A pulsar's beat goes on, but slower.

A pulsar's best goes on, but slower

Every 2 minutes or so, the X-ray signal from a stellar source known as GX1+4 dims and then brightens. During the 1970s, the rate of those pulsations was slowly increasing. Around 1980, the source unexpectedly became so faint even sensitive satellite instruments couldn't detect it. Now the Japanese satellite Ginga has observed a newly strengthened X-ray signal coming from GX1+4. This time, the pulsations appear to be gradually slowing--an unusual shift that astronomers find difficilt to explain.

GX1+4 is an X-ray pulsar -- a dense, compact, rapidly spinning neutron star that draws gaseous material from a nearby companion staar. The captured material forms into a disk or ring, and the neutron star's intense magnetic field funnels some of the gas into the star's polar regions, where the falling gas generates X-rays. As the star spins, X-ray beams from these regions sweep the sky like lighthouses beacons to produce the characteristic pulsed signed seen from earth. By observing the pulses, astronomers can determine the star's spin rate.

Matter falling onto a star can alter its rotation period. As the material spirals in from a ring rotating in the same direction as the star, it adds its spin to the star's spin, and the star's rotation rate increases. How such a process slows a star's spin is not as well understood.

One possibility is that the neutron star's magnetic field is so strong it actually disrupts the surrounding ring of material. that interaction increases the drag on the star, even when the ring is rotating in the same direction as the star. Another possibility is that the stellar wind streaming from the companion star to the neutron star changes direction, forcing material to spin down to the neutron star in opposite directions at different times.

However, the spin behavior of GX1+4 fits neither model well. K. Makishima of the University of Tokyo and his colleagues, who report the Ginga results in the June 23 NATURE, argue that the ring orbiting the neutron star has probably reversed its direction, and the incoming matter is slowing the star's spin. But unlike other X-ray pulsars, GX1+4 has a red giant star as a partner, and a red giant produces a slow, weak stellar wind unlikely to alter the ring's motion.

Theoretical calculations indicate that magnetic braking may be equally implausible. This mechanism would require a magnetic field significantly larger than any yet observed for a neutron star. The lack of a good explanation leaves astronomers puzzled, but excited by the chance to study in detail how a star spins down.