Shadow matter and 'black widow' pulsars.

Shadow matter and 'black widow' pulsars

Radio pulses streaming from a source 3,000 light-years away from Earth tell the dramatic story of a rapidly spinning neutron star that seems to be destroying its stellar companion. Designated PSR1957+20, this recently discovered "black widow" pulsar now spins at the dizzying rate of 622 revolutions per second, completing a rotation in 1.6 milliseconds. Only one known pulsar spins faster.

Astrophysicists speculate that when it first formed 100 million years ago, the pulsar spun more slowly and its companion was a normal star much like the sun. As the companion star exhausted its hydrogen fuel, it swelled into a red giant, and gas from its outer layers began to funnel toward the pulsar. The addition of this gaseous material made the pulsar whirl faster and emit increasing amounts of radiation. That radiation is now blasting what's left of the companion star, forcing it to slough off even more of its mass. The radiation is so intense the companion is likely to vanish within the next 100 million years (SN: 7/30/88, p.72).

This scenario and the pulsar's observed characteristics provide a rich ground for theoretical speculation. In the Dec. 8 NATURE, David Eichler of Ben-Gurion University in Beer Sheva, Israel, suggests looking in the vicinity of isolated millisecond pulsars for traces of dark matter--invisible material, detectable only through its gravitational influence, that could make up as much as 90 percent of the mass in the universe.

"Some forms of dark matter could collect in stellar cores and settle into a sufficiently compact form that it would survive disruption of the host stars," Eichler says. This "shadow" matter could take the form of quark nuggets that become coupled with ordinary matter, or miniature black holes created by the stellar capture of hypothetical particles known as WIMPs (weakly interacting massive particles).

The PSR1957 scenario indicates that isolated millisecond pulsars probably once had companions. Carefully measuring the spin rates of such pulsars may permit researchers to detect the influence of an exotic companion as small as one-billionth of a solar mass. "If pulsars evaporate their companions, leaving behind exotic matter remnants in places where they can be detected, then a survey of pulsars should be capable of finding the remnants, or of determining that they do not exist," Eichler says.

The characteristics of rapidly spinning neutron stars, composed almost entirely of densely packed neutrons, may also provide useful information about the equation of state for nuclear matter. That equation expresses the relationship between the pressure in a material and its density. Experiments to determine this pressure-density relationship for nuclear matter -- and whether neutron stars are "stiff" or "soft" -- are practically impossible to perform on Earth.

Nothing that the two fastest known pulsars may be spinning near or at their limiting frequencies, John L. Friedman of the University of Wisconsin-Milwaukee and his colleagues suggest, also in the Dec. 8 NATURE, that the equation of state for a neutron star must be highly incompressible, or stiff. In other words, if material at the star's equator is rotating near the maximum velocity it can have without flying off, then such a star's nuclear matter would strongly resist compression. This answer has important astrophysical implications because the outcome of supernova explosions--whether the result is a pulsar or a black hole -- depends on how nuclear matter responds to gravitationally induced compression.

However, the argument presented by Friedman and his colleagues doesn't settle the question. The discovery of a pulsar with an even shorter period would negate their argument, whereas the discovery of a neutron star more massive than 1.5 times the sun's mass could destroy other arguments in favor of a "soft" equation of state. Nevertheless, says Gerald E. Brown of the State University of New York at Stony Brook, "their conclusion will invigorate the continuing debate on whether nuclear matter is stiff or soft."