Where does the high-energy cosmic rays come from? Researchers have now found an answer to these questions, thereby confirming a long-held assumption by astronomers. The radiation apparently comes from natural particle accelerators in the vicinity of the remains of exploded stars, the scientists report in the journal Physical Review Letters.
Particles – mainly protons, but also heavier atomic nuclei – are constantly raining down on our earth from space. Most of them have relatively low energies of a few gigaelectronvolts or less. In large part, they come from flares on the surface of the sun. But there are also particles with energies in the range of one petaelectron volt – this corresponds to an energy that is a million times higher.
Astronomers have long suspected that there must be natural particle accelerators in space. There, for example, supernova explosions trigger strong shock waves, which then accelerate electrically charged particles to high energies. Due to the enormous energies around one petaelectronvolt – PeV for short – such particle accelerators are also called PeVatrons. But so far no one has been able to track them down. The problem: charged particles such as the protons of cosmic rays do not move in a straight line through space because strong magnetic fields deflect these particles. As a result, it is not possible simply to deduce the origin of the particles from the direction from which they come.
Astronomers therefore have to search for PeVatrons indirectly: They search for high-energy gamma rays, which are produced when cosmic rays interact with interstellar gas, i.e. the gas between stars. However, this connection is not clear: Even if the cosmic background radiation – the radiation echo of the Big Bang – is scattered by electrons in space, such gamma radiation can arise.
University of Wisconsin’s Ke Fang and her colleagues have now analyzed 12 years of data collected by the Fermi Satellite Observatory and other telescopes of G106.3+2.7, the supernova remnant 2,600 light-years away. While Fermi specializes in gamma rays, the other observatories provided additional data on X-rays and radio waves emanating from the same observed object.
The researchers’ study shows that the energy distribution of the radiation from the supernova remnant over this wide range from radio waves to gamma rays fits very well with a PeVatron – but not with the alternative scenario of scattering of background radiation. Therefore, according to Fang and her colleagues, G106.3+2.7 seems to be one of the most important sources of high-energy cosmic particles. Now the researchers hope to use their method to track down other cosmic particle accelerators.