Illustration of NASA’s Fermi Gamma-ray Space Telescope in action. Credit: NASA Goddard Space Flight Center Concept Imaging Laboratory
Astronomers have long searched for the launch sites of some of our galaxy’s highest-energy protons. 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 has shown that shock waves from exploding stars boost particles to speeds comparable to that of light. Called cosmic rays, these particles mostly take the form of protons, but can include atomic nuclei and electrons. Because they all carry an electrical charge, their paths twist as they pass through our galaxy’s magnetic field. Since we can no longer tell from which direction they originated, this masks their birthplace. But when these particles collide with the interstellar gas near the supernova remnant, they produce a telltale glow in gamma rays—the highest-energy light there is.
“Theorists think that the highest-energy cosmic ray protons in the Milky Way reach a million billion electron volts, or PeV energies,” said Ke Fang, assistant professor of physics at the University of Wisconsin, Madison. “The precise nature of their sources, which we call PeVatrons, has been difficult to determine.”
Trapped by chaotic magnetic fields, the particles repeatedly cross the supernova shock wave, gaining speed and energy with each step. Eventually, the remnant can’t hold them anymore and they approach interstellar space.
Boosted to about 10 times the energy accumulated by the world’s most powerful particle accelerator, the Large Hadron Collider, PeV protons are about to escape our galaxy.
Explore how astronomers located a supernova remnant that fires protons at energies 10 times greater than Earth’s most powerful particle accelerator. Credit: NASA Goddard Space Flight Center
Astronomers have identified a few suspicious PeVatrons, including one at the center of our galaxy. Naturally, supernova remnants top the list of candidates. However, of about 300 known remnants, only a few have been found to emit gamma rays at sufficiently high energies.
One stellar wreck in particular has attracted a lot of attention from gamma-ray astronomers. Named G106.3+2.7, it is 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 think 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 remnant’s extended tail. (For comparison, the energy of visible light measures about 2 to 3 electron volts.) The Very Energetic Radiation Imaging Telescope Array System (VERITAS) at the Fred Lawrence Whipple Observatory in southern Arizona recorded energy gamma rays even higher in the same region. And both the High Altitude Water 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 considerable interest for some time, but to crown it as a PeVatron, we need 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. Flight Center in Greenbelt, Maryland. “The problem 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 believe we have shown 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.
This sequence compares the Fermi results in three energy ranges. Pulsar J2229 + 6114 is the bright 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 number of gamma rays and then the excess amounts compared to the expectations of a background model. Brighter colors indicate greater numbers of gamma rays or excessive amounts. At higher energies, a new source of gamma rays emerges, produced when protons accelerated by the supernova’s shock wave collide with a nearby gas cloud. Credit: NASA/Fermi/Fang et al. 2022
The pulsar, J2229+6114, emits its own gamma rays in a lighthouse-like flare as it rotates, and this glow dominates the region at 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 the gamma rays coming from the last part of the cycle. Below 10 GeV, there is no significant tail emission from the remainder.
Above this energy, the pulsar interference is negligible and the additional source becomes apparent. The team’s detailed analysis overwhelmingly favors PeV protons as the particles driving this gamma-ray emission.
“So far, G106.3+2.7 is unique, but it may turn out to be the brightest member of a new population of gamma-ray-emitting supernova remnants reaching TeV energies,” notes Fang. “More may be revealed through future Fermi observations and very high-energy gamma-ray observatories.”
NASA explores cosmic 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 review letters (2022). DOI: 10.1103/PhysRevLett.129.071101
Citation: NASA’s Fermi telescope confirms stellar wreckage as source of extreme cosmic particles (2022, August 10) Retrieved August 10, 2022 from
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