New research conclusively proves that cosmic rays come from the explosion of distant exploded stars otherwise known as supernovae, say scientists.
Cosmic rays are a form of high-energy radiation made up of [mostly] ultra-fast moving protons that constantly hit earth.
The rays stream through the galaxy interacting with air molecules in Earth’s upper atmosphere creating showers of charged particles. They can have energies much higher than those produced in the Large Hadron Collider.
The mystery of where they come from has long puzzled scientists. While it was suspected that many of the cosmic rays detected at Earth were accelerated in the intense blasts from the death of big stars, the evidence was ambiguous.
Part of the reason is the protons’ positive charge when interact with the earth’s atmosphere. Because they get deflected by any magnetic field they encounter as they move through space, it made it impossible for scientists to accurately track them back to their point of origin.
Researchers’ solution was to look for a characteristic light signature it could lock onto and link to known space events.
BBC News reports that the discovery was made using Nasa’s Fermi telescope which records events up to 10,000 light years away. Scientists studied the unique light that is produced when protons crash into other particles in space.
The telescope looked for a phenomenon known as pion decay.
In simple terms, collisions between the cosmic rays and the slower-moving protons that already exist in the gas and dust around a supernova produce subatomic particles called neutral pions.
The pions then decay rapidly into very high-energy light, or gamma rays. Such rays are not affected by magnetic fields and will travel in straight lines until they fall on Fermi’s main instrument, its Large Area Telescope.
Professor Stefan Funk of the Kavli Institute and Stanford University, who led the study, along with colleagues at the SLAC National Accelerator Laboratory analysed Fermi’s observations of the debris left by two exploded stars named IC 433 and W44.
Both lie within in our galaxy — IC 443 is about 5,000 light-years from Earth; W44 is located about 10,000 light-years away.
Both these supernova remnants are strong sources of the type of gamma rays one would expect from neutral pion decay, says Funk, before explaining “They are not rays at all, they are particles.”
“The interesting thing is that the protons don’t get accelerated in the supernova explosion itself, but they get accelerated in what we call the remnant – the shockwave that is created in the explosion and then moves away through the interstellar medium.
“The process by which the particles are accelerated in this shockwave is actually a slow one. The particles get a little kick in energy every time they cross the shock front, and eventually they get accelerated to these massive energies that we detect here at Earth.
“This acceleration process was first theorised by Enrico Fermi and therefore it is very fitting that the Fermi Gamma Ray Telescope has now found the evidence for it.”
Scientists now believe Professor Funk’s study provides “incontrovertible evidence” of the source of cosmic rays, said The Guardian.
Dr Patrick Slane from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who was not involved in the study, said:
“It’s a clear demonstration of the physical principles that we really thought were happening here; that we really have protons being accelerated to high energies with high efficiencies in these two supernova remnants, and by inference many other supernovas remnants that show similar signals are probably doing the same as well.”
It’s worth noting that cosmic rays can threaten life on earth were it not for the shielding of our planet’s atmosphere and magnetic field. Rays can still cause genetic mutations, The Telegraph notes.
Although exploded stars may account for most of the cosmic rays that are produced in our Milky Way Galaxy, there is another class of cosmic rays that exist at even higher energies. Presently, scientists believe these rays have their origins further afield. One source could be the powerful swathes of energy emitted by giant black holes.
The new study has been published in this week’s Science magazine. Professor Funk
presented it in Boston at the annual meeting of the American Association for the Advancement of Science.