Extreme Gamma Rays

The highest-energy gamma ray photon ever recorded was recently observed by the Large High Altitude Air Shower Observatory (LHAASO) on Haizi Mountain, Sichuan province, China, during its first year of operation.

1.42 ± 0.13 PeV

That is 1.4 petaelectronvolts = 1.4 × 1015 eV! The origin of this fantastically energetic photon hasn’t been localized, but possible candidates are the Cygnus OB2 young massive cluster (YMC), the pulsar PSR 2032+4127, or the supernova remnant candidate SNR G79.8+1.2.

The LHAASO observatory, in China, observes ultra high-energy light using detectors spread across a wide area that will eventually cover more than a square kilometer. Institute of High Energy Physics/Chinese Academy of Sciences

How much energy is 1.4 PeV, actually?

We can calculate the frequency of this photon using

\textup{E}=h\nu


where
h = Planck’s constant = 4.135667696 × 10-15 eV·Hz-1
ν = the photon’s frequency
E = the photon’s energy

Solving for ν, we get

ν = 3.4 × 1029 Hz

Next, we’ll calculate the photon’s wavelength using

c=\lambda \nu

where
c = the speed of light = 299792458 m·s-1
λ = the photon’s wavelength

Solving for λ, we get

λ = 8.9 × 10-22 m

To give you an idea of just how tiny 8.9 × 10-22 meters is, the proton charge radius is 0.842 × 10-15 m, so 1.9 million wavelengths of this gamma ray photon would fit inside a single proton! An electron has an upper limit on its radius—if it can be said to have a radius at all—between 10-22 and 10-18 m. So between 1 and 2000 wavelengths of this gamma ray photon would fit inside a single electron.

Using Einstein’s famous equation E = mc2 we can find that each eV has a mass equivalent of 1.78266192 × 10-36 kg. 1.4 PeV then gives us a mass of 2.5 × 10-21 kg. That may not sound like a lot, but it is 1.5 million AMUs (Daltons), or a mass comparable to a giant molecule (a protein, for example) containing ~200,000 atoms.

This and other extremely high energy gamma ray photons are not directly detected from the Earth’s surface. The LHAASO detector array in China at 14,500 ft. elevation detects the air shower produced when a gamma ray (or cosmic ray particle) hits an air molecule in the upper atmosphere, causing a cascade of subatomic particles and lower-energy photons, some of which reach the surface of the Earth. It is the Cherenkov photons produced by the air shower secondary charged particles that LHAASO collects.

References
Conover, E. (2021, June 19). Record-breaking gamma rays hint at violent environments in space. Science News, 199(11), 5.
https://www.sciencenews.org/article/light-energy-record-gamma-ray

Z. Cao et al. Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sourcesNature. Published online May 17, 2021. doi: 10.1038/s41586-021-03498-z.

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