Ultra-high-energy gamma rays are

gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

s with photon energies higher than 100 TeV (0.1 PeV). They have a frequency higher than 2.42 × 10²⁸ Hz and a wavelength shorter than 1.24 × 10⁻²⁰ m. The existence of these rays was confirmed in 2019. In a 18 May 2021 press release, China's

Large High Altitude Air Shower Observatory 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 cos ...

(LHAASO) reported the detection of a dozen ultra-high-energy gamma rays with energies exceeding 1 peta-electron-volt (quadrillion electron-volts or PeV), including one at 1.4 PeV, the highest energy photon ever observed. The authors of the report have named the sources of these PeV gamma rays PeVatrons.

Importance

Ultra-high-energy gamma rays are of importance because they may reveal the source of

cosmic ray Cosmic rays 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 ...

s. Discounting the relatively weak effect of gravity, they travel in a straight line from their source to an observer. This is unlike cosmic rays which have their direction of travel scrambled by magnetic fields. Sources that produce cosmic rays will almost certainly produce gamma rays as well, as the cosmic ray particles interact with nuclei or electrons to produce photons or neutral

pion In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more gene ...

s which in turn decay to ultra-high-energy photons. The ratio of primary cosmic ray

hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ...

s to gamma rays also gives a clue as to the origin of cosmic rays. Although gamma rays could be produced near the source of cosmic rays, they could also be produced by interaction with

cosmic microwave background In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...

by way of the

Greisen–Zatsepin–Kuzmin limit 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 (50 EeV), or about 8 j ...

cutoff above 50 EeV. Ultra-high-energy gamma rays interact with magnetic fields to produce positron-electron pairs. In the Earth's magnetic field, a 10²¹ eV photon is expected to interact about 5000 km above the earth's surface. The high-energy particles then go on to produce more lower energy photons that can suffer the same fate. This effect creates a beam of several 10¹⁷ eV gamma ray photons heading in the same direction as the original UHE photon. This beam is less than 0.1 m wide when it strikes the atmosphere. These gamma rays are too low-energy to show the

Landau–Pomeranchuk–Migdal effect In high-energy physics, the Landau–Pomeranchuk–Migdal effect, also known as the Landau–Pomeranchuk effect and the Pomeranchuk effect, or simply LPM effect, is a reduction of the bremsstrahlung and pair production cross sections at high energi ...

. Only magnetic field perpendicular to the path of the photon causes pair production, so that photons coming in parallel to the geomagnetic field lines can survive intact until they meet the atmosphere. These photons coming through the magnetic window can produce Landau–Pomeranchuk–Migdal showers.

References

External links

Search for Galactic PeV Gamma Rays with the IceCube Neutrino ObservatoryAir shower detection of diffuse PeV gamma-rays
{{Electromagnetic spectrum Gamma rays