TIFR discovers sources of ultra high energy cosmic rays

Cosmic rays are energetic sub-atomic particles,usually charged,originating from outside the solar system,that impinge on the Earth’s atmosphere. They hit the Earth from space and may produce secondary particles that may penetrate to the Earth’s surface,and deeper underground. Some of these particles can pack a considerable amount of energy. These unique particles,with an energy of more than 10^18 (10 followed by 18 zeros) volts,are dubbed ultra high energy cosmic rays.

Last year,the international team—comprising Chakraborti,professor Alak Ray from TIFR’s Department of Astronomy & Astrophysics,Alicia Soderberg and Abraham Loeb from Harvard University,USA,and Poonam Chandra from the Royal Military College,Canada—that was involved in this study had discovered a new class of supernovae. Called SN 2009bb,this was the first supernova with material outflow at relativistic speeds to be detected exclusively through its radio emission. When a massive star blows up in a very short span,it is called a supernova. They are of two kinds—those that have a plasma outflow of speeds much less than that of light (called non-relativistic speeds),and those which blow up at relativistic speeds.

While ordinary supernovae eject stuff at around 10,000 km/s,this is small compared to the speed of light. Relativistic supernovae,on the other hand,throw out stuff at speed comparable to the speed of light,say about half that speed. When a relativistic supernova explodes,it creates a shock and that generates a magnetic field. This magnetic field,gives rise to high energy cosmic rays. The sub-atomic particles go back and forth across this shock,and gain energy,which the magnetic fields keep confined.

“The sources of the highest energy cosmic rays pose an intriguing problem,as the magnetic fields in our galaxy are neither strong enough to contain them nor enough to bend them sufficiently,” says the paper. So far,says the team,nearby gamma ray bursts (GRBs),hypernovae,active galactic nuclei and their flares have been suggested and debated as possible sources. However,the team of scientists have now found clues which indicate that the recently discovered relativistic supernovae could be sources of these enigmatic particles. “We find that a new class of supernovae,discovered last year,satisfy for the first time,all the criteria required for the acceleration of ultra high energy cosmic rays. We hope to address the most important unsolved puzzle in astro-particle-physics,namely,where do the highest energy cosmic rays come from?” said Chakraborti.

This work,says Ray,will offer a new avenue for research in ultra high energy cosmic rays. “Since such high energies of particles are not attainable by terrestrial accelerators,understanding their origin is a crucial step,” said Ray. “While the known laws of physics have been tested by accelerating particles to high energies in particle accelerators such as the Large Hadron Collider (LHC),cosmic rays are helping physicists probe even higher energies,” said Chakraborti.

The study,titled “ultra-high-energy cosmic rays from relativistic supernovae”,was initiated a year ago. “It’s a very interesting paper and is at the frontier area of science. The team has got some vital clues which point to the origin of such rays and it needs to be explored more,” said professor Govind Swarup,former director of TIFR’s National Centre for Radio Astrophysics.MIHIKA BASU