Pulsar pas de deux.

Pulsars are the whirling dervishes of stars. These ultracompact bodies rotate hundreds of times each second, sweeping radio waves across space. One pulsar is exotic enough, but a closely orbiting pair offers more than double the information about the extreme gravity of these dense bodies.

A newly discovered pulsar pair, which lies some 1,800 light-years from Earth, is the only known duo among the more than 1,400 pulsars found since 1967. Andrew G. Lyne of the University of Manchester in England and his colleagues report the find in an upcoming Science.

The researchers had originally thought that the pair consists of a single pulsar spinning 44 times a second around a neutron star that isn't sending out radio pulses. Those initial findings were reported in 2002. Follow-up observations with radio telescopes in New South Wales, Australia, and at the University of Manchester revealed that the companion also pulses radio waves, but at a slower rate of once every 2.8 seconds. The interaction of these two ultradense bodies provides a unique opportunity to test the general theory of relativity, Lyne says.

By chance, the orbits of the partners lie nearly edge-on to Earth, so that one pulsar's signal periodically eclipses the other's. Moreover, the intensity of radio waves from the slower pulsar varies dramatically, almost petering out before returning to full strength. The variation is probably due to the blasts of radiation and the relentless wind of particles that this pulsar must withstand as its elongated orbit takes it alternatively nearer to and farther from its faster, brighter partner, the team suggests. The eclipsing and the signal variation offer astronomers the first opportunities to probe a pulsar's outer atmosphere.--R.C.