Cosmic rays point to Orion

Scientists at Los Alamos National Laboratory's Milagro observatory and their collaborators have pinpointed two side-by-side sectors in our region of the Galaxy that are generating more than their share of cosmic rays. Seven years of recording over 200 billion cosmic ray collisions with the Earth's atmosphere went into the calculations.

"Earth is always bombarded by cosmic rays," said Brenda Dingus, who heads operations at the Milagro observatory. "Now we've found that instead of being uniform, there are localized sources."

Dingus is one of 33 co-authors from 16 national and international institutions of a paper published this week in Physical Review Letters. The announcement was the second finding in a week that suggested cosmic rays might have something other than a random distribution.

"The source of cosmic rays has been a 100-year-old problem for astrophysicists," said John Pretz, another LANL scientist on the team. "With the Milagro observatory, we identified two distinct regions with an excess of cosmic rays. These regions are relatively tiny bumps on the background of cosmic rays, which is why they were missed for so long. This discovery calls into question our understanding of cosmic rays and raises the possibility that an unknown source or magnetic effect near our solar system is responsible for these observations."

The two sources appear to be on either side of the constellation of Orion in a direction opposite from the galactic center and farther out than the sun. One theory that hasn't been proven is that cosmic rays are born some time after a supernova, in the turbulence of the shock wave perhaps tens of thousands of years after the explosion.

"You can see the optical light, the brightening and fading," said Dingus. "You can make guesses, but you can't see the cosmic rays." After they are born, they would usually seem to wander randomly through the universe. Because a cosmic ray is made of the nucleus of an atom that has been stripped of its electrons, it is a charged particle easily deflected by magnetic fields and hard to trace to a source. "Here we've found something pointing back," Dingus said. "Either it was produced within a few light months of the Earth, or a magnetic field is aligning these particles to come straight at us."

A light-month is the distance light travels in one month in a vacuum, about 500 billion miles. The disk of the Milky Way Galaxy is about 100,000 light years across. Our sun is in the Orion arm of the galaxy; Orion is 2,000 light years out.

"There is currently no compelling explanation for the excess in Region A and Region B," according to the authors of the paper, who describe them as "two unexplained regions of excess with high significance." Along with Dingus and Pretz, the Los Alamos researchers included Gus Sinnis, Gary Walker and Petra Huntemeyer.