Revving up a neutron star.

Like souped up lighthouses, millisecond radio pulsars rotate hundreds of times a second, sending out a radio beacon that sweeps across the sky. Ever since the first millisecond pulsars were discovered in the early 1980s, astronomers have suspected that these compact beasts--rapidly whirling neutron stars--acquired their spin during an earlier stage of evolution, when they stole matter from a companion star.

Such activity should reveal itself by the emission of X rays, generated when the stolen material heats up and spirals onto the neutron star. The intensity of X rays should fluctuate at a rate related to the neutron star's spin.

After 15 years of searching, astronomers have finally confirmed that scenario. Recent observations with the Rossi X-ray Timing Explorer spacecraft revealed a pair of stars whose X-ray emissions vary in intensity every few thousandths of a second. One of the stars is a neutron star.

Astronomers suggest that the X rays will be generated only as long as material continues to fall onto the neutron star. They propose that when the transfer is complete and the star has revved up to its maximum spin, the system will switch on as a millisecond radio pulsar. Radio waves cannot be detected before then because they cannot penetrate the infalling material.

Frederick K. Lamb of the University of Illinois at Urbana-Champaign cited the findings in an overview talk but had not taken part in the study. In an upcoming Nature, Rudy Wijnands of the University of Amsterdam and the Center for High Energy Astrophysics in Amsterdam and Michiel van der Klis of the University of California, Berkeley detail the discovery. In an accompanying article, Deepto Chakrabarty and Edward H. Morgan of the Massachusetts Institute of Technology interpret the results.