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Orbiting Stars Flooding Space With Exotic Gravitational Waves
05.30.05
 
A scientist using NASA's Chandra X-ray Observatory has found that two white dwarf stars orbiting each other in a death grip, destined to merge spectacularly, may be flooding space right now with gravitational waves. These waves are ripples in space-time predicted by Einstein but never detected directly.

Still from animation showing the white dwarf stars in a death spiral. Image/animation to right: As a pair of white dwarfs steadily spiral inward, they churn the sea of space-time. The gravitational waves become more intense as the stars accelerate faster and faster, edging ever closer to a merger. Stars in the binary system RX J0806.3+1527 are only 50,000 miles apart. Merging white dwarfs might create a neutron star. Although they won't merge for another few hundred thousand years, these stars now might be one of the brightest sources of gravitational waves known. The Chandra X-ray Observatory has found indirect evidence for this; direct detection could come with the launch of the LISA mission. Click on image to view animation (no audio) Credit: NASA/Dana Berry, Sky Works Digital

Dr. Tod Strohmayer of NASA Goddard Space Flight Center has found that the system's orbit appears to be shrinking about one inch per hour. This is consistent with Einstein's equations. The stars -- only about 50,000 miles apart, a fifth of the distance from the Earth to the Moon -- are whipping around in excess of a million miles per hour. As the stars steadily spiral inward, they churn the sea of space-time and release more and more gravitational waves.

If these measurements are confirmed, this star system would be one of the brightest known sources of gravitational waves and among the first to be detected directly with an upcoming space mission called LISA, the Laser Interferometer Space Antenna.

Still collage from animation showing the collision of the white dwarf stars. Image to left: These are three stills from the movie noted above which show the white dwarfs circling each other and then colliding. Click on image to enlarge. Credit: NASA/Dana Berry, Sky Works Digital

"This source might be releasing more gravitational waves right now than light waves," said Strohmayer, who presents his results today at the American Astronomical Society meeting in Minneapolis, Minn. "When LISA searches for gravitational wave sources, this one might stick out like a sore thumb."

Einstein predicted that accelerating, massive objects emit gravitational waves, which propagate through space at light speed. A passing wave will cause the Earth, Moon and all matter to bob, like a buoy on the ocean. Chandra cannot detect these waves directly; special instruments like LISA are needed.

White dwarfs are remnants of stars like our Sun that have used up all their fuel. White dwarfs, along with neutron stars and black holes, are called compact objects because they pack a lot of mass into a small volume. White dwarfs have about half the mass of the Sun, yet are only about the size of Earth. Being compact, white dwarfs can find themselves in very tight orbits.

The orbit of this white dwarf pair, called RX J0806.3+1527, might be the smallest of any binary in the Galaxy. The system's orbital period is 321.5 seconds -- barely more than five minutes -- and is decreasing by 1.2 milliseconds every year. The five-minute period is likely the system's orbital period, but definitive confirmation of this fact awaits. The other possibility is that it represents the spin period of one of its white dwarfs.

"It's either the most compact binary known or one of the most unusual systems we've ever seen," said Strohmayer. "Either way it's got a great story to tell."

Strohmayer's Chandra X-ray observation tightens orbital decay estimates made through optical observations in recent years independently by teams led by GianLuca Israel of the Astronomical Observatory of Rome and by Pasi Hakala of the University of Helsinki.

Still image from the animation showing the Chandra spacecraft in orbit. Image/animation to right: This animation shows the Chandra spacecraft in orbit. Click on image to view animation (no audio). Credit: NASA

This white dwarf pair is similar to the orbiting neutron stars leaking gravitational waves discovered by Russell Hulse and Joseph Taylor, who won the 1993 Nobel Prize in Physics. Similar binary stars have been detected since then. While this new star system is not as well characterized as the Hulse-Taylor system, the stars could be about 1,000 times brighter in gravitational waves if the orbit is indeed shortening and quickening, as the data suggest.

With Chandra data, Strohmayer calculated that the system's five-minute period is speeding up at a rate of 3.6x10^-16 hertz per second. Although small sounding, this number is big in the realm of orbital decays -- 100,000 times larger than that of the Hulse-Taylor pulsar with its eight-hour orbit, for example.

Still from an animation depicting LISA, the Laser Interferometer Space Antenna missionImage/animation to left: This animation shows the LISA spacecraft in orbit. Click on image to view animation (no audio). Credit: NASA

LISA is a joint venture between NASA, the European Space Agency and European national space agencies. LISA works by setting out three spacecraft -- buoys in spacetime -- and measuring the change in their separation as they bob in response to passing gravitational waves. Launch of this revolutionary observatory is planned for early next decade.

Strohmayer's results will appear in an upcoming issue of The Astrophysical Journal.

 
 
Christopher Wanjek
Goddard Space Flight Center