Puzzling pulsar offers opportunities.

Puzzling pulsar offers opportunities

In announcing recently the first discovery of a pulsar in one of the globular clusters of stars that form a halo around our galaxy, Shrinivas R. Kulkarni of Caltech in Pasadena pointed out that it was not entirely unexpected. But the manner of its exising, as a millisecond pulsar alone--without a binary companion star--raises problems for the theory of pulsar formation. At the same time it offers opportunities for astronomers who study the globular clusters, which contain the oldest stars associated with out galaxy, for information about the evolution of the galaxy and various cosmological questions.

A pulsar is a neutron star, probably formed in the supernova exploson of an ordinary star. In the magnetized atmosphere of charged particles surrounding the neutron star, a spot produces a beam of radiation: radio waves, sometimes light, sometimes X-rays. The rotation of the star swings this beam around, giving an observer the effect of pulses.

Millisecond pulsars--those with pulse periods of a few milliseconds -- are thought to develop out of a certain kind of binary star system that at first emits X-rays. Because these X-ray binaries are known to be in globular clusters, scientists have mounted a number of searches for pulsars in the clusters. In this one, Andrew G. Lyne and A. Brinklow of the University of Manchester, England, recorded multichannel radio date (that is, everything they could get) from the target globular cluster, known as Messier 28 or NGC 6628, with a 76-meter radio-telescope at the Nuffield Radio Astronomy Observatory at Jodrell Bank, England.

These date had to be searched for correlations revealing the presence of a pulsed signal. It took a supercomputer--a Cray computer at Los Alamos (N.M.) National Laboratory with a program developed by John Middleditch of Los Alamos, Donald C. Backer and Trevor R. Clifton of the University of California at Berkeley and Kulkarni. They found correlations pointing to a pulsed signal of 3 milliseconds period. Given this information, Lyne and Brinklow looked for a signal at that period and found it. The discovery was reported recently in Vancouver, British Columbia, at the meeting of the American Astronomical Society and the Canadian Astronomical Society and at a simultaneous meetng of the Royal Astronomical Society in England.

However, the new pulsar, now called PSR 1821-24, stands alone without a companion star. According to theory, a millisecond pulsar should not be able to spin so fast unless it has a companion from which it can draw matter and rotary motion and so increase its spin rate. Roger W. Romani, Kulkarni and Roger D. Blandford, all of Caltech, in a letter submitted to NATURE, propose two ways in which this pulsar might have come to stand alone.

Both take advantage of the dense packing of stars in globular clusters. In a globular cluster, 100,000 to 1 million stars are packed together in a volume small by astronomical standards. Close encounters between stars are statistically probable in a globular cluster, but virtually impossible in the more normal parts of the galaxy.

In one scenario the neutron star, which is the pulsar, formed alone. Through a close encounter it gained a red giant as a companion. In spinning up the neutron star, the red giant turned into a white dwarf. As the red giant lost more and more mass, the bond between it and the neutron star got weaker until finally a close encounter with a third star detached it.

The second scenario has the neutron star hitting an ordinary star head on. The direct hit disrupts the ordinary star, and its material becomes a disc around the neuron star. The material in the disc gradually falls onto the neutron star and spins it up. After 10 million years the disc disappears.

In whatever way the origin of the millisecond pulsar in the globular cluster is explained, its presence will be a boon to people interested in globular clusters. By its precise timing they can track the motions of the globular cluster in space and so learn about its relation to the galaxy, possibly some of its history and so on.

The precise timing also may be of use to those interested in gravity waves. These are cyclic disturbances of gravitational forces that travel through space; another way of describing them is as ripples in the fabric of space-time itself. If such a wave passes by, stars and similar objects will move slightly, like chips on an undulating ocean. By comparing the timing rates of two or three millisecond pulsars, observers may be able to detect the passage of such a ripple.