Two Stars Poised to Merge

The expected merging of two white dwarf stars would unleash a burst of gravitational waves. CREDIT: NASA/GSFC/T. Strohmayer

MINNEAPOLIS, MN - Two dense stars whipping around each other at breakneck speed may be the strongest known source of Einstein's space-trembling gravity waves.

The double star - called RX J0806 - was discovered in 1994 in X-rays. Later shown to be blinking on and off every 5.4 minutes, the two-star setup is believed to be a pair of white dwarfs - the dense ashes of burnt-out stars - rotating around each other.

The implied separation is just 50,000 miles - a mere one-fifth the distance between the Earth and the Moon, making this the closest stellar pair ever observed. The tangled duo should be booming out gravity waves - undulations in the fabric of space and time predicted by Einstein's theory of general relativity.

"Those waves have still not been detected directly, but there is indirect evidence," said Tod Strohmayer, who presented the results here last week at a meeting of the American Astronomical Society meeting.

Strohmayer, of NASA's Goddard Space Flight Center, presented data from the Chandra X-ray Observatory that shows the time between the X-ray blips is decreasing by 1.2 milliseconds every year. The implication is that the dwarfs are orbiting faster and faster, as they gradually fall into each other at a rate of one inch per hour.

This "spin-up" is consistent with rotational energy being lost to gravity waves. The amount of energy radiated in gravity waves in all directions could be 100 times the energy our Sun puts out in light, according to Strohmayer.

The study will be detailed in the Astrophysical Journal.

Gravitational ticking

This is not the first time that two objects have been seen spinning faster over time. The radio pulses from the Hulse-Taylor pulsar were found to be getting closer together due to gravity wave emission between the pulsar and an orbiting neutron star companion.

Also presented at this meeting, Ingrid Stairs of the University of British Columbia and colleagues found that the distance between a double-pulsar system (PSR J0737-3039A and B) is shrinking at about two and a half inches per day, just as expected from energy loss to gravity waves.

"Things are ticking along just as Einstein would have predicted," Stairs said.

Although gravity waves have yet to be detected directly, future missions will be sure to look at this newly-identified white dwarf pair, which is 1,600 light years from Earth in the constellation Cancer.

"When LISA [the Laser Interferometer Space Antenna] searches for gravitational wave sources, this one might stick out like a sore thumb," Strohmayer said.

LISA, scheduled for 2012 launch, will involve three satellites orbiting 3 million miles (5 million kilometers) apart in a triangle formation. As gravity waves - traveling at the speed of light - wash up on the Earth's shores, the satellites can detect a change in their separation far less than the width of an atom.

"As a gravitational wave source J0806 is very bright," said Lee Finn, the director of the Center for Gravitational Wave Physics at Penn State University, in an email message.

Finn expects that - relative to each other - the LISA satellites will bob one quadrillionth of a meter (roughly the size of an atomic nucleus) every 160 seconds in the J0806 surf.

Opposing faces

The white dwarfs are assumed to each have about half the mass of the Sun, but their radii are likely comparable to that of the Earth, which means their matter is densely packed. The compact size allows the dwarfs to orbit far closer than normal stars could.

Questions remain as to what mechanism is causing the observed X-rays to go on and off. One possibility is that a hot X-ray spot on one of the dwarfs comes in and out of our view as the dwarfs rotate around each other.

Evidence for this hypothesis comes from the fact that the pair blinks in visible light as well. The timing of the visible flashes are opposite that of the X-ray flashes: just as the X-rays turn on/off, the visible turns off/on.

This offset might be because the hot X-rays from the spot heat up the opposing face of the other dwarf - causing it to emit visible light. It is as if the white dwarfs are two dancers twirling around each other - one with a smile beaming in X-rays, the other reflecting that gaze in the visible.

When will this dance end?

Strohmayer said the two dwarfs should continue losing energy to gravity waves and merge between 500,000 and one million years from now. That event, theory says, would unleash a colossal burst of gravitational waves.

A movie depicting how the merger might play out is available here.

This article is part of SPACE.com's weekly Mystery Monday series.

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