The most energetic gamma ray ever seen could be the strongest evidence yet that high-energy cosmic rays are produced within our Milky Way galaxy, where they spend millions of years accumulating, forming a “cosmic-ray pool”.

Now, the Tibet ASγ Collaboration, which has hundreds of detectors located on the Tibetan Plateau, has observed 23 extremely high-energy gamma rays, with energies ranging from 400 to 955.7 TeV, the latter being the most energetic gamma ray ever detected.

Gamma rays formed in this manner are about an order of magnitude less energetic than their cosmic-ray parents, which means that those cosmic rays reached energies far in excess of one peta-electronvolt (1015 eV).

Many of the higher energy gamma rays from extragalactic PeVatrons may just not be reaching us, although Hanna does note that a few of the gamma rays detected by the Tibet ASγ Collaboration don’t align with the Milky Way.

The idea is that cosmic rays become contained in our galaxy by the Milky Way’s powerful magnetic fields, and they circle the galaxy for millions of years before coincidentally colliding with an atom or molecule in interstellar space, releasing a gamma ray.

For example, earlier this year the Tibet ASγ Collaboration detected gamma rays with energies up to 100 TeV originating from the supernova remnant G106.3+2.7, which is just 2600 light years away.

The Pierre Auger Observatory in Argentina can detect gamma rays with energies of hundreds of TeV and has hunted for gamma rays even more powerful, while the Large High Altitude Air Shower Observatory in China, which has just begun observing, may be able to detect gamma rays with energies above 1 PeV