NASA’s Fermi Gamma-ray Space Telescope has confirmed one supernova remnant as a launch site for some of our galaxy’s most energetic protons. Astronomers have long sought the origins of some of our galaxy’s most powerful protons. A new study based on 12 years of data from NASA’s Fermi Gamma-ray Space Telescope confirms that one supernova remnant is just such a place.
Fermi demonstrated that the shock waves of exploding stars accelerate particles to speeds comparable to those of light. Cosmic rays are particles that mostly consist of protons but can also contain atomic nuclei and electrons. Because they all have an electric charge, their paths become jumbled as they pass through our galaxy’s magnetic field. Since we can no longer tell which direction they originated from, this masks their birthplace. But when these particles collide with interstellar gas near the supernova remnant, they produce a tell-tale glow in gamma rays — the highest-energy light there is.
“Theorists think the highest-energy cosmic ray protons in the Milky Way reach a million billion electron volts, or PeV energies,” said Ke Fang, an assistant professor of physics at the University of Wisconsin, Madison. “The precise nature of their sources, which we call PeVatrons, has been difficult to pin down.”
Trapped by chaotic magnetic fields, the particles repeatedly cross the supernova’s shock wave, gaining speed and energy with each passage. Eventually, the remnant can no longer hold them, and they zip off into interstellar space.
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.Ke Fang
PeV protons have been boosted to 10 times the energy of the world’s most powerful particle accelerator, the Large Hadron Collider, and are on the verge of escaping our galaxy entirely.
Astronomers have discovered a few possible PeVatrons, including one at the heart of our galaxy. Naturally, supernova remnants are at the top of the list of possible candidates. However, only a few of the approximately 300 known remnants have been found to emit sufficiently high-energy gamma rays.
Gamma-ray astronomers have focused their attention on one particular star wreck. It’s a comet-shaped cloud called G106.3+2.7 that’s about 2,600 light-years away in the constellation Cepheus. The northern end of the supernova remnant is capped by a bright pulsar, and astronomers believe both objects formed in the same explosion.
Fermi’s primary instrument, the Large Area Telescope, detected billion-electron-volt (GeV) gamma rays from the remnant’s extended tail. (By comparison, the energy of visible light is between 2 and 3 electron volts.) Even higher-energy gamma rays were detected by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) at the Fred Lawrence Whipple Observatory in southern Arizona. In addition, photons with energies of 100 trillion electron volts (TeV) have been detected by the High-Altitude Water Cherenkov Gamma-Ray Observatory in Mexico and the Tibet AS-Gamma Experiment in China from the area studied 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,” explained co-author Henrike Fleischhack at the Catholic University of America in Washington and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “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.”
A paper detailing the findings, led by Fang, was published Aug. 10 in the journal Physical Review Letters. The pulsar, 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 team 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 level, the pulsar’s interference is negligible, and the additional source is easily discernible. According to the team’s detailed analysis, PeV protons are the particles responsible for this gamma-ray emission.
“So far, G106.3+2.7 is the only one,” Fang says, “but it may turn out to be the brightest member of a new population of supernova remnants that emit gamma rays with TeV energies.” “Future observations by Fermi and very-high-energy gamma-ray observatories may reveal more of them.”