Cosmic rays are born in supernovas

Explosions of giant stars accelerate energy particles powerfully enough to spread them across the entire galaxy, striking the Earth constantly, say scientists

By Heitor Shimizu, from Boston

Agência FAPESP – Cosmic rays sound like something out of sci-fi books and films, but they are actually quite common occurrences. These rays exist throughout the galaxy and approach Earth from all sides, hitting the planet’s surface and everything found on it. A new study has just confirmed the origins of these rays.  

Cosmic rays are born as a result of the violent explosions of giant stars, known as supernovas. This conclusion comes from a study announced during the annual meeting of the American Association for the Advancement of Science (AAAS) held in Boston, Massachusetts, in the United States, in February. 

The study was led by an international group of scientists who analyzed four years of data obtained through NASA’s FERMI gamma-ray space telescope. The results represent the first evidence to be considered unequivocal on the origin of cosmic rays. The results were also published in the new edition of the journal Science. 

The scientists identified signals from two old supernovas (W44, located 5,000 light years from Earth, and IC 443, located 10,000 light years away) whose shock waves (produced during their explosions) accelerated protons at velocities near the speed of light, transforming the particles into what have come to be known as cosmic rays. 

According to the study’s leader, German astrophysicist Stefan Funk of Stanford University and the United States Department of Energy’s Kavli Institute for Particle Physics and Cosmology, when these energy-charged protons bombard static protons in gas or stellar dust, the result is gamma rays with distinct characteristics. 

“Cosmic rays aren’t exactly rays, but actually protons. However, not all subatomic particles accelerated in a supernova explosion become cosmic rays—only a small portion do,” said Funk.

According to the scientist, protons make up over 90% of the cosmic rays that hit Earth’s atmosphere in the form of particle showers, producing radiation. 

“They hit us all the time, though they don’t harm us, and they make up an insignificant part of the radiation on the planet. However, these particles have been very important throughout the history of the Solar System because of their influence on the galaxy’s evolution,” said Funk. “One thing I find special about cosmic rays is that they originate during the largest explosions that happen in our galaxy, which accelerate the smallest of particles.” 

Pions

For years, scientists had agreed that the two most probable sources of cosmic rays were either supernova explosions in the Milky Way or jets of energy from black holes beyond the galaxy. These hypotheses were due to the magnitude of the phenomena, as a source would have to possess a great deal of energy to spread particles throughout the entire galaxy. However, until now, no evidence had been found confirming these hypotheses. 

The stellar explosion known as a supernova is able to radiate as much energy as our Sun will emit during its entire existence. The shock waves from a supernova accelerate protons until they are transformed into cosmic rays. During this process, the protons are stuck in shock regions—which accelerate increasingly—by magnetic fields.

“The energies of these protons are much greater than the largest particle accelerators on Earth (the LHC for example) are able to produce,” said Funk.

“It is important, however, to note that these cosmic rays have very low energy levels, less than 10^{16} eV. The strongest cosmic rays like the ones studied at the Pierre Auger Observatory with FAPESP funding are not generated in supernovas and can’t be explained in this article,” states Luiz Vitor de Souza Filho, professor at the Universidade de São Paulo’s Institute of Physics in São Carlos. 

Collisions between the accelerating protons and the protons that are moving much more slowly—and which are found in clouds of dust and gas—lead to the formation of neutral particles called pions. 

These subatomic particles (discovered in the 1940s by Brazil’s Cesar Lattes, Italy’s Giuseppe Occhialini and England’s Cecil Powell) decay into gamma rays, a highly energetic form of light. This decay with its specific gamma-ray signature could be identified by space telescopes such as Fermi,  which proved the origin of the cosmic rays. 

“Until now, we had only theoretical calculations and common sense to guide us in the idea that cosmic rays came from supernovas. Detection directly from the pion decay signatures in supernova remnants closes the circuit in providing observational evidence of a significant component of cosmic rays,” said Jerry Ostriker from Columbia University, who was also involved in the discovery. 

In the next phase of the study, the group will seek to understand the details of the acceleration mechanism and the maximum energies at which a supernova explosion can accelerate protons. 

“It’s interesting that this discovery came at the same time we were celebrating the 100th anniversary of the discovery of cosmic rays,” said Roger Blandford, director of the Kavli Institute, which took part in analysis of the data obtained from the Fermi Large Area Telescope. 

The article Detection of the Characteristic Pion-Decay Signature in Supernova Remnants (10.1126/science.1231160) can be read by Science subscribers at:  www.sciencemag.org.