Where do high-energy cosmic rays come from? A star’s last breath

Gamma rays from this supernova remnant have been seen by telescopes since 2007, but exceptionally energetic light was not discovered until 2020 when it was imaged by the HAWC observatory in Mexico, piqued the interest of scientists searching for galactic PeVatrons. When gamma rays reach our atmosphere, they can produce showers of charged particles that can be measured with ground-based telescopes. Using data from HAWC, scientists were able to work backwards and determine that these showers came from gamma rays emanating from the supernova remnant. However, they could not tell whether the light was produced by protons or fast electrons, which can also emit gamma rays and lower energy X-rays and radio waves.

To prove that PeV protons were the culprits, Fang’s research team compiled data from a wide range of energies and wavelengths collected by 10 different observatories over the past decade. Then they turned to computer simulations. By changing various 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. No matter what they set, electrons couldn’t be the only source. Their simulations would only fit the highest energy data if they included PeV protons as an additional light source.

“We were able to rule out that this emission is mainly caused by electrons because the spectrum we got just didn’t match the observations,” says Henrike Fleischhack, an astronomer at the Catholic University of America, who first performed this analysis two years ago Tried only with the HAWC record. Multi-wavelength analysis was key, says Fleischhack, because they were able to show, for example, that increasing the number of electrons at one wavelength led to a discrepancy between data and simulation at a different wavelength — meaning the only possibility is the whole Spectrum of light was explained by the presence of PeV protons.

“The result required very careful attention to energy budgeting,” says David Saltzberg, an astrophysicist at the University of California, Los Angeles, who was not involved in the work. “It really goes to show that it takes a lot of experiments and a lot of observatories to answer the big questions.”

Looking ahead, Fang hopes more supernova remnants of PeVatron will be found, which will help them determine if this discovery is unique or if all stellar corpses have the ability to accelerate particles to such speeds. “That could be the tip of the iceberg,” she says. Emerging instruments like the Cherenkov Telescope Array, a gamma-ray observatory with over 100 telescopes being built in Chile and Spain, could even 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) coming from supernova remnants. Finding these with the IceCube neutrino observatory searching for their tracks at the South Pole would be even more evidence that these locations are PeVatrons, as it would indicate the presence of pions. And Fang agrees: “It would be fantastic if telescopes like IceCube could see neutrinos directly from the sources, because neutrinos are clean probes for proton interactions – they cannot be produced by electrons.”

Ultimately, finding the PeVatrons of our universe is crucial to discovering how the relics of stellar death pave the way for the birth of new stars — and how the most energetic particles help power this cosmic cycle. Cosmic rays affect pressure and temperature, drive galactic winds, and ionize molecules in star-rich regions like supernova remnants. Some of these stars could form their own planets or one day explode into supernovae themselves and start 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. “It’s all an energetic system, which is very complicated. And we’re just beginning to understand it.”

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