Astronomers have long sought to understand the sites of origin of ultra-high energy protons in our galaxy, the Milky Way. A study led by Ke Fang of the IceCube Particle Astrophysics Center of the University of Wisconsin at Madison, US, using 12 years of data from NASA’s Fermi Gamma-ray Space Telescope (FGST) confirmed that a remnant of a supernova (star explosion) named G106.3+2.7 is such a place, solving a decade-long cosmic mystery. The finding was published in a recent issue of Physical Review Letters.
Nine years ago, the Fermi telescope showed that the shock waves of exploded stars boost particles to speeds comparable to that of light. These cosmic ray particles mostly comprise protons but can also include atomic nuclei and electrons. Since they are all charged particles, their paths become scrambled as they travel through the galactic magnetic field, which masks their origins. But when these particles collide with interstellar gas near a supernova remnant (SNR), they produce a telltale glow in gamma rays, the highest-energy light wave.
“Theorists think the highest-energy cosmic ray protons in the Milky Way reach a million billion electronvolt, or PeV (for peta-electronvolt), energies,” said Fang. “The precise nature of their sources, which we call PeVatrons, has been difficult to pin down.”
The team identified a few suspected PeVatrons, including one at the centre of the Milky Way. SNRs top the list of candidates as sources of cosmic rays, according to Fang, because of how efficiently they bounce charged subatomic particles such as protons back and forth until they reach very high energy levels, a process called diffusive shock acceleration. However, of about 300 known SNRs, only a few have been found to emit gamma rays with sufficiently high energies.
SNR G106.3+2.7 has attracted a lot of attention from gamma-ray astronomers. It is a comet-shaped cloud located about 2,600 light years away in the constellation Cepheus. A bright pulsar caps the northern end of the SNR, and astronomers believe that both objects formed in the same explosion.
“... only a couple [of] SNRs have been observed above 1 TeV (a tera or 1 trillion eV),” Fang says. “Our work provides observational evidence for one of them as a PeVatron.” Fermi’s Large Area Telescope, FGST’s primary instrument, detected billion eV (giga-eV, or GeV) gamma rays from within the remnant’s extended tail. (For comparison, the energy of visible light ranges between about 2 eV and 3 eV.)
“So far, G106.3+2.7 is unique, but it may turn out to be the brightest member of a new population of SNRs that emit gamma rays reaching TeV energies,” said Fang. “More of them may be revealed through future observations by Fermi and very-high-energy gamma-ray observatories.”