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MIT researchers help fill in 'missing link' in evolution of stars

More than 12,000 light years away in the constellation Sagittarius, an extremely rapidly spinning neutron star is providing a long-sought "missing link" in our understanding of the evolution of stars, MIT and competing Dutch researchers reported in the July 23 issue of Nature.

This star, spotted by NASA's Rossi X-ray Timing Explorer (RXTE) satellite during a month-long outburst of X-rays that began in April, could be a millisecond radio pulsar in the making. It provides proof for the theory that these unusual neutron stars are propelled to mind-boggling speeds by the force of material spiraling onto their surfaces from a companion star.

Pulsars are rotating, magnetized neutron stars--very dense, compact objects only as big as Boston but as massive as our sun--that often emit radio waves. Ordinary radio pulsars, born spinning up to 100 times a second in supernova explosions, gradually lose steam over tens or hundreds of millions of years.

But millisecond radio pulsars are the "Energizer bunnies" of neutron stars. Even though they are billions of years old, they spin nearly 1,000 times a second--much faster than the birth speed of pulsars and so fast that their surface is moving at almost one-fifth the speed of light.

Since the first millisecond radio pulsar was discovered more than 15 years ago, scientists have sought to prove that these stars are "spun up" by matter spiraling down onto them from a binary companion, a normal adjacent star, according to postdoctoral fellow Deepto Chakrabarty and research scientist Edward H. Morgan, both of the Center for Space Research.

Every 2.5 milliseconds, the new star found by the RXTE emitted clock-like X-ray pulses thought to be caused by a "hot spot" on the spinning neutron star's surface from the impact of the companion star's matter. Drs. Chakrabarty and Morgan noticed that these pulsations underwent periodic Doppler shifts that mirrored the neutron star's revolving dance with its companion.

They deduced that this new millisecond X-ray pulsar is part of a highly compact binary system, with an orbital period of just two hours and a companion star that is much less massive than our sun. Because the two stars are so close together, mass is lost from the low-mass companion onto the pulsar, generating X-rays from the monstrous heat of the impacts and "spinning up" the pulsar through the "accretion" of angular momentum, like a top given an extra twirl.

The two stars in this binary system are so close that Dr. Chakrabarty and Dr. Morgan believe that this low-mass X-ray binary, as it is known, may be related to "black widow" radio pulsars, which accumulate matter from their companions while gradually evaporating away these companions through intense radiation.

Although tantalizing hints of rapidly spinning neutron stars have been detected during the past two years by the RXTE in several of the more than 30 low-mass X-ray binaries, this newly discovered star--designated SAX J1808.4-3658--is the first clearly emitting the regular X-ray pulsations characteristic of a rapidly spinning star accumulating material from its companion.

"This was not an easy hunt," said Dr. Chakrabarty, who will join the MIT faculty as an assistant professor of physics in January. "Over the past 15 years, many competing groups have searched more than 30 sources with a series of X-ray satellites without detecting persistent, coherent X-ray pulsations." The RXTE's discovery of "the real McCoy" in April is "a dramatic vindication of the basic model theorists had put forward for millisecond pulsars."

"This is a triumph for the RXTE," said Physics Professor Hale Bradt, who helped design instruments for the satellite named for Bruno B. Rossi, an MIT pioneer in the field of X-ray astronomy. "The holy grail of this mission was to find an X-ray-emitting pulsar spinning as fast as one revolution in several milliseconds. This fills an important niche in the history of stars."

Pulsars emit radio waves generated by their powerful magnetic fields rotating the particles in their atmosphere. Earthbound radio telescopes can detect these pulses each time one of the star's magnetic poles swing by. However, if the pulsar is accreting material from a companion star, these radio pulses can be smothered by the infalling matter, although X-ray pulsations may still be detected.

If radio pulsations commence from SAX J1808.4-3658 now that the transient X-ray outburst has ended, the star could officially take its place on Hollywood Boulevard as the first millisecond radio pulsar to provide conclusive proof for the theory of how these unusual neutron stars are propelled to mind-boggling speeds.

Dr. Victoria Kaspi, an assistant professor of physics at MIT, is leading an international team, including collaborators from Italy, Australia and Great Britain, which is searching for radio pulsations from SAX J1808.4-3658 using the Parkes radio telescope in Australia. Even if this search proves unsuccessful, the discovery of an accretion-powered millisecond X-ray pulsar is powerful evidence in favor of the theory.

Dr. Morgan had been looking for just this phenomenon since the RXTE was launched in December 1995. Car trouble kept him out of his office on the Monday after SAX J1808.4-3658 was first noticed by the RXTE. That Tuesday, he glanced over the weekend data and was shocked to see a big pulsation leap off the screen in the first minute of data. "We had been looking for something like this in weeks and months of data, and there this was, right in the first minute," he said.

A day earlier, while Dr. Morgan was dealing with flat tires and tow trucks, competing Dutch astronomers caught the same pulsations and made the first announcement of the detection. The following day, the MIT team, though scooped on the pulsation discovery, announced the discovery of the two-hour orbital modulation in these pulsations, establishing the evolutionary status of the system. Dr. Rudy Wijnands and Dr. Michiel van der Klis of the University of Amsterdam detail the pulsation discovery in a separate paper in the same issue of Nature as the Chakrabarty and Morgan paper on the orbit discovery.

Drs. Morgan and Chakrabarty hope this star system will provide an important future laboratory for the study of the effects of general relativity. It has spawned a host of new questions; most important, why is it pulsing while none of the more than 30 other known low-mass X-ray binary systems have detectable pulsations? A future X-ray outburst, which may occur as matter again accumulates in the accretion disk surrounding the neutron star, may help solve some of the mysteries.

MIT graduate student Michael Muno is working with Drs. Morgan and Chakrabarty to determine if our viewing geometry could play a role. Many other aspects of this puzzling source are being studied as well. MIT research scientist Wei Cui, along with Dr. Morgan and Dr. Lev Titarchuk of NASA's Goddard Space Flight Center, has written a paper reporting an unexpected phase delay of the low-energy X-ray pulses with respect to the high-energy pulses that may provide an important clue about the environment near the pulsar.

In addition to Professor Bradt, Dr. Chakrabarty, Dr. Cui, Dr. Morgan and Mr. Muno, the RXTE instrument team at MIT includes principal research scientists Alan M. Levine and Ronald Remillard; scientist Robert Shirey; graduate students Donald Smith and Linqing Wen; programmer Douglas Alan; data technicians Joan Quigley and Alan Wood; project manager William F. Mayer, associate director of the Center for Space Research; and project engineer Robert F. Goeke.

A version of this article appeared in MIT Tech Talk on September 12, 1998.

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