A localised cosmic ray influx
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: 28 November, 2008

Research conducted at the Milagro observatory has uncovered two nearby regions in space that exhibit unusually high readings of cosmic rays.

This is the second finding of a source of near-Earth galactic cosmic rays announced in the past week; scientists working on the ATIC experiment reported a surplus of cosmic ray electrons near to the Earth in the 20 November issue of the journal Nature. You can read our report here.

"These two results may be due to the same, or different, astrophysical phenomenon,” says Jordan Goodman of the University of Maryland and principal investigator for Milagro. "However, they both suggest the presence of high-energy particle acceleration in the vicinity of the Earth. Our new findings point to general locations for the localised excesses of cosmic-ray protons observed with the Milagro observatory."

Showers of high energy particles occur when high energy particles strike the top of the atmosphere. Image: Simon Swardy/U. Chicago/ NASA.

Cosmic rays are charged particles, including protons and
electrons, that are accelerated to high energies from sources both outside and inside our Galaxy. It is unknown exactly what creates these cosmic rays, but likely culprits may include supernovae, quasars or even more exotic processes involving dark matter. Until recently, it was widely believed that cosmic ray particles bombarded the Earth uniformly from all directions. These new findings are the strongest indications yet that the distribution of cosmic rays is much more variable.

"Whatever the source of the protons we observed with Milagro, their path to Earth is deflected by the magnetic field of the Milky Way so that we cannot directly tell exactly where they originate," says Goodman. "And whether the regions of excess seen by Milagro actually point to a source of cosmic rays, or are the result of some other unknown nearby effect is an important question raised by our observations."

Based on seven years worth of observations of the entire sky above the northern hemisphere, and over 200 billion cosmic ray collisions with the Earth's atmosphere, the researchers could see statistical peaks in the number of cosmic ray events originating from relatively small regions of the sky. An excess of cosmic ray protons were found in an area above and to the right of Orion, near the constellation Taurus. The other hot spot was identified as comma-shaped region in the sky near the constellation Gemini.

The Milagro Observatory is located under several metres of water to enhance cosmic ray particle detections. Image: Milagro Observatory.

The Milagro observatory is located in a 60 x 80 x 8 metre covered pond in the Jemez Mountains near Los Alamos, New Mexico, and detects cosmic rays by observing the energetic secondary particles that make it to the surface. The Earth’s atmosphere protects us from direct strikes of high energy cosmic ray particles and when a high-energy cosmic ray enters the atmosphere it loses its energy via interactions with the nuclei that make up the air. These interactions create a large cascade of secondary particles in an ‘air shower’. The particles in the air shower interact much more quickly with water than air, and generate more detectable particles in water, which is why cosmic ray detectors are usually encased in water.

Future observations of cosmic rays may come in the form of a new observatory that Goodman and colleagues have proposed to the National Science Foundation. This second-generation experiment named the High Altitude Water Cherenkov experiment (HAWC) would be built at a high altitude site in Mexico.