Where Do High-Energy Cosmic Rays Come From? A Star’s Last Gasp

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Gamma rays from this supernova remnant have been seen with telescopes since 2007, but the exceptionally energetic light was not detected until 2020, when it was picked up by Mexico’s HAWC Observatory, sparking scientists’ interest who were looking for galactic PeVatrons. When gamma rays reach our atmosphere, they can produce showers of charged particles that can be measured by ground-based telescopes. Using the HAWC data, scientists were able to work backwards and determine that these showers came from gamma rays emanating from the supernova remnant. But they couldn’t say whether the light was generated by protons or fast electrons, which can also radiate gamma rays, as well as X-rays and lower-energy radio waves.

To show that PeV protons were the culprit, Fang’s research team compiled data from a wide range of energies and wavelengths that had been collected by 10 different observatories over the past decade. Then they moved on to computer simulations. By adjusting different values, such as the strength of the magnetic field or the density of the gas cloud, the researchers tried to reproduce the conditions necessary to account for all the different wavelengths of light they had observed. Regardless of what they fit, electrons could not be the only source. Their simulations would match the higher energy data only if they included PeV protons as an additional source of light.

“We were able to rule out that this emission is mainly produced by electrons because the spectrum we came up with would not match the observations,” says Henrike Fleischhack, an astronomer at the Catholic University of America who had first attempted this analysis two years only with the HAWC data set. Doing a multi-wavelength analysis was key, says Fleischhack, because it allowed them to show, for example, that increasing the number of electrons at one wavelength caused a mismatch between the data and the simulation at a other wavelength, which means the only way to explain the full spectrum. of light was in the presence of PeV protons.

“The result required very careful attention to the energy budget,” says David Saltzberg, an astrophysicist at the University of California, Los Angeles who was not involved in the work. “What this really shows is that you need a lot of experiments and a lot of observatories to answer the big questions.”

Looking ahead, Fang is hopeful that more supernova remnant PeVatrons will be found, which will help them find out if this discovery is unique or if all stellar bodies have the ability to accelerate particles to these speeds. “This could be the tip of the iceberg,” he says. Emerging instruments such as the Cherenkov Telescope Array, a gamma-ray observatory with more than 100 telescopes erected in Chile and Spain, may even be able to locate PeVatrons beyond our own galaxy.

Saltzberg also believes that next-generation experiments should be able to see neutrinos (tiny, neutral particles that can also be produced when pions decay) arriving from supernova remnants. Detecting them with the IceCube Neutrino Observatory, which looks for their signatures at the South Pole, would be even more of a smoking gun proving that these sites are PeVatrons because it would indicate the presence of pions. And Fang agrees: “It will be great if telescopes like IceCube can see neutrinos directly from the sources because neutrinos are pure probes of proton interactions; they cannot be made by electrons.”

Ultimately, finding the PeVatrons in our universe is crucial to figuring out how relics of stellar death pave the way for new stars to be born, and how higher-energy particles help fuel this cosmic cycle. Cosmic rays influence pressure and temperature, drive galactic winds, and ionize molecules in fertile regions of stars such as supernova remnants. Some of these stars may go on to form their own planets or one day explode in supernovae, starting the process all over again.

“Studying cosmic rays is almost as important to understanding the origins of life as studying exoplanets or anything else,” says Kerr. “Everything is a very complicated energy system. And we’re just now coming to understand it.”

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