Cosmic rays zapping the Earth over the South Pole appear to be coming from particular locations, rather than being distributed uniformly across the sky. Cosmic ray “hotspots” have also been seen in the northern skies too, yet there is no source close enough to produce this strange pattern.
“We don’t know where they are coming from,” says Stefan Westerhoff of the University of Wisconsin, who used the IceCube neutrino observatory at the South Pole with a team of colleagues to create the most comprehensive map to date of the arrival direction of cosmic rays in the southern skies.
IceCube detects muons produced by neutrinos striking ice, but it also detects muons created by cosmic rays hitting Earth’s atmosphere. These cosmic ray muons can be used to figure out the direction of the original cosmic ray particle.
Between May 2009 and May 2010, IceCube has detected 32 billion cosmic-ray muons, with a median energy of about 20 teraelectronvolts (TeV). These muons revealed, with extremely high statistical significance, a southern sky with some regions of excess cosmic rays (“hotspots”) and others with a deficit of cosmic rays (“cold” spots).
Over the past two years, a similar pattern has been seen over the northern skies by the Milagro observatory in Los Alamos, New Mexico, and the Tibet Air Shower array in Yangbajain.
The mystery remains perplexing because the hotspots must be produced within about 0.03 light years of Earth. Further out, galactic magnetic fields should deflect the particles so much that the hotspots would be smeared out across the sky. But no such sources are known to exist.
One of the hotspots seen by IceCube points in the direction of the Vela supernova remnant, a possible source of cosmic rays, but it’s almost 1000 light years away. Its source supernova exploded about 800 light years away in the southern constellation, Vela. The Vela pulsar, made by astronomers at the University of Sydney in 1968, was the first direct observational proof that supernovae form neutron stars.
The stunning image above from the orbiting Chandra X-ray Observatory is the Vela pulsar — the collapsed stellar core within the Vela supernova remnant. The Vela pulsar is a neutron star. More massive than the Sun, it has the density of an atomic nucleus. About 12 miles in diameter it spins 10 times a second as it hurtles through the supernova debris cloud. The pulsar’s electric and magnetic fields accelerate particles to nearly the speed of light, powering the compact x-ray emission nebula revealed in the Chandra picture. The Vela pulsar and the supernova remnant was created by a massive star which exploded over 10,000 years ago.
Cosmic rays coming from such large distances should be constantly buffeted and deflected by galactic magnetic fields on route, and should thus have lost all directionality by the time they reach Earth. In other words, such long-distance cosmic rays should appear to come from all parts of the sky.
Milagro has also seen hotspots that appear to come from implausibly distant sources. As an explanation, Felix Aharonian of the Dublin Institute for Advanced Studies in Ireland and colleagues have suggested that there could be a “tube” of magnetic field lines extending between the source and our solar system, funnelling the cosmic rays towards us. However, Aharonian admits the theory is highly speculative.
Others have proposed that a local phenomenon called magnetic reconnection –- in which solar magnetic field lines cross and rearrange, converting magnetic energy to kinetic energy –- could be accelerating local cosmic rays to energies in the TeV range and beaming them towards Earth, creating the observed hotspots. “It implies that we have a Tevatron in the solar system,” says Aharonian, referring to the particle accelerator at Fermilab in Batavia, Illinois. “That’s also crazy, but it is at least less crazy than other explanations.”