X-ray source with a quasi beat.

Bright celestial X-ray sources tend to be binary star systems. Astrophysicists tend to believe that one member of such binaries is a neutron star. In the cases of the X-ray sources found in the "galactic bulge," the fat central region of our galaxy, researchers find reason to believe that such neutron stars should be "spun up" by the dynamics of the binary system until they are rotating from hundreds to a thousand times a second. Such neutron stars could emit X-ray pulses with a precisely even period, timed according to their rotation. Now, a mostly European group looking for such pulsations in the X-ray sources GX5-1 has found instead "quasiperiodic emissions," chains of pulses with varying periods. This seems to make GX5-1 the first example of a newly discovered class of celestial objects, which the discoverers of the pulsations call "quasiperiodic objects," or QPOs.

The observers used the European Space Agency's EXOSAT satellite. The story proves the adage: If at first you don't succeed, try at least once again. Searches of data taken by various pieces of X-ray observing equipment had failed to find any fast periodic pulsations in galactic bulge sources. Astronomers thought that if there were any such things, EXOSAT, which is a particularly sensitive X-ray detector, should find them, but even a 1983 search with EXOSAT failed to turn up evidence of periodic pulses. However, the present observing group decided that a second try was warranted. They got authorization to do this and made their observations in September 1984.

Analysis of the data seemed at first disappointing, as the sought-for precise periodicity did not appear. However, a closer look brought out the evidence for the quasiperiodic pulses, trains of pulses with periods ranging from 20 to 40 oscillations per second. The results were reported in the July 18 NATURE by Michiel van der Klis of the European Space Agency's Space Science Department in Noordwijk, the Netherlands; F. Jansen of the Laboratory for Space Research in Leiden, the Netherlands; J. van Paradijs and E.P.J. van den Heuvel of the University of Amsterdam; Walter H.G. Lewin of Massachusetts Institute of Technology; and J.E. Trumper and M. Sztajno of the Max Planck Institute for Extraterrestrial Physics in Garching, West Germany.

The discovery has reportedly caused great excitement in the X-ray astronomical community. As the news has spread, quasiperiodic pulsations have also been reported in two long-known X-ray sources, Scorpio X-1 and Cygnus X-2. The GX5-1 group took a second look on April 29, 1985, and again found the quasiperiodic oscillations in that object.

A number of theoretical suggestions to explain the quasiperiodic pulsations have already been put forth. The one most favored by the GX5-1 observing group comes from M. Ali Alpar of the University of Illinois at Urbana and Jacob Shaham of Columbia University in New York City. This theory proposes that the quasiperiodic pulses represent a "beat" frequency, a combination of the precise rotations of the neutron star with the variable rotations of an accretion disk around it.

Astrophysicists' general picture of these binary X-ray sources is that of a neutron star gravitationally bound to a more ordinary star. The strong gravity of the neutron star pulls material from the companion. Because of the rotations and orbital motion of the system, this material does not fall directly onto the neutron star but forms an accretion disk around it. The accretion disk is tied to the neutron star's magnetic field and rotates with it. Matter moves gradually through the disk before falling on the neutron star.

What Alpar and Shaham propose is that material on the inside edge of the accretion disk is perturbed by the strength of the neutron star's magnetic field and breaks into clumps. The formation and rotation of these clumps is irregular, depending on the infalling material's flow rate, which changes from time to time. The quasiperiodic pulsations would then be a combination of the neutron star's precise rotation (which is hidden from observation from earth) and the clumps' irregular rotations.

The GX5-1 observers say that from this theory one can calculate that the neutron star's rotation period is about 10.5 milliseconds, or about 100 rotations per second, and that the lifetimes of the blobs vary between 0.1 and 0.2 second.