A Tera-Electronvolt Gamma Ray Originating in the Milky Way

The most energetic parcels of electromagnetic radiation -- Tera-electronvolt gamma rays -- ever determined to have originated in the plane of our home galaxy were observed recently by the Milagro detector, located at high mountain elevations in New Mexico. The potent photons are believed to have been part of the debris spawned when even more energetic cosmic rays struck the matter-dense heart of the Milky Way.

Photons in the TeV range arrive at the Earth very rarely, not often enough to attempt observation from a space-based gamma telescope: for a wristwatch-sized detector you'd have to wait about 10 years, on average. Therefore, terrestrial gamma observations are usually carried out by large-area-arrays attached to the ground.

Milagro, operated by scientists from nine institutions, records the arrival of energetic photons at Earth by observing the air shower of secondary particles generated when the gamma rays hit the atmosphere. These particles betray their presence by the light (Cerenkov radiation) emitted when the particles pass through a 6-million-gallon pond instrumented with photo detectors. This method of observation offers a rough ability to determine the direction of arrival.

For the Milagro experiment so far, 70,000 TeV photon events from within a region of the Milky Way plane were culled from an inventory of about 240 million TeV-level events seen so far seen from the same region. These numbers, says team member Roman Fleysher of New York University (roman.fleysher@physics.nyu.edu), are consistent with theoretical estimates for cosmic ray production.

And where do the cosmic rays get their 100-TeV-and-more energies? Fleysher recalled an analogy due to Enrico Fermi: imagine a pingpong ball trapped between two hole-filled walls zooming toward each other. The ball will ricochet many times, gaining energy, before eventually escaping out one of the holes. Ions in the interstellar medium, perhaps near a collapsed star or an active galactic nucleus (AGN), can get caught up by shock waves and accelerated to high energies like the pingpong ball.

Atkins et al., Physical Review Letters, 16 December 2005