An illustration of NASA’s Fermi Space Gamma-ray Telescope in action. Credit: NASA Goddard Space Flight Center Concept Imaging Laboratory
Astronomers have long searched for the launch sites of some of the highest-energy 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 just such a place.
Fermi showed that shock waves from exploding stars accelerate particles to speeds comparable to the speed of light. Called cosmic rays, these particles are mostly in the form of protons, but can include atomic nuclei and electrons. Because they all carry an electrical charge, their paths are shuffled as they move through our galaxy’s magnetic field. Since we can no longer tell which direction they came from, this masks their birthplace. But when these particles collide with interstellar gas near the supernova remnant, they produce a telltale glow in gamma rays—the highest-energy light.
“Theorists believe that 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 exact nature of their sources, which we call PeVatron, has been difficult to determine.”
Trapped by chaotic magnetic fields, the particles repeatedly cross the supernova shock wave, gaining speed and energy with each pass. Eventually the remnant can no longer contain them and they drift away into interstellar space.
Boosted to about 10 times the energy collected by the world’s most powerful particle accelerator, the Large Hadron Collider, PeV protons are on the verge of escaping our galaxy altogether.
Take a look at how astronomers discovered a supernova remnant that shoots protons to energies 10 times greater than the most powerful particle accelerator on Earth. Credit: NASA Goddard Space Flight Center
Astronomers have identified several suspected PeVatrons, including one at the center of our galaxy. Naturally, supernova remnants top the list of candidates. Yet of the 300 or so known remains, only a few have been found to emit gamma rays of sufficiently high energy.
One particular piece of stellar debris has attracted a lot of attention from gamma-ray astronomers. Named G106.3+2.7, it is a comet-shaped 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 both objects formed in the same explosion.
The Fermi Large Telescope, its primary instrument, detected billion-electronvolt (GeV) gamma rays from the remnant’s elongated tail. (For comparison, the energy of visible light is measured between about 2 and 3 electron volts.) The Very Energy Radiation Telescope System (VERITAS) at the Fred Lawrence Whipple Observatory in southern Arizona recorded even higher-energy gamma rays from the same region. And both the high-altitude Cherenkov Gamma-ray Observatory in Mexico and the Tibet AS-Gamma Experiment in China have 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 significant interest for some time, but in order to crown it as a PeVatron, we need to prove that it accelerates protons,” explained co-author Henrike Fleischhack of the Catholic University of America in Washington and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “The catch is that electrons accelerated to several hundred TeV can produce the same emission. Now, using 12 years of Fermi data, we think we have made the case that G106.3+2.7 is indeed a PeVatron.”
A paper describing the findings, led by Fang, was published Aug. 10 in the journal Physical examination letters.
This sequence compares the Fermi results in three energy ranges. Pulsar J2229+6114 is the brilliant source at the top, northern end of the supernova remnant G106.3+2.7 (outlined in green). In each energy range, the sequence first shows the gamma-ray counts and then the excess amounts compared to expectations from a background model. Brighter colors indicate higher numbers of gamma rays or excess amounts. At the highest energies, a new source of gamma rays appears, produced when protons accelerated by the supernova shock wave hit a nearby gas cloud. Credit: NASA / Fermi / Fang et al. 2022
The pulsar, J2229+6114, emits its own gamma rays in a lighthouse-like beacon as it spins, and this glow dominates the region up to energies of a few GeV. Most of this emission occurs during 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 tail.
Above this energy the pulsar interference is negligible and the additional source becomes readily apparent. The team’s detailed analysis overwhelmingly favors PeV protons as the particles driving this gamma-ray emission.
“For now, 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,” notes Fang. “More of these may be revealed by future observations from Fermi and very high-energy gamma-ray observatories.”
NASA is investigating space mysteries—and this particular puzzle took more than a decade of cutting-edge observations to solve.
Unraveling a century-old mystery: Where the Milky Way’s cosmic rays come from More information: Ke Fang et al, Evidence for PeV proton acceleration from Fermi-LAT observations of SNR G106.3+2.7, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.129.071101
Citation: NASA’s Fermi Telescope Confirms Star Debris Is Source of Extreme Cosmic Particles (2022, August 10) Retrieved August 10, 2022, from
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