Let There Be Spin

Revving up neutron stars

1:48pm, July 10, 2002
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While most of us slow down as we become senior citizens, some elderly stars rotate faster and faster as they age. Ultimately, they become the most swiftly twirling stars known in the universe. Called millisecond pulsars, these stars can spin up to several hundred times per second and broadcast strong beams of radio waves that sweep across the sky like lighthouse beacons. An outpouring of energetic radiation from several pairs of stars in our Milky Way is providing new clues about how these whirling dervishes got their rapid spin. The observations support the theory that these aging stars didn't start life in the fast lane. Rather, they rev up their spin as they devour a closely orbiting companion.

According to this theory, as gas from a companion spirals onto a pulsar, it imparts angular momentum, accelerating the pulsar's spin. Ultimately, the pulsar can entirely consume its companion. The theory could also explain why slow-spinning pulsars are commonly found to have partners while many older, more rapidly spinning pulsars do not.

Strange orbs

Aside from their breathtaking spin, millisecond pulsars have several other remarkable traits. Like all pulsars, these world-record rotators are neutron stars–the dense remains of massive stars destroyed by supernova explosions. Not only do such explosions jettison a star's outer layers, but they also squeeze the remaining material so tightly that electrons and protons fuse to form a giant ball of neutrons. The result is an object so dense that more than a sun's worth of matter crams into a sphere only about 20 kilometers in diameter.

If the revving-up model is correct, then astronomers should be able to catch rapidly rotating pulsars in the act of stealing material from a closely orbiting partner. Fortunately, that activity has a telltale signature.

The stolen gas forms a disk around the neutron star. As it does so, it begins spiraling inward, heating up, and emitting X rays. Because the pulsar's strong magnetic field channels the X-ray-emitting material, the gas falls only onto the star's magnetic poles. These hot spots rotate along with the pulsar, so that the X rays appear to flicker on and off at the speed of the star's rotation. While the pulsar sups on its companion, the infalling material stifles the pulsar's radio emissions, notes Craig Markwardt of NASA's Goddard Space Flight Center in Greenbelt, Md.

Although millisecond pulsars were discovered in 1982, it was not until 1998 that astronomers found the first X-ray evidence that one of these stars was indeed eating its companion–and presumably speeding up its rotation (SN: 7/4/98, p. 11). However, the dimness of that X-ray signal and its rapid fading made the observations difficult to interpret.

This spring, researchers using NASA's Rossi X-ray Timing Explorer (RXTE) satellite announced they had found two more millisecond pulsars pulling in matter from a close partner.

In April, researchers analyzing RXTE data detected a 10-day burst of X rays from a source near the Milky Way's center. Markwardt and Jean Swank of Goddard identified the source as material falling onto a millisecond pulsar from its companion. From their observations, they determined that the pulsar, dubbed XTE J1751-305, spins 435 times per second, while the companion star whirls around it once every 42 minutes. That's a much closer pairing than the 2-hour orbit observed in the pulsar system reported in 1998.

Markwardt and Swank also determined that the stellar companion is puny, weighing just 1 percent as much as the pulsar. Given the companion's low mass and its proximity to the pulsar, the researchers suggest the pulsar has already stripped its small companion of outer layers of hydrogen. Markwardt reported the findings in April at a joint meeting in Albuquerque of the American Physical Society and the American Astronomical Society.

"This system has to be close to the endpoint" of making a millisecond pulsar, he says. With so little of its partner left, the pulsar has nearly attained its maximum spin. However, it will take the pulsar hundreds of millions of years to gobble up what remains of its companion, notes Markwardt.

This newly detected system puts the revving-up theory of millisecond pulsars on surer footing, says Deepto Chakrabarty of the Massachusetts Institute of Technology, one of the discoverers of the X-ray emission reported in 1998.

In May, Chakrabarty and several colleagues found evidence of a third X-ray-emitting millisecond pulsar pulling material from a partner. Ronald A. Remillard of MIT, along with Swank and Tod Strohmayer of Goddard, discovered the pulsar during a routine sky survey with RXTE. The X-ray emissions indicate that the star, known as XTE J0929-314, spins 185 times a second. The researchers reported the findings in several circulars of the International Astronomical Union (IAU) posted by E-mail in early May.

In follow-up observations of the pulsar's X-ray radiation, MIT astronomers including Remillard and Chakrabarty deduced that the star has a companion, and they inferred its mass. Just as the gravitational tug of an orbiting planet causes its parent star to wobble back and forth, thereby shifting the frequency of the light emitted by the star, this companion star induces a slight shift in the frequency of the pulsar's X-ray radiation. The shift indicates that the pulsar has whittled its companion from the original weight–half the mass of the sun–to one-fiftieth that amount.

Ultimately, the companion will completely disintegrate. Not only does the pulsar seize matter from the lighter-weight body, but the X rays generated by that infalling material also further erodes it.

"This pulsar has been accumulating gas donated from its companion for some time now," says Duncan Galloway of MIT. "It's exciting that we are finally discovering pulsars at all stages of their evolution, that is, some that are quite young and others that are transitioning to a final stage of isolation."

The findings will "help us to understand the link between slow-spinning pulsars in binary systems, which are quite common, and fast-spinning, isolated pulsars, which are commonly seen by radio astronomers," says Chakrabarty.

Terminal velocity

Another result reported at the April meeting lends support to the revving-up theory. Also using RXTE, Strohmayer and Markwardt observed a rare, 3-hour thermonuclear explosion on a rapidly rotating neutron star. Such explosions are caused by the accumulation of a critical mass of material falling onto the neutron star from a partner. As fresh matter piles onto the pulsar, it buries material that arrived earlier, subjecting it to such extreme pressures and temperatures that some of the nuclei fuse together, unleashing enormous amounts of energy.

Periodic variations in the brightness of the X-ray emission observed for about 20 minutes during the explosion provide a measure of the pulsar's spin rate, the astronomers say. Known as 4U 1636-53, the pulsar rotates at 582 times per second, which makes it one of the fastest known.

Its partner weighs somewhere between 30 to 80 percent of the mass of the sun and takes about 3 hours to orbit the pulsar. Although this companion has plenty of mass left to donate, the pulsar may have reached its rotational speed limit. Energy continuously radiated away by the pulsar puts a cap on its spin rate, Markwardt notes.

In one respect, the results pose a puzzle. Even if a thermonuclear explosion begins at the spot beneath the star's surface where the density and temperature are highest, the conflagration rapidly engulfs the entire surface. When it does so, the accompanying X rays should no longer flicker on and off in sync with the rotation of the pulsar. Instead, they should show up as a relatively steady X-ray source for the duration of the explosion.

This pulsar's magnetic field may solve the puzzle. It could confine the explosion to particular regions on the surface for a longer period of time than astronomers had expected, notes Cole Miller of the University of Maryland in College Park. On the other hand, the field can't be strong enough to direct matter ripped from the companion to fall onto the magnetic poles. Otherwise, the X-ray signal would always pulsate at the same rate as the star's rotation, not just when a thermonuclear explosion occurs.

Spin and bones

NASA scientists aren't the only ones finding evidence of revved-up millisecond pulsars. In February, Francesco Ferraro of the Bologna Astronomical Observatory in Italy and his colleagues announced observations of a millisecond pulsar known as PSR J1740-5340. Using the Parkes Radio Telescope in Australia and the Hubble Space Telescope, the scientists found that the millisecond pulsar spins 274 times a second. Images from Hubble show that this pulsar's partner is a bloated, red star with a girth about 2.6 times that of the sun. Material from the puffed-up companion, which takes 1.35 days to orbit the pulsar, is easily stripped away and so is the probable source of the pulsar's high spin rate, Ferraro and his collaborators say.

All of these observations, Markwardt notes, are painting a consistent portrait of the evolution of millisecond pulsars. "First, we saw just the old bones–millisecond pulsars in isolation," he says. "Now, we're seeing how they acquired their spin."

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