Pulsar Has Almost Completely Devoured a Star

by Fraser Cain on September 12, 2007

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Artist impression of the SWIFT J1756.9-2508 system. Image credit: NASA
Astronomers using NASA’s Swift and Rossi X-ray Timing Explorer satellites have discovered a very bizarre object. It only has about 7 times the mass of Jupiter, but instead of orbiting a star, it’s orbiting a pulsar. Oh, and it’s not a planet. It’s all that’s left from a star after the pulsar siphoned away most of its material.

The ghastly duo was discovered on June 7 when Swift picked up a burst of X-rays and gamma rays coming from the direction of the galactic centre. Rossi turned to gaze at the source as well, and confirmed that it’s pulsing out X-rays 182.07 times a second. These are the classic characteristics of a pulsar – the rapidly spinning remains of a massive star.

Normally a pulsar like this is slowing down over time, releasing energy that decreases its rotational velocity. However, in the case of SWIFT J1756.9-2508, it’s actually speeding up. This means that some source is supplying the pulsar with additional material to increase its spin rate.

The researchers were able to detect that a low mass object is orbiting the pulsar, tugging it back and forth, towards and away from the Earth. Astronomers were then able to calculate that this binary companion has somewhere between 7 and 30 times the mass of Jupiter.

The system probably formed billions of years ago as a very massive star and a smaller companion with 1 to 3 solar masses. The massive star evolved quickly and then detonated as a supernova. The smaller star eventually died as well, becoming a red giant, and encompassing the pulsar. This slowed down their orbits enough to begin them spiraling inward.

Today they’re so close that the pulsar produces a tidal bulge on the surface of the dead star, siphoning material away. Sometimes there’s so much mass accumulated that it piles up and explodes as the outburst that led astronomers to the discovery in the first place.

Don’t think of the companion as a planet. “Despite its extremely low mass, the companion isn’t considered a planet because of its formation,” says researcher Christopher Deloye of Northwestern University. “It’s essentially a white dwarf that has been whittled down to a planetary mass.”

Original Source: NASA News Release

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Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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