Using telescopes at the High Energy Stereoscopic System (HESS) observatory in Namibia, an international team of researchers have discovered that the highest-energy cosmic rays found in the Milky Way are not the result of supernova explosions, as previously believed.
Rather, as reported in Wednesday’s edition of the journal Nature, the HESS Collaboration team has found a different, somewhat unexpected new source of the rays: the supermassive black hole located at the center of the galaxy, which reportedly accelerates cosmic radiation to an energy 100 times thought possible at Earth’s largest particle accelerator, the Large Hadron Collider.
While most cosmic rays are produced by supernova, these explosions cannot explain the highest energy cosmic rays – those with energy levels measured in peta-electronvolts (PeV), Science and Cosmos explained. The cosmic rays were found as they collided and interacted with surrounding gases to produce gamma rays, which astronomers can use to trace their source.
Scientists have known about these high-energy particle waves, which bombard our planet from space, for more than a century. However, their origins have long been the topic of debate. With their recent observations from the HESS telescope array, the authors of the new study found that the distribution of gamma rays coming from the heart of the galaxy matches up with what would be expected if a process occurring near the black hole was emitting PeV cosmic rays.
Pinpointing the exact source of the acceleration will have to wait
“It really demonstrates that there is a central source” of cosmic rays, Werner Hofmann from the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, told Science. Unfortunately, he added that the research provides “very few clues about what the actual accelerator is.” But he and his colleagues will continue monitoring the galactic center to see if they can figure it out.
One possibility presented in the paper is that there may be a region around the black hole where dust and gas are being pulled in by its gravity, and during this process, the electric and magnetic fields of the superheated material is somehow causing protons to accelerate to high energies. An effort to track changes in luminosity and gamma ray distribution in the upcoming weeks, months and even years ahead could confirm or disprove this possibility, the researchers said.
“The supermassive black hole located at the center of the Galaxy, called Sgr A, is the most plausible source of the PeV protons,” Felix Aharonian, also of the Max Planck Institute, said in a statement. “Several possible acceleration regions can be considered, either in the immediate vicinity of the black hole, or further away, where a fraction of the material falling into the black hole is ejected back into the environment, thereby initiating the acceleration of particles.”
The authors explained that the HESS readings of the gamma-ray emissions infer the spectrum of protons accelerated by the central black hole and reveal that it is likely the cause of those rays reaching PeV energy levels. The protons alone are unable to account for all of the cosmic rays detected, but if the black hole had been more active in the past than it is now, it may be responsible for most of the rays being observed from the Earth today, they added.
A definitive answer will have to wait until the construction of a new detector, the Cherenkov Telescope Array (CTA), is detected, Hoffman told Science. The CTA will have more than 100 mirrors distributed across multiple sites in the Northern and Southern hemispheres, and will be capable of producing higher-resolution images than it currently possible, he said.
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Image credit: ESO/L. Calçada
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