Astronomers have long searched for the launch sites of some of the most energetic protons in our galaxy. Now, a study using 12 years of data from NASA’s Fermi Gamma-ray Space Telescope confirms that a supernova remnant is one such place.
Fermi has shown that the shock wave of an exploding star boosts the particle’s velocity to a speed comparable to the speed of light. Known as cosmic rays, these particles come primarily in the form of protons, but can also include atomic nuclei and electrons. Because they both carry an electric charge, their paths get chaotic as they pass through our galaxy’s magnetic field. This obscures their birthplace since we can no longer tell which direction they came from. But when these particles collide with interstellar gas near the supernova remnant, they produce a distinct glow of gamma rays — the most energetic light out there.
“Theorists believe that the most energetic cosmic ray protons in the Milky Way reach energies of one million electron volts, or PeV,” said Ke Fang, assistant professor of physics at the University of Wisconsin-Madison. “The exact nature of the source of what we call PeVatrons is difficult to pin down.”
Trapped by the chaotic magnetic field, the particles repeatedly travel through the supernova’s shock wave, gaining speed and energy with each pass. Eventually, the remnants could no longer hold them, and they sped off into interstellar space.
Boosted by about 10 times the energy collected by the Large Hadron Collider, the world’s most powerful particle accelerator, PeV protons are on the cusp of escaping our galaxy entirely.
Astronomers have identified some suspicious PeVatrons, including one at the center of our galaxy. Naturally, supernova remnants topped the shortlist. However, of the roughly 300 known remnants, only a few have been found to emit gamma rays with sufficiently high energies.
A peculiar stellar remnant has caught the attention of gamma-ray astronomers. Known as G106.3+2.7, it’s a comet-like cloud located about 2,600 light-years away in the constellation Cepheus. A bright pulsar covers the northern end of the supernova remnant, and astronomers believe the two objects formed in the same explosion.
Fermi’s Large Area Telescope, its main instrument, detected billion-electron-volt (GeV) gamma rays from the tail extending from the wreckage. (In comparison, visible light has about 2 to 3 electron volts.) The High Energy Radiation Imaging Telescope Array System (VERITAS) at the Fred Lawrence Whipple Observatory in southern Arizona recorded higher-energy gamma rays from the same region . The high-altitude water Cherenkov Gamma-ray Observatory in Mexico and the AS-Gamma Experiment in Tibet in China both detected photons with energies of 100 trillion electron volts (TeV) from the region probed by Fermi and VERITAS.
“This object has generated considerable interest now, but to name it a PeVatron, we have to show that it is accelerating protons,” explained co-author Henrike Fleischhack of the Catholic University of America in Washington and NASA’s Goddard Space, Maryland. Flight Center in Greenbelt, Fla. “The problem is that electrons accelerated to a few hundred TeV can produce the same emission. Now, with 12 years of Fermi data, we think we’ve shown that G106.3+2.7 is indeed a PeVatron.”
A paper detailing the findings, led by Fang, was published in the journal Aug. 10. Physical Review Letters.
Pulsar J2229+6114 emits its own gamma rays in beacon-like beacons as it rotates, and this glow dominates the region with energies of a few GeV. Most of this emission occurs during the first half of the pulsar’s rotation. The team effectively shut down the pulsar by analyzing only the gamma rays arriving from late in the cycle. Below 10 GeV, there is no apparent emission from the residue tail.
Above this energy, the pulsar’s interference is negligible, and additional sources become apparent. The team’s detailed analysis overwhelmingly supports PeV protons as the particles driving this gamma-ray emission.
“So far, G106.3+2.7 is unique, but it could be the brightest member of the new group of supernova remnants emitting gamma rays up to TeV energies,” Fang noted. “More may be discovered through future observations by Fermi and the Ultra-High Energy Gamma-ray Observatory.”
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Ke Fang et al, Evidence for PeV Proton Acceleration from Fermi-LAT Observations of SNR G106.3+2.7, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.071101
Citation: NASA’s Fermi Telescope confirms debris as source of extreme cosmic particle (10 Aug 2022), 11 Aug 2022 from https://phys.org/news/2022-08-nasa-fermi -telescope-star-source.html retrieved
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