A Puzzling Pulsar

A discovery by Australian scientists will force astronomers to rethink their models of pulsars, tiny stars that fling radio beams into the universe as they spin. In this week's Nature, the team reports finding a pulsar that spins so slowly that, according to current theories, it can't be a pulsar at all.

When astronomers detected the first pulsar signal in 1967, they figured that only a technologically advanced civilization could send out radio blips with such great regularity, and kept the discovery secret for a while. Now, they know that pulsars are neutron stars, made up of tightly packed neutrons; they have a diameter of less than 20 kilometers and are remnants of supernova explosions. Most known pulsars spin very fast, with periods of less than 1 second. That speed, together with their extremely strong magnetic fields, causes neutron stars to accelerate charged particles, which emit radio waves in a narrow beam that sweeps across space like the rotating beam of a lighthouse. Radio telescopes can capture the pulses every time the beam hits Earth.

Theorists had calculated that neutron stars would not emit radio waves at all if either their magnetic field or rotation speed was too low. But a team led by astronomer Matthew Young of the University of Western Australia in Nedlands has remeasured the rotation rate of a pulsar called J2144-3933 using the Parkes radio telescope in New South Wales. When the pulsar was first studied in the early 1990s, it seemed to send out a pulse every 2.84 seconds, fast enough to satisfy theorists. But Young and his colleagues think a software problem may have thrown off that measurement. They clocked a rotation period of 8.51 seconds--a speed at which the neutron star shouldn't produce radio waves at all.

"This [finding] is very surprising and leaves a lot of theorists scratching their heads," says Vicky Kaspi, an astrophysicist at the Massachusetts Institute of Technology. As soon as she learned about the study, says Kaspi, she submitted a grant proposal to check the periods of other pulsars using the Arecibo Telescope in Puerto Rico. The new finding proves that radio emission by pulsars is still not very well understood, says Kaspi.