Cosmic gamma rays with energies as high as 450 TeV (1012 eV) have been observed by the ASgamma observatory in Tibet – which is run jointly by China and Japan. This shatters the previous record of 75 TeV, which was set by the High-Energy-Gamma-Ray Astronomy observatory on the Canary Islands.

ASgamma detected 24 gamma rays with energies in the 100-450 TeV range. The particles appear to originate in the Crab Nebula, which is a supernova remnant about 6000 light-years away. It is home to a pulsar – a rapidly rotating neutron star that broadcasts a bright beam of electromagnetic radiation.

Astronomers believe that gamma rays in the 100-450 TeV range are created when much higher energy electrons in the petaelectronvolt (1015 eV) range interact with the cosmic microwave background – radiation released just after the Big Bang that permeates the universe. These electrons are believed to be accelerated to such high energies by the swirling magnetic fields generated by the pulsar. Indeed, the ASgamma researchers describe the Crab Nebula pulsar as “the most powerful natural electron accelerator known so far in our galaxy”.

Cosmic shower

ASgamma looks for high-energy gamma rays by detecting the shower of secondary particles that rain down on Earth when a cosmic ray interacts with the atmosphere. By analysing the components of a shower, physicists can work out what type of cosmic ray caused the shower (gamma ray or charged particle) and the energy of the cosmic ray. Located at 4300 m above sea level, ASgamma does this using two types of detectors – plastic scintillators on the surface and Cherenkov detectors buried underground.

Now that cosmic gamma rays with energies above 100 TeV have been detected, ASgamma scientists are keen to find other regions where electrons are accelerated to petaelectronvolt energies. This could help solve one of the most important mysteries of astrophysics – what are the origins of the very highest energy cosmic rays?

A paper describing the observations has been accepted for publication in Physical Review Letters and a preprint is available on arXiv.