Eclipsing Pulsar Promises Clues to Crushed Matter
Astronomers using NASA's Rossi X-ray Timing Explorer (RXTE) have found the first fast X-ray pulsar to be eclipsed by its companion star. Further studies of this unique stellar system will shed light on some of the most compressed matter in the universe and test a key prediction of Einstein's relativity theory. The pulsar is a rapidly spinning neutron star the crushed core of a massive star that long ago exploded as a supernova. Neutron stars pack more than the sun's mass into a ball nearly 60,000 times smaller. With estimated sizes between 10 and 15 miles across, a neutron star would just span Manhattan or the District of Columbia.

"It's difficult to establish precise masses for neutron stars, especially toward the higher end of the mass range theory predicts," said Craig Markwardt at NASA's Goddard Space Flight Center in Greenbelt. "As a result, we don't know their internal structure or sizes as well as we'd like. This system takes us a step closer to narrowing that down." Known as Swift J1749.4-2807 J1749 for short the system erupted with an X-ray outburst on April 10. During the event, RXTE observed three eclipses, detected X-ray pulses that identified the neutron star as a pulsar, and even recorded pulse variations that indicated the neutron star's orbital motion.

J1749 was discovered in June 2006, when a smaller eruption brought it to the attention of NASA's Swift satellite. Observations by Swift, RXTE and other spacecraft revealed that the source was a binary system located 22,000 light-years away in the constellation Sagittarius and that the neutron star was actively capturing, or accreting, gas from its stellar partner. This gas gathers into a disk around the neutron star. "Like many accreting binary systems, J1749 undergoes outbursts when instabilities in the accretion disk allow some of the gas to crash onto the neutron star," said Tod Strohmayer, RXTE's project scientist at Goddard.

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