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January 2, 2006

Astrophysicists detect very high-energy gamma rays from the Milky Way

Milagro image

Milagro consists of a large pool of water (about the size of a football field), covered with a light-tight barrier, and instrumented with 723 light-sensitive detectors. This photo was taken when the cover was inflated to allow scientists to work on the detector.
Photo: Rick Dingus

By Tim Stephens

Scientists using the Milagro Gamma-ray Observatory in New Mexico have captured evidence of radiation emitted from the plane of our home galaxy at extremely high energies. The researchers detected "TeV gamma rays"--electromagnetic radiation in the one trillion electronvolt energy range, about a trillion times more energetic than visible light--and determined that the gamma rays were coming from the Milky Way.

The TeV gamma rays are thought to be generated when even more energetic cosmic rays plow into clouds of gas in the galaxy. The new findings, published December 16 in the journal Physical Review Letters, finally confirm theoretical predictions after years of efforts to make the necessary experimental measurements.

"The theoretical expectation was that cosmic rays interacting with material in the galaxy would produce high-energy gamma rays as a result of those collisions. This is the first confirmation of this mechanism at the highest energies," said David Williams, an adjunct professor of physics at UCSC and a coauthor of the paper.

Milagro, operated by scientists from nine institutions, is the first telescope capable of continuously monitoring the northern sky in the TeV energy range. It records the arrival of the highly energetic photons (particles of radiation) by observing the air shower of secondary particles generated when a gamma ray hits the atmosphere. These particles emit light (Cerenkov radiation) when they pass through water. Milagro consists of a huge pool of water (about the size of a football field and holding 6 million gallons), sealed to keep out light and instrumented with light-sensitive detectors.

The Milagro experiment has been in operation since 1999. The Santa Cruz Institute for Particle Physics (SCIPP) at UCSC was one of the founding groups in the collaboration and has played a central role in the design and development of the electronics and other hardware components and in the ongoing operation of the experiment. Seven SCIPP scientists, including Williams, are among the coauthors on the paper describing the new findings.

The analysis of the data was led by Roman Fleysher and Peter Nemethy of New York University. They culled 70,000 TeV photon events from within a region of the Milky Way plane out of a total inventory of about 240 million TeV-level events seen from the same region so far in the Milagro experiment.

"This was a very difficult analysis, because the flux of gamma rays is so small compared to the flux of charged cosmic rays, so it has taken a long time to be able to make the measurement," Williams said.

Previous experiments have detected gamma ray emissions from the galactic plane at energies up to 30 GeV (30 billion electronvolts). This was achieved by the EGRET satellite experiment on the Compton Gamma Ray Observatory. The photons detected by Milagro are on average 100 times more energetic than the highest-energy gamma rays detected by EGRET, Williams said.

"With its wide field of view and long exposure, Milagro has different capabilities from other experiments used to study high-energy gamma rays," he said. "We are achieving sensitivities never before achieved in this energy range, and we hope to extend these measurements to test even more precisely the scientific models for high-energy cosmic rays in the galaxy."

In addition to NYU and UCSC, the other institutions involved in the Milagro collaboration are the University of Wisconsin, University of Maryland, Los Alamos National Laboratory, George Mason University, Michigan State University, University of New Hampshire, and UC Irvine.


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