A Star That's All Awhirl

Outburst. X-rays from a neutron star surged in Apri

Astronomers have discovered a "missing link" that could explain the formation of the bizarre celestial beacons called millisecond radio pulsars. These neutron stars spin hundreds of times a second and give off a radio blip with each rotation. New observations from NASA's Rossi X-ray Timing Explorer (XTE) satellite, reported in International Astronomical Union (IAU) Circulars and in scientific talks, seem to show a millisecond pulsar's birth process: infalling material spiraling down onto a neutron star, spinning it like a top and spewing out x-rays.

Radio pulsars are thought to consist of a spinning neutron star--a collapsed star about 10 kilometers across but more massive than our sun--sprouting a magnetic field about a billion times more intense than Earth's. Ordinary radio pulsars spin once a second or so, but some pulsars spin nearly 1000 times faster. A popular theory holds that these stars began as slower pulsars that pulled material from a companion star. The material spirals inward, applying a torque to the pulsar's magnetic field and spinning it up.

Bursts of x-rays, emitted from the superheated material that crashes to the neutron star's surface, should signal the spin-up. And that's what Rudy Wijnands and Michiel van der Klis of the University of Amsterdam saw when they analyzed XTE observations of an x-ray beacon that lies perhaps 12,000 light-years away in the direction of the galactic center. The x-rays from the object slightly dim and brighten every 2.5 milliseconds, or thousandths of a second. The variation probably arises as the "hot spot" from infalling material whirls around with the neutron star.

The rapid-fire x-ray pulses are only one sign of a millisecond pulsar being born. The midwife--a companion star--appears to be present as well. In a second result reported in IAU Circulars, Deepto Chakrabarty and Edward Morgan of the Massachusetts Institute of Technology describe slower variations in the x-ray emission, which seem to show that the pulsar is tightly orbiting a companion star. "The missing link has now been found," says Frederick Lamb, a theorist at the University of Illinois, Urbana-Champaign. "It's kind of a dream come true."

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