Ultra-high-energy cosmic ray

Unsolved problem in physics:

Why is it that some cosmic rays appear to possess energies that are theoretically too high?

In high-energy physics, an ultra-high-energy cosmic ray (UHECR) or extreme-energy cosmic ray (EECR) is a cosmic ray (subatomic particle) which appears to have extreme kinetic energy, far beyond both its rest mass and energies typical of other cosmic rays. These particles are significant because they have energy comparable to (and sometimes exceeding) the Greisen–Zatsepin–Kuzmin limit.

Observational history

The first observation of a cosmic ray with an energy exceeding 10²⁰ electronvolts was made by John Linsley at the Volcano Ranch experiment in New Mexico in 1962.^[1]^[2]

Cosmic rays with even higher energies have since been observed. Among them was the Oh-My-God particle (a play on the nickname "God particle" for the Higgs boson) observed on the evening of 15 October 1991 over Dugway Proving Grounds, Utah. Its observation was a shock to astrophysicists, who estimated its energy to be approximately 3 × 10²⁰ electronvolts^[3] (300 EeV or 50 joules)—in other words, a subatomic particle with kinetic energy equal to that of a baseball (142 g or 5 ounces) traveling at 96 km/h (60 mph).

It was most probably a proton with a speed very close to the speed of light. So close to the speed of light was the proton tavelling - in fact traveling at 1 − (5×10⁻²⁴) times c, in the course of a year the proton would travel only 46 nanometers (5×10⁻²⁴ light-years) short of a full light year's distance span, that is only 0.15 femtoseconds (1.5×10⁻¹⁶ of a second) worth of a year in light travelling equivalent distance.^[4]

The energy of this particle is some 40 million times that of the highest energy protons that can currently be produced in any 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 7.5 × 10¹⁴eV, roughly 50 times the collision energy of the LHC.

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.

Active galactic cores as one possible source of the particles

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).^[5] Interactions with blue-shifted 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.

AGN have been proposed as likely sources of ultra-high-energy cosmic rays, and results from the Pierre Auger Observatory suggest that these objects may be their source. However, since the angular correlation scale is fairly large (3 degrees or more) these results do not unambiguously identify the origins of such cosmic rays. In particular, the AGN could merely be closely associated with the actual sources, which may be found, for example, in galaxies or other astrophysical objects that are clumped with matter on large scales within 100 Mpc.

Additional data collection will be important for further investigating a possible AGN source for these highest energy particles, which might be protons accelerated to those energies by magnetic fields associated with the rapidly growing black holes at the AGN centers. According to a recent study,^[6] short-duration AGN flares resulting from the tidal disruption of a star or from a disk instability can be the main source of the observed flux of super GZK cosmic rays.

Some of the supermassive black holes in AGN are known to be rotating, as in the Seyfert galaxy MCG 6-30-15^[7] with time-variability in their inner accretion disks.^[8] Black hole spin is a potentially effective agent to drive UHECR production,^[9] provided ions are suitably launched to circumvent limiting factors deep within the nucleus, notably curvature radiation^[10] 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.^[11] The corresponding electric fields are commensurably 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 LINERs.^[12]

Other possible sources of the particles

Other possible sources of the UHECR are:^[13]

radio lobes of powerful radio galaxies
intergalactic shocks created during the epoch of galaxy formation
hypernovae
gamma-ray bursts
decay products of supermassive particles from topological defects, left over from phase transitions in the early universe
Particles undergoing the Penrose effect.

Relation with dark matter

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.^[14]

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.^[15]

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 Project, 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.^[16]

Pierre Auger Observatory

Pierre Auger Observatory is an international cosmic ray observatory designed to detect ultra-high-energy cosmic rays (sub-atomic 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 Island, 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.