Pulsar hints at supernova asymmetry.

The geometry of a fanshaped supernova remnant called G5.4-1.2 has fascinated radio astronomers for nearly a decade: Its structure resembles a crossbow launching an arrow. In 1985, Australian researchers linked a nebula sitting at the tip of the arrow with a nearby pulsar--a rapidly rotating, radio-emitting neutron star.

Now, two radio astronomers in the United States have determined that the pulsar lies within the arrow, where its energetic emissions likely power the nebula's radio broadcasts.

Using the Very Large Array radio telescope near Socorro, N.M., the U.S. team also traced an elongated trail of radio signals leading from the crossbow to the arrow. These suggest that the pulsar was indeed shot out of the supernova remnant. By estamating the pulsar's age (about 15,000 years) and the distance it seems to have traveled from the crossbow to the arrow, the scientists infer that this relatively youthful pulsar races along at an astonishing 2,300 kilometers per second, or 0.5 percent of the speed of light. This would make it by far the fastest neutron star ever found.

The pulsar, a Milky Way resident known as PSR 1757-24, appears to move so rapidly that it has overtaken the expanding shell of debris created at its birth during the supernova explosion. Pulsars form as gravity squeezes the remains of a massive, exploded star. Researchers often think of this squeezing as s symmetric process in which all sides of the dying star experience the same amount of compression.

But the speed and direction of PSR 1757-24 suggest that the compression may have occurred asymmetrically -- for example, with more material raining onto the shrinking star from above than from below. Such a scenario may best explain this pulsar's unusually forceful expulsion from the remnant, asserts Shrinivas R. Kulkarni of the California Institute of Technology in Pasadena. He coauthored the new study with Dale A. Frail of the National Radio Astronomy Observatory in Socorro. They describe their findings in the Aug. 29 NATURE.

The pulsar's velocity, the precise nature of its association with the supernova remnant, and the validity of the asymmetric model all remain uncertain, observes Adam Seth Burrows, a theoretical astrophysicist at the University of Arizona in Tucson. Nonetheless, he says, the new work suggests that "symmetric models might have to be discarded."

Burrows notes that many effects, such as the transfer of heat and mass during a supernova explosion, could create asymmetric conditions. Indeed, studies over the past few years have convinced him that asymmetry may be "central to the [supernova] mechanism," he told SCIENCE NEWS.

If other pulsars are as speedy as the bow-and-arrow pulsar appears, this might help explain why astronomers find only a handful of them in the vicinity of supernova remnants, Burrows adds.

Kulkarni and Frail plan to continue radio observations of PSR 1757-24 for several more years in order to monitor its motion and directly measure its velocity.