The most powerful cosmic rays ever observed have astonished astronomers, leading to a search for the locations that could have produced them, with 12 candidates identified so far.
The world is bombarded with cosmic rays, particles accelerated to close to the speed of light by astronomical phenomena such as supernovas and black holes. Rather than detecting these directly, we witness the effects on our atmosphere as the rays trigger “air showers” of particles that cause nitrogen to fluoresce or stimulate Cherenkov light.
Cosmic rays may have an important influence on the Earth, for example by triggering lightning, but they also represent an invaluable tool to help us explore some of the most extreme conditions in the universe, where they originate. That's not a simple process, however. Since they carry electric charge, their paths are curved by magnetic fields, so we can't just look back along their path to know where they came from.
China's Large High Altitude Air Shower Observatory (LHAASO) has been established in Sichuan at an altitude of 4,410 meters (14,470 feet). Although not yet complete, it has already been used to identify gamma rays from 530 rays detected with energies of more than 0.1 PeV. One Peta electronvolts, or PeV, is equivalent to 1 million billion electronvolts (eV). Ultra-high-energy gamma rays are in the 0.1-1 PeV range. Older observatories have seen rays that powerful before, but the LHAASO set also includes some an entire order of magnitude higher. One, carrying a record-breaking 1.4 PeV was so surprising that China Daily reports the chief scientists put six exclamation marks after an order to “further verify its authenticity.”
For comparison, the maximum energy produced by the Large Hadron Collider is 0.013 PeV, and that's done deliberately.
Almost 300 authors have published a paper in Nature describing these rays and attempting to identify their sources. Even if an individual ray's exact direction cannot be identified, we can tell the rough part of the sky it comes from, to an area about the size of the full Moon. For rays originating from the center of the galaxy, this tells us little. That area is so crowded with potential accelerators there is no way to determine the right one. Elsewhere, however, there may be only one or two likely-looking sources in a field that size, at least without going beyond our galaxy.
Two months ago NASA scientists published a paper arguing the highest energy rays we have detected come not from supernova remnants, as previously suspected, but from star clusters. The new paper offers some support for that. The highest energy rays come from Cygnus and the authors identify the Cygnus Cocoon, a nursery for new stars 4,600 light-years from us as, the likely source. However, rays of up to 0.43 PeV come from the area of the sky around the Crab nebula, a recent supernova remnant with a pulsar at its core the authors consider the only confirmed source.
In both cases, we still don't understand what could accelerate particles to such immense energies but with LHAASO set to reach full power this year, we may soon find out.
