Mystery gamma rays yield nearest pulsar.

For nearly two decades, Geminga--one of the three brightest sources of gamma rays in the sky--stood alone. Unlike the other two sources, associated with the Crab and Vela pulsars, Geminga apparently showed no pulsations and no traces of accompanying X-rays, radio waves or visible-light emissions.

"This was a big mystery for a long time because Geminga could not be identified with anything else at any other wavelength," says astronomer Charles D. Bailyn of Yale University.

Using data collected by instruments aboard orbiting observatories, astronomers have now detected pulsations in Geminga's gamma-ray emissions, allowing them to match Geminga with a weak, pulsating X-ray source and an extremely faint, visible-light source in the same part of the sky. The evidence also suggests that Geminga lies closer to Earth than any other known pulsar.

"With this discovery, we consider the mystery of Geminga largely solved," Jules P. Halpern of Columbia University in New York City and Stephen S. Holt of NASA's Goddard Space Flight Center in Greenbelt, Md., conclude in the May 21 NATURE.

Three sets of observations proved crucial to making the identification. Using data from the orbiting X-ray observatory ROSAT, Halpern and Holt showed that the X-ray source in the same neighborhood as Geminga pulsates with a 0.237-second period. Prompted by this finding, Goddard's David L. Bertsch and his collaborators looked for and found pulsations with an identical period in Geminga's gamma-ray emissions, as detected by NASA's Gamma Ray Observatory.

Finally, a team of scientists in Italy demonstrated that earlier Geminga gamma-ray data also showed signs of this periodicity, but the evidence had been overlooked. From the historical record available, the researchers could deduce that the Geminga pulsar, already spinning less rapidly than the Vela and Crab pulsars, has continued to slow gradually--just as expected for an ordinary, spinning neutron star.

The dearth of radio emissions probably indicates that Geminga's magnetic field points in the wrong direction for Earth-based observers to intercept its signals. Moreover, the pulsar's long period and the absence of a visible supernova remnant suggest that Geminga is significantly older than the much more energetic Vela and Crab pulsars.

"The reason we see it at all at these other wavelengths ... is that it's probably very close to us," Bailyn says. Astronomers estimate that Geminga may lie no more than 30 parsecs, or about 100 light-years, from Earth. That puts it much closer than any other known neutron star.

Geminga's value to astronomers now rests on its very ordinariness. It's much more typical than the Crab and Vela pulsars. "It's nice that what seemed such a bizarre object fits into the scheme of things so well," Bailyn says.

Theorists have proposed that all pulsars emit most of their energy as gamma rays. They can now use the Geminga gamma-ray data to refine their ideas of how pulsars manage to generate such prodigious outpourings of radiation.