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Ultra-high-energy gamma ray

Ultra-high-energy gamma rays are gamma rays 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.[1] In a 18 May 2021 press release, China's Large High Altitude Air Shower Observatory (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.


Ultra-high-energy gamma rays are of importance because they may reveal the source of cosmic rays. 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 pions which in turn decay to ultra-high-energy photons.[2]

The ratio of primary cosmic ray hadrons 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 by way of the Greisen–Zatsepin–Kuzmin limit cutoff above 50 EeV.[3]

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. 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.[3]

Class energy (TeV) energy (eV) energy (μJ) frequency (YHz) wavelength (am) comparison properties
10−12 1 1.602 × 10−13 2.418 × 10−12 1.2398 × 1012 near infrared photon (for comparison)
0.1 1 × 1011 0.01602 24.2 12 Z boson
Very-high-energy gamma rays
1 1 × 1012 0.1602 242 1.2 flying mosquito produces Cherenkov light
10 1 × 1013 1.602 2.42 × 103 0.12 air shower reaches ground
100 1 × 1014 16.02 2.42 × 104 0.012 ping pong ball falling off a bat causes nitrogen to fluoresce
Ultra-high-energy gamma rays
1000 1 × 1015 160.2 2.42 × 105 1.2 × 10−3
10 000 TeV 1 × 1016 1602 2.42 × 106 1.2 × 10−4 potential energy of golf ball on a tee
100 000 1 × 1017 1.602 × 104 2.42 × 107 1.2 × 10−5
1 000 000 1 × 1018 1.602 × 105 2.42 × 108 1.2 × 10−6
10 000 000 1 × 1019 1.602 × 106 2.42 × 109 1.2 × 10−7 air rifle shot
1.22091×1016 1.22091 × 1028 1.95611 × 109 1.855 × 1019 1.61623 × 10−17 explosion of a car tank full of gasoline
Planck energy


  1. ^ Yrika, Bob (June 26, 2019). "Highest energy photons ever recorded coming from Crab Nebula". Retrieved December 20, 2019.
  2. ^ Aharonian, Felix (24 August 2010). "The Fascinating TeV Sky" (PDF). The Twelfth Marcel Grossmann Meeting. pp. 368–380. Bibcode:2012mgm..conf..368A. doi:10.1142/9789814374552_0016. ISBN 978-981-4374-51-4. Archived from the original (PDF) on 2012-05-29. Retrieved 27 November 2011 – via WSPC - Proceedings.
  3. ^ a b Vankov, H. P.; Inoue, N.; Shinozaki, K. (2 February 2008). "Ultra-High Energy Gamma Rays in Geomagnetic Field and Atmosphere" (PDF). Retrieved 3 December 2011.
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Ultra-high-energy gamma ray
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