SNR G106.3+2.7, a supernova remnant located about 2,600 light-years away in the constellation of Cepheus, is producing protons with energies 10 times greater than the most powerful particle accelerator on Earth, CERN’s Large Hadron Collider, according to an analysis of 12 years of gamma-ray data from NASA’s Fermi Gamma-ray Space Telescope.
In this multiwavelength view, radio emission from SNR G106.3+2.7 is blue and green, while yellow, orange, and red reveal radio waves from a vast cloud of interstellar gas; at center, the pink region shows where protons boosted by the supernova’s shock wave strike the gas, emitting a tell-tale glow of gamma rays. Image credit: Jayanne English, University of Manitoba / NASA / Fermi / Fang et al. / Canadian Galactic Plane Survey / DRAO / FCRAO.
“Theorists think the highest-energy cosmic ray protons in the Milky Way reach a million billion electron volts, or PeV energies,” said Dr. Ke Fang, a physicist at the University of Wisconsin, Madison.
“The precise nature of their sources, which we call PeVatrons, has been difficult to pin down.”
Astronomers have identified a few suspected PeVatrons, including one at the center of our Galaxy.
Naturally, supernova remnants top the list of candidates. Yet out of about 300 known remnants, only a few have been found to emit gamma rays with sufficiently high energies.
One of the best candidates, SNR G106.3+2.7, is located about 2,600 light-years away in the constellation of Cepheus.
A bright pulsar caps the northern end of SNR G106.3+2.7, and astronomers think both objects formed in the same explosion.
Using Fermi’s Large Area Telescope, Dr. Fang and colleagues detected billion-electron-volt (GeV) gamma rays from within the remnant’s extended tail.
The Very Energetic Radiation Imaging Telescope Array System (VERITAS) at the Fred Lawrence Whipple Observatory recorded even higher-energy gamma rays from the same region.
And both the High-Altitude Water Cherenkov Gamma-Ray Observatory and the Tibet AS-Gamma Experiment detected photons with energies of 100 trillion electron volts (TeV) from the area probed by Fermi and VERITAS.
“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,” said Dr. Henrike Fleischhack, an astronomer at the Catholic University of America and NASA’s Goddard Space Flight Center.
“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 SNR G106.3+2.7 is indeed a PeVatron.”
The pulsar, designated PSR J2229+6114, emits its own gamma rays in a lighthouse-like beacon as it spins, and this glow dominates the region to energies of a few GeV.
Most of this emission occurs in the first half of the pulsar’s rotation.
The authors effectively turned off the pulsar by analyzing only gamma rays arriving from the latter part of the cycle. Below 10 GeV, there is no significant emission from the remnant’s tail.
Above this energy, the pulsar’s interference is negligible and the additional source becomes readily apparent.
The detailed analysis favors PeV protons as the particles driving this gamma-ray emission.
“So far, SNR G106.3+2.7 is unique, but it may turn out to be the brightest member of a new population of supernova remnants that emit gamma rays reaching TeV energies,” Dr. Fang said.
“More of them may be revealed through future observations by Fermi and very-high-energy gamma-ray observatories.”
The study was published in the journal Physical Review Letters.
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Ke Fang et al. 2022. Evidence for PeV Proton Acceleration from Fermi-LAT Observations of SNR G106.3+2.7. Phys. Rev. Lett 129 (7): 071101; doi: 10.1103/PhysRevLett.129.071101
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