Thanks to a study that used 12 years of data from NASA’s Fermi Gamma-ray Space Telescope, scientists are finally getting close to accurately identifying PeVatrons, the source of some of the highest energy particles that whip across our galaxy. The study has been documented in a research article published in Physical Review Letters.
Streams of particles called cosmic rays travel at breakneck speeds around our galaxy and they also strike our planet’s atmosphere. They typically consist of protons but sometimes also include atomic nuclei and electrons. They all carry an electric charge, this means that their paths deviate and scramble as they go through our galaxy’s magnetic field.
This means that we can no longer tell which direction they originally came from, effectively masking their birthplace. But when the particles that are part of cosmic rays collide with the gas near supernova remnants, they produce gamma rays; some of the highest-energy forms of radiation that exist.
“Theorists think the highest-energy cosmic ray protons in the Milky Way reach a million billion electron volts, or PeV energies. The precise nature of their sources, which we call PeVatrons, has been difficult to pin down,” said Ke Fang, an assistant professor of physics at the University of Wisconsin, Madison, in a NASA press statement.
These particles get trapped by the chaotic magnetic fields near supernova remnants. They pass through the supernova’s shock wave multiple times and each time they do, they gain speed and energy. Eventually, they can no longer be held by the supernova remnant and will careen off into deep space. These particles are boosted to 10 times the energy that the Large Hadron Collider, the most powerful man-made particle accelerator, can generate.
Scientists have identified a few locations that could be PeVatrons, generating these high-energy extreme cosmic particles. Many of these candidates are naturally supernova remnants. But out of the 300 known remnants, only a few emit gamma rays with sufficiently high energies to be considered as a PeVatron candidate.
G106.3+2.7, a comet-shaped cloud located about 2,600 light years away from us in the direction of the Cepheus constellation, is one of the prime candidates. The northern end of the supernova remnant is marked by the presence of a bright pulsar and astronomers believe both objects formed in the same explosion.
“This object has been a source of considerable interest for a while now, but to crown it as a PeVatron, we have to prove it’s accelerating protons. The catch is that electrons accelerated to a few hundred TeV can produce the same emission. Now, with the help of 12 years of Fermi data, we think we’ve made the case that G106.3+2.7 is indeed a PeVatron,” explained Henrike Fleischhack at the Catholic University of America in Washington and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in a press statement. Fleischhack is one of the co-authors of the research article.
Fermi’s primary instrument, its Large Area Telescope detected GeV (billion electron volt) gamma rays from G106.3+2.7’s extended tail. The VERITAS system at Fred Lawrence Whipple Observatory in southern Arizona recorded even higher-energy gamma rays from the same region. TeV (100 trillion electron volt) readings have been observed by observatories in Mexico and China, in the area probed by Fermi and Veritas.
J2229+6114, the pulsar at the northern end of the supernova remnant emits its own gamma rays as it spins, just like a lighthouse emits light. The glow from the pulsar dominates the region during the first half of the rotation as it emits energies in the range of a few GeV. The research term only analysed gamma rays arriving from the remnant during the latter part of the cycle, effectively turning off the pulsar.
There was no significant emission from the remnant’s tail below 10 GeV. Above that energy, the pulsar’s interference is negligible and it became clear that there is an additional source of radiation. The team conducted detailed analysis that overwhelmingly favours PeV protons as the particles driving the gamma-ray emission.