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January 19, 2012

Scientists Discover New Gamma-Ray Binary Star System

Contact: Nicole Ruediger, Communications Manager
email: nruedige@umbc.edu
phone: 410-455-5791

Star systems seem like the stuff of Star Trek. But, although we can’t yet travel to them, we can find them. And that’s just what Robin Corbet of UMBC and his colleagues have done -- discovered a rare gamma-ray binary star system.

Gamma-ray binary systems consist of a star and a neutron star or a star and a black hole orbiting each other. A neutron star is more massive than the sun but has been compressed by gravity to matter so dense that its diameter is the size of Washington, D.C.

Corbet and his fellow scientists used the large area telescope, or LAT, on the Fermi satellite to make the discovery. Since it’s launch in 2008 Fermi has been building up a collection of gamma-ray sources in the sky. Researchers have classified a lot of these sources as pulsars or galaxies containing massive black holes.

To prove the existence of this new gamma-ray binary the scientists collected two years of data using the LAT. In gamma-ray binaries, the stars revolve around each other in an orbit that is typically eccentric and the intensity of the gamma-rays will vary as the stars comes closer and then move farther away. The scientists exploited this variability in gamma-ray intensity to look for new binary systems. Once the data was collected they used a mathematical technique called a Fourier transform to look for variations in gamma-ray levels. “This technique has been around since the 1800’s however this is the first time it’s been used to identify a gamma-ray binary star system,” says Corbet.

The data showed regular brightening of gamma-rays about every 17 days, suggesting a highly elliptical orbit. To confirm their findings, the team brought in experts studying radio waves, X-rays and optical light. They then used optical telescopes in South Africa and Chile, a radio telescope in Australia and a NASA X-ray satellite called Swift. All observations confirmed the presence of the gamma-ray binary.

Because the new system, published in the January 13 issue of Science, can't readily be explained by the current models it will help scientists better understand how these systems work.

The more systems that are discovered, says Corbet, the better they can be understood because the models are tested and adjusted with each new discovery. The more systems that a given model fits, the more accurate it is. Gamma-ray binaries, Corbet says, tell us about the origin of neutron stars and the death of the hot large stars in supernova explosions that lead to the birth of a neutron star or a black hole.

Gamma-ray binaries are rare and only a handful have been discovered so far. However scientific models suggest that there should be more. This new system is the one of the brightest sources known to date.

Its discovery suggests there may be more systems out there waiting to be found. “We’re really hoping that this is the tip of the iceberg,” says Corbet, “we’ve got our fingers crossed that as Fermi continues its mission for more years and our measurements get more and more sensitive, we’ll find even more binaries.”

Posted by nruedige at January 19, 2012 4:07 PM