Astronomers find best gravitational wave prospect

 作者:霍镙     |      日期:2019-02-27 07:01:02
By Maggie McKee Two burned-out stars are spiralling towards each other so fast they may ripple the fabric of space-time more than any other source near Earth, suggest new observations. A future space mission may detect the ripples – or gravitational waves – within 10 years. Massive, accelerating objects such as black holes and the dense corpses of stars are thought to release gravitational waves as they orbit each other. This allows them to fall inwards until they eventually collide and merge – unleashing even more powerful gravitational radiation. Though widely theorised, no such waves have yet been detected. But new observations with the Chandra X-ray Observatory may have identified the most likely candidate for a future detection. The space telescope has confirmed previous observations suggesting two white dwarfs – the burned-out embers of stars like our Sun – are whipping around each other every 321.5 seconds. Tod Strohmayer, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, US, used Chandra to reveal that the pair’s X-ray emission varies on that timescale – and is gradually quickening. Strohmayer thinks the emission comes from matter dropping from one star onto the other, and says the stars may be edging closer to each other by about 3 centimetres per hour. The pair, called RX J0806.3+1527, appear to be separated by just 80,000 kilometres – five times closer than the distance between the Earth and Moon. That makes them the closest of about 10 known white dwarf binaries. “It’s either the most compact binary known or one of the most unusual systems we’ve ever seen,” says Strohmayer. “Either way it’s got a great story to tell.” The pair lies just 1600 light years from Earth. Other, denser, types of stellar cadavers called neutron stars are thought to be more powerful sources of gravitational waves. But pairs of these stars are rarer, with only three known. And all three pairs are at least tens of times further from Earth – and each other – than RX J0806.3+1527. That means the nearer white dwarf duo may be the strongest source of gravitational waves detectable by astronomers. “There are more of the white dwarf pairs out there,” says Matthew Benacquista, an astrophysicist at Montana State University in Billings, US. “It’s quantity rather than quality.” He says ground-based gravitational wave detectors would be capable of picking up signals from the apocalyptic final seconds before a pair of neutron stars collide. But a future US-European space-based detector called LISA (Laser Interferometer Space Antenna) should be able to detect gravitational waves with frequencies thousands of times lower than those of ground-based instruments. That means LISA should be able to detect the gravitational waves leaking from this pair of white dwarfs. The effect should show up as a small change in the relative spacing of the three spacecraft in the LISA fleet. “When LISA searches for gravitational wave sources, this one might stick out like a sore thumb,” says Strohmayer, who presented the Chandra observations at a meeting of the American Astronomical Society in Minneapolis, Minnesota, US, on Monday. “It is currently the best candidate,” agrees Benacquista, who is involved in planning for LISA, which could launch around 2012. But he says yet more massive pairs of white dwarfs – producing “louder” gravitational waves – could remain to be discovered. That is because astronomers can only detect light from systems where one white dwarf is dumping matter onto the other. “There may be something that hasn’t yet begun transferring mass and so we simply can’t see it,