Two white dwarfs have been discovered on the brink of a merger. In just 900,000 years, material will start to stream from one star to the other (as shown in this artist's conception), beginning the process that may end with a spectacular supernova explosion. Watching these stars fall in will allow astronomers to test Einstein's general theory of relativity as well as the origin of a special class of supernovae.
Credit: David A. Aguilar (CfA)
LONDON: A newly-discovered pair of white dwarfs engaged in a dizzy dance could provide an unprecedented test for Einstein's theory of general relativity.
These burned out stars are set to merge in about 900,000 years - just around the corner in astronomical time - but for now, their tango is tearing up the floor, sending out ripples in space-time.
The find was a by-product of another search, intent on hypervelocity stars - stars moving so fast that they escape their galaxies. Warren Brown, an astrophysicist at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts and his colleagues kept turning up unwanted white dwarfs, what he calls "boring remnants" of stars too small to become supernovae.
These discarded stars caught the eye of Brown's Smithsonian colleague Mukremin Kilic, who noticed that some were extremely underweight - just 20% of the Sun's mass. These stars likely had unseen companions which had stolen the rest of their mass, and upon closer scrutiny, the team found that this was often the case.
And one of these pairs was "a real record-breaker", said Brown, lead author of a new study in The Astrophysical Journal Letters, adding, "These things are something like a third of the Earth-Moon distance away from each other, and they go around each other every 12 minutes."
A sped-up cosmic dance
Pairs of stars, called binaries, are a classic testing ground for general relativity. According to Einstein's theory, the gravity of heavy bodies warps the space-time around them. As they orbit one another, they make waves in space-time that radiate outwards.
The bodies lose energy by creating the gravitational waves, so they fall nearer to one another, speeding up the dance. Neutron star pairs, such as the Hulse-Taylor binary in discovered in 1974, have provided evidence in favour of gravitational waves through their shortening orbits.
But the stars discovered by Brown and his colleagues are 10 times closer and are orbiting 10 times faster. Their proximity means the gravitational attraction between them is extraordinarily powerful, and so they should be losing more energy by sending out stronger waves.
And since Earth-bound astronomers are seeing this orbit edge-on, they can time it very accurately by watching for one star passing in front of the other. "The eclipsing system acts as a great clock," said Brown, allowing the team to closely monitor any changes in the stars' orbit.
Cleanest evidence for gravitational waves
But the binary's best asset is that, in spite of their closeness, neither star is siphoning matter off the other. The transfer of mass between stars can also change the time needed to complete an orbit, making it difficult to disentangle the effects of gravitational waves.
Because this does not occur in the new binary, Brown and his colleagues believe that it could offer the cleanest evidence for gravitational waves yet, through observations of a shortening orbit.
Joel Weisberg, an astrophysicist at Carleton College in Northfield, Minnesota, calls the binary a "remarkable system", but wonders if the researchers aren't missing some other interactions between the stars that could muddy the indirect observation of gravitational waves.
Brown and Kilic acknowledge that this is possible. "The exciting part is we should know the answer later this year," said Brown. They plan to observe the binary again in the autumn. If the system is as clean as they believe, the time between eclipses should be falling at a rate of 5.5 seconds per year. If they measure some other change, they will need to reconsider other factors that could alter the rate of the orbit, such the colossal tides on the smaller of the two stars.
Measuring subtle space-time ripples
"It's a very nice test system for general relativity, and even more importantly, it's a very nice system for a future space-based gravitational wave detector," said Gijs Nelemans, an astrophysicist at Radboud University in Nijmegen, the Netherlands.
He is involved with the developing the new detector, the Laser Interferometer Space Antenna (LISA), which could be launched by the European Space Agency in 10 years.
LISA would measure the subtle ripples in space-time with three spacecraft arranged in an equilateral triangle, one million kilometres to a side. The interference patterns in laser beams sent among the spacecraft would monitor any minute shifts in distance between them, caused by the ebb and flow of gravitational waves.
The signal from the new binary would be so strong that it would be detected in LISA's first week of operation, said the researchers, helping to calibrate the detector.
