamic (MHD) forces from the quasi-neutral fluid of superconducting protons and electrons existing in a neutron superfluid accelerate iron nuclei to UHECR velocities. The magnetic field produced by the neutron superfluid in rapidly rotating stars creates a magnetic field of 10⁸ to 10¹¹ teslas, at which point the neutron star is classified as a magnetar. This magnetic field is the strongest stable field in the observed universe and creates the relativistic MHD wind believed to accelerate iron nuclei remaining from the supernova to the necessary energy.

Another hypothesized source of UHECRs from neutron stars is during neutron star to strange star combustion. This hypothesis relies on the assumption that strange matter is the ground state of matter which has no experimental or observational data to support it. Due to the immense gravitational pressures from the neutron star, it is believed that small pockets of matter consisting of ''up'', ''down'', and ''strange'' quarks in equilibrium acting as a single hadron (as opposed to a number of baryons). This will then combust the entire star to strange matter, at which point the neutron star becomes a strange star and its magnetic field breaks down, which occurs because the protons and neutrons in the quasi-neutral fluid have become strangelets. This magnetic field breakdown releases large amplitude electromagnetic waves (LAEMWs). The LAEMWs accelerate light ion remnants from the supernova to UHECR energies.

"Ultra-high-energy cosmic ray ''electrons''" (defined as electrons with energies of ≥10¹⁴ eV) might be explained by the Centrifugal mechanism of acceleration in the magnetospheres of the Crab-like Pulsars. The feasibility of electron acceleration to this energy scale in the Crab pulsar

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