The 10,000 year old A supernova remnant 2,600 light-years away is acting like a giant particle accelerator and is spewing cosmic rays across the galaxy, much like the Large Hadron Collider on an astronomical scale.
Data from NASA’s Fermi Space Telescope on supernova remnant G106.3+1.7 is the first time astrophysicists have identified an actual source for high-speed energetic particles called cosmic rays, stripped-back particles (mostly protons) They move at almost the speed of light. They published their findings in a recent article in the journal Physical Review Letters.
What’s new – Instruments on Earth and in space have measured the speed and other properties of incoming cosmic rays, but have never been able to trace a cosmic ray back to its source or identify a specific object in space that produces it. Physicists call these hypothetical objects “PeVatrons,” because they accelerate protons until they carry several PeV of energy (one PeV is 1000 trillion electron volts, which can also be written as one trillion electron volts). And physicist Ke Fang of the University of Wisconsin, Madison, and his colleagues say they’ve finally found one.
A supernova remnant 2,600 light-years from Earth, with the not-so-poetic name G106.3+1.7, emits gamma rays of extremely high energy, up to trillions of electron volts (eV). Fang and his colleagues’ analysis suggests that the best explanation for these high-energy gamma rays is that cosmic rays, launched by the shock wave from the supernova explosion, collide with gas clouds in the surrounding nebula. .
Why does it matter? One hundred and ten years after physicist Victor Hess discovered cosmic rays while carrying scientific instruments 5,300 meters into the sky in a hot air balloon, we understand these high-energy, high-speed particles and their effects on Earth’s atmosphere, computer chips and astronauts. in space, reasonably well.
When they collide with Earth’s atmosphere, cosmic rays ionize nitrogen and oxygen molecules, triggering chemical reactions that have helped shape our planet’s chemistry. They also produce the radioactive isotope carbon-14, which is used in radiocarbon dating. In space, these high-energy particles can be harmful to astronauts outside of Earth’s magnetic field. And when a cosmic ray hits a computer chip or microprocessor, it can cause tiny memory errors.
But where cosmic rays come from remains a mystery.
Previous research suggests that these particles are accelerated to near the speed of light at the edges of black holes in active galactic nuclei and, more often, in shock waves from massive stellar explosions.
“The precise nature of these sources, which we call PeVatrons, has been difficult to pin down,” Fang says in a statement.
When a massive star explodes, subatomic particles like protons get caught up in the shock wave from that explosion as it travels out into space. As the protons bounce back and forth through the shock wave, they gain speed and energy until finally moving fast enough to break free of the shock wave’s magnetic field and out into space. Sometimes they move almost fast enough to escape our galaxy’s gravity altogether.
The problem is that cosmic rays don’t fly in straight lines. Those speedy particles are also electrically charged, so they interact with the galaxy’s magnetic field, which causes them to bounce around so much that it’s nearly impossible to trace a cosmic ray back to its source. And that, in turn, makes it hard for scientists to confirm their theories about where and how these super-energetic, super-fast particles get supercharged in the first place.
Here is the background: When cosmic rays collide with other matter, such as the expanding cloud of gas around a supernova remnant, the collision produces a bright flash of gamma rays. Those light rays, unlike the charged particles that make up cosmic rays, follow a straight path through the galaxy, meaning astronomers can see where they come from.
In other words, searching for high-energy gamma-ray sources offers astrophysicists like Fang and his colleagues the best chance of finding a PeVatron.
A massive star explodes in a supernova every 50 to 100 years in our galaxy, and so far astronomers have found the remains of about 300 such exploding stars. Of those 300, only a few emit gamma rays at energies high enough to make PeVatron’s list of possible suspects. G106.3+2.7 has been at the top of that list for over a decade.
G106.3+2.7 is a vaguely comet-shaped cloud of gas with a bright pulsar (known as J2229+6114) at its northern end. Both the expanding gas cloud and the pulsar are the remains of a massive star that died a fiery, explosive death about 10,000 years ago. Fermi and several other observatories detected high-energy gamma rays from within the supernova, and Fang and his colleagues suspected they might be the product of newly minted cosmic rays colliding with the gas cloud.
But first, they had to rule out the pulsar’s low-energy gamma rays, which were bright enough to drown out almost everything else. A pulsar sends out beams of radiation in only two directions, making it a bit like a spinning beacon in space, so Fang and colleagues looked at data for periods when the pulsar’s beam was pointed away from Earth. Discarding the pulsar’s radiation allowed astrophysicists to see a gamma-ray source of even higher energy (10 to 500 billion electron volts) somewhere in the middle of the cloud.
“The problem is that electrons accelerated to a few hundred trillion electron volts can produce the same emission,” says Henrike Fleischack, a physicist at NASA’s Goddard Space Flight Center and co-author of the study, in a statement. In other words, protons with a few thousand trillion electron volts of energy, or electrons with a few hundred trillion electron volts of energy, can collide with gas clouds to produce gamma rays with 10 to 500 trillion electron volts of energy.
It took 12 years of Fermi data and much analysis to show that the accelerated protons colliding with the gas cloud were probably producing the high-energy gamma rays that Fermi and other observatories had detected.
“With the help of 12 years of Fermi data, we think we have shown that G06.3+2.7 is indeed a PeVatron,” says Fleischack.
Whats Next – Fang and his colleagues, along with other teams of astronomers, will continue to search the skies for high-energy gamma rays that might reveal cosmic rays crashing into the clouds of a supernova.
“More of them could be revealed through future observations of Fermi and very high-energy gamma-ray observatories,” says Fang.
The results could eventually shed light on how many sources of these objects there are in the galaxy and what types of supernovae are most likely to produce them.