Cosmic First: Pulsing Neutron Star Eclipsed By Stellar Companion
J1749 is the first accreting millisecond pulsar to undergo eclipses. The pulsar and its companion star are separated by 1.22 million miles, or about five times the distance between Earth and the moon.

In a strange cosmic first, astronomers have discovered a pulsing X-ray neutron star ? the ultra-dense leftovers from a supernova explosion ? being eclipsed by another companion star in deep space.

The unique star pair sits 22,000 light-years from Earth and was discovered using NASA's Rossi X-ray Timing Explorer satellite. It offers astronomers a rare chance to study the inner workings of neutron star systems.

In this binary star system, known as Swift J1749, a normal star is eclipsing a pulsar ? a fast-spinning neutron star that emits rotating beams of light like a lighthouse. Scientists can only see this light when the beams are pointing toward Earth, so its light appears to pulse ? hence the name.

Swift J1749's pulsar is only 12 miles (20 km) across, but 60,000 times denser than our sun. It spins 518 times per second ? as fast as the blades of a kitchen blender. The beams the pulsar emits are in the X-ray range of the electromagnetic-radiation spectrum.

In April, the Rossi satellite observed three eclipses in the Swift J1749 system over the course of one weel, as the pulsar passed behind the normal star. Each event lasted 36 minutes. [Graphic: Swift J1749 in a Nutshell]

X-ray pulsar first

The eclipses mark the first time a fast X-ray pulsar has been observed being eclipsed by its companion star.

But the eclipses had more than mere gee-whiz appeal for astronomers. Tracking the pulsar's motion revealed a great deal about Swift J1749 ? and could shed light on other neutron-star systems like it.

"Using this information, we now know the size and mass of the companion star with unprecedented accuracy," said researcher Craig Markwardt of NASA's Goddard Space Flight Center in a statement.

The normal star is about 70 percent as massive as the sun, but it's about 20 percent larger than its mass and apparent age suggest it should be. Markwardt thinks he knows why.

"We believe that the star's surface is 'puffed up' by radiation from the pulsar, which is only about a million miles away from it," Markwardt said. "This additional heating probably also makes the star's surface especially disturbed and stormy."

More observations needed

The next step for astronomers may be to keep their eyes on Swift J1749, but to focus on the normal star this time.

"We need to detect the normal star with optical or infrared telescopes," said Tod Strohmayer, Rossi satellite project scientist at Goddard. "Then we can measure its motion and extract the same information about the pulsar that the pulsar's motion told us about the star."

Doing this would help nail down the mass of the pulsar, which is estimated to be between 1.4 and 2.2 times that of the sun.

Markwardt and Strohmayer describe their results in the July 10 issue of The Astrophysical Journal Letters.

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