A large collaboration of researchers from China and Japan have detected the highest-energy photons ever recorded, tens of times higher than the energy we can give to particles in the Large Hadron Collider. The findings are published in Physical Review Letters.
The photons come from the Crab Nebula, the remnant of a supernova that exploded less than a thousand years ago. The nebula is a large cloud of material with a dense and pulsating neutron star at its center that spins on its axis 30 times every second. For decades now, researchers have observed powerful photons coming from it, but none as energetic as the one recently announced.
First, some energy conventions. In particle physics, physicists use the unit electronvolt (eV), given that tiny energies are at play. A red photon is roughly over 1.7 eV. By comparison, particles in accelerators such as the Large Hadron Collider (LHC) are accelerated to much higher energies in the range of trillions of electron volts (TeV). Compared to the macro world we experience, these are still small energies but not that small anymore – one TeV is roughly the kinetic energy of a mosquito in flight.
The Crab Nebula is one of the most studied celestial objects, with powerful TeV photon emissions observed coming from it since 1989. The year after, China and Japan built the Tibet Air Shower (AS) Gamma experiment, which successfully detected many TeV photons from this object and others, including from the center of the Milky Way. In the current experiment, they detected 24 photons with energies higher than 100 TeV, one of which registered an incredible value of 450 TeV.
The experiment doesn’t observe the photons directly, but instead looks at particle showers. When one of these highly energetic photons hits matter in our atmosphere, it generates a shower of secondary particles with much lower energy. By studying these cascades or particles, researchers can work out the energy of the original photon as well as its direction of origin. The team is confident about their results, saying the source of these photons is indeed the Crab Nebula.
The Tibet AS Gamma experiment is located in Yangbajing, 4,300 meters (14,100 feet) above sea level. The high altitude allows the array to better detect high-energy photons. Since 1990, it has been upgraded several times and now sports 733 detectors.