A surfeit of millisecond pulsars.

A surfeit of millisecond pulsars

Finding a millisecond pulsar -- a rapidly spinning neutron star -- is about as difficult as locating the proverbial needle in a celestial haystack. Nevertheless, recent systematic searches have uncovered nearly a dozen of these faint objects within the Milky Way galaxy and in associated concentrations of stars known as globular clusters. These few, fast-spinning pulsars represent only a tiny fraction of the thousands likely to be found in globular clusters, says astronomer Ramesh Narayan of the University of Arizona in Tucson. Narayan and his collaborators base their estimate on a statistical study of which pulsars are likely to be discovered first.

Of the nearly 500 pulsars now known, millisecond pulsars represent a special class. These stars spin so rapidly -- with periods ranging from 1.6 milliseconds to tens of milliseconds -- that astronomers suspect they achieved their high spin rates by gravitationally stealing matter from nearby companion stars. Such a transfer of matter typically would be marked by the emission of X-rays. When the X-ray emissions eventually cease, the neutron star, now spinning much faster than at earlier stages of its life, would presumably begin sending out radio waves again. Astronomers have identified nearly 100 bright X-ray sources that may end up as "recycled" pulsars.

However, whereas conventional theory suggests that the number of millisecond pulsars and X-ray sources should be approximately equal, Narayan's population estimates indicate 100 times more pulsars than X-ray sources. "There are too many pulsars and not enough X-ray sources," he says.

One way out of the discrepancy is to say that only a small fraction of the recycled pulsars come from bright X-ray sources. For example, the collapse of a star in a supernova may itself produce a millisecond pulsar. A not-yet-confirmed observation that supernova 1987A produced a pulsar with a period of only 0.5 millisecond supports this scenario (SN: 2/18/89, p.100). However, astronomers at Columbia University in New York City contend the observed flashes from 1987A may actually represent rapid vibrations of a slowly spinning neutron star.

Another possibility is that X-ray sources that become pulsars last only 10 million years, rather than 1 billion years as astronomers have assumed. Such a short life is possible if radiation from the vicinity of a neutron star heats the surface of its stellar companion, generating a "wind" that accelerates the companion's evaporation, says Columbia's Jacob Shaham (SN: 12/10/88, p.374). Previous theoretical models of binary X-ray systems assumed the stars interacted only gravitationally.