Strongest Source of Gravitational Waves?

Binary white dwarf
A close pair of orbiting white dwarf stars throw off spiral waves of gravitational radiation in this NASA illustration.
Courtesy GSFC / D. Berry.
Astronomers have new evidence that a 21st-magnitude speck in Cancer may be the strongest source in our sky of gravitational waves — weak, elusive ripples in the fabric of space-time that should be washing through the solar system, as predicted by Einstein's general theory of relativity. If the ultrasensitive LISA gravitational-wave detector gets built, this object could be the first thing it sees.

At the American Astronomical Society meeting now under way in Minneapolis, Tod Strohmayer (NASA/Goddard Space Flight Center) reported on periodic X-ray pulsations from a source known as RX J0806.3+1527. The pulsations, found with the Chandra X-Ray Observatory, agree with earlier, visible-light observations indicating that the source is a pair of white-dwarf stars orbiting each other in a tight binary system. The two collapsed stars are separated by only 80,000 kilometers (50,000 miles, or one-fifth the Earth-Moon distance) and circle each other every 5.36 minutes. No known binary star has a shorter orbital period (Sky & Telescope, June 2002, page 20).

From the X-ray and visible-light observations, Strohmayer determined that the system's period is speeding up at a rate of 1.2 milliseconds per year, which translates to an orbital decay of about 2.5 centimeters (1 inch) per hour. That might not sound like much, but it's just the amount of orbital energy that such a system should be losing in the form of gravitational waves, according to general relativity.

"If confirmed, J0806 could be one of the brightest sources of gravitational waves in our galaxy," says Strohmayer. And at a distance of only about 1,600 light-years from us, that could make it the strongest such source in our sky.

The gravitational waves would have the system's orbital frequency of one cycle per 5.36 minutes. Unfortunately, this frequency is much too low to be detected by current, ground-based detectors such as LIGO (the Laser Interferometer Gravitational-Wave Observatory, in Louisiana and Washington). But the proposed ESA/NASA space mission known as LISA (Laser Interferometer Space Antenna) should be able to measure these slow waves with relative ease.

In another session at the meeting, Deepto Chakrabarty (MIT) offered hope that LIGO may detect gravitational waves from a different, much faster kind of source: millisecond pulsars. Like whirling dervishes on steroids, these pulsars spin at least 100 times per second. When pulsars are born they spin very fast, but not for long. The millisecond pulsars that we see were apparently "spun-up" later by mass spiraling in from a binary companion.

Chakrabarty pointed out that none of 100 or so known millisecond pulsars spin faster than 641 times per second, even though they should be able to do so without breaking apart. He noted that if the neutron stars are even slightly irregular they would shed gravitational waves, and this could be the limiting mechanism that prevents even faster spins. If so, high-frequency detectors such as LIGO should be able to detect these waves after several months of sustained observations.