An artist's concept of the Rossi X-ray Timing Explorer. Credit: NASA

Scientists see a few glitches in an otherwise flawless spin of a pulsar, and this is a good thing. These glitches, a term given to the sudden change in pulsar spin frequency, are revealing the strange physics of the high-pressure interior of the pulsar, a long-standing mystery.

A pulsar named PSR J0537-6910 appears to have the highest rate of glitches of any known pulsar, according to a team led by Frank Marshall of NASA Goddard Space Flight Center in Greenbelt, Md. These glitches occur seven times more frequently than in the Vela pulsar, the former "glitcher king," and are also among the strongest ever observed.

The high rate and strength of these glitches suggest that they are caused by movements in a frictionless superfluid of neutrons in the pulsar's interior. These results, the product of a 2.6-year observational campaign with NASA's Rossi X-ray Timing Explorer, are presented Monday at the 199th Meeting of the American Astronomical Society in Washington, D.C.

A pulsar is a neutron star that emits steady pulses of radiation with each spin. A neutron star is the core remains of a star once several times more massive than our Sun. In an event called a supernova, such a large star -- having expended its nuclear fuel -- expels its outer shell of gas into the surrounding medium. The core, still containing about the mass of Sun, collapses to form a densely packed sphere no wider than about 10 miles.

Pulsars spin with clock-like precision, keeping time to within a millisecond over the course of a year as the spin very gradually slows down. During a glitch, the pulsar suddenly starts to spin a little faster. Rarely do astronomers see more than one or two big glitches in a pulsar over many years of observation. Yet PSR J0537-6910 has had six big glitches in about two-and-a-half years. Each glitch releases more energy than our Sun does in 3,000 years. All these glitches make PSR J0537-6910 a "pretty horrible clock," according to Marshall. Yet there is a plus side: "Observing glitches is one of the few ways we have to study the interior of a neutron star," he said.

Scientists say that the crust of the pulsar, a dense plate of crystalline matter, spins more slowly and slows down at a different rate than the immediate interior, a superfluid of neutrons. (A superfluid is a fluid without friction. Certain liquids can become superfluid under tremendous pressure and heat; other liquids, such as helium, become superfluid at extremely cold temperatures.) As the value of the two speeds widens, the superfluid reaches a critical moment and, like a flywheel, suddenly transfers its built-up angular momentum to the crust, causing the "exterior" of the neutron star to spin faster.

The glitch's size -- that is, how much it quickens the pulsar spin -- is a reflection of the mass, density and other characteristics of the theorized superfluid within. The Rossi team has also observed tiny changes in the pulsar spin rate just before a glitch, which has never been recorded before, and this too may provide new insight to the glitch phenomenon.

What scientists know about pulsar interiors so far has come largely from the study of a handful of "glitchers," particularly the Vela pulsar, which has exhibited 13 glitches in the past 25 years. Newcomer PSR J0537-6910, discovered by the Rossi Explorer in 1998 and now known to glitch so frequently, will likely become a major source for exploration into the subsurface of pulsars to understand how the pressure of pulsar matter depends on its density. This is called the equation of state, a long-standing mystery for pulsars that would reveal the pulsar's mass-to-radius characteristic.

PSR J0537-6910 is already in the record books as the fastest young (5,000 years old) pulsar, spinning once every 16 milliseconds, 6 million times faster than the Earth's spin. PSR J0537-6910 is also an X-ray pulsar not visible in other wavelengths. It is located in our neighboring Large Magellenic Cloud, an irregular galaxy visible to the naked eye from the Southern Hemisphere.

Other members of the observation team -- the same team that discovered this pulsar in 1998 -- are William Zhang of NASA Goddard, Eric Gotthelf of Columbia University, John Middleditch of the Los Alamos National Laboratory, and Q. Daniel Wang of the University of Massachusetts. Continued observations of PSR J0537-6910 with the Rossi Explorer are planned for 2002.