Astronomers have identified what's possibly the tightest pair of stars ever seen: two white dwarfs swooping around each other every 5 minutes. If confirmed, the whirling dervishes are revolving twice as fast as the next closest pair. Moreover, the system could be the strongest known source of continual gravitational waves--eerie ripples in space-time that a planned orbiting observatory will chase.
Many stars exist as binaries. If each partner is about as massive as the sun, they collapse when they run out of hydrogen fuel and become white dwarfs--dense Earth-sized remnants of their cores. Perhaps 100 million such pairs fleck our galaxy. Most take years to complete an orbit, but the closest together take mere hours or minutes. In the tightest of these pairs, astrophysicists believe, the more massive dwarf rips matter from its partner. When the gas crashes onto the dominant star, it emits x-rays.
Such x-rays may stream from RX J0806.3+1527, a source spotted by the German satellite ROSAT in the 1990s. In 1999, astronomers realized its signal fluttered every 321 seconds. The x-rays vanished for half that time, as if their source were rotating into and out of view. Now, two independent teams have studied the system with optical telescopes. Gianluca Israel, of the Astronomical Observatory of Rome in Italy, and his colleagues used the European Southern Observatory's Very Large Telescope array in Chile and other instruments to monitor a faint blue star that fluctuates with the same 321-second period in the same position. A team led by astronomer Gavin Ramsay of University College London also detected that cycle 2 months ago with the 2.5-meter Nordic Optical Telescope in the Canary Islands.
A single star, such as a slowly spinning neutron star, cannot explain the x-ray and optical patterns, both teams maintain in reports scheduled to appear in Astronomy & Astrophysics and the Monthly Notices of the Royal Astronomical Society, respectively. Rather, they think the blue star and another white dwarf are locked in a sizzling tango about 80,000 kilometers apart--just one-fifth of the distance from Earth to the moon. X-rays from gas flowing onto the more massive dwarf strike its companion and make it glow blue.
The dwarfs' breakneck pace should whip the fabric of space like an eggbeater and churn out "easily detectable" gravitational waves, says astrophysicist E. Sterl Phinney of the California Institute of Technology in Pasadena. The Laser Interferometer Space Antenna, planned for launch within a decade, should detect these relatively nearby waves and "hear" thousands of similar systems, Phinney says.