Astrophile: The case of the disappearing pulsar

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: pulsar in a double system
Orientation: out of view

The crime scene: 1600 light years away in the constellation Puppis. The victim: radio pulsar J0737-3039B. For five years, Pulsar B had blinked faithfully at a team of astronomers watching from radio telescopes on Earth. The star was a spectacular find: unlike every other pulsar ever observed, this one was in a close binary orbit with another pulsar. Together, the pair provided a precise laboratory to test Einstein's theory of general relativity, and a means of detailing how pulsars behave.

But in March 2008, Pulsar B went dark.

"We weren't able to detect the pulsar at all," says Maura McLaughlin of West Virginia University in Morgantown. "It's the first time we've ever seen a pulsar disappear."

This cosmic whodunit is without a culprit, however. No one snuffed out Pulsar B ? it just rotated out of view.

J0737-3039A/B, the first double-pulsar system ever, was discovered in 2003 by the Parkes Radio Telescope in Australia.

Pulsar A spins once every 23 milliseconds. Pulsar B is slower, completing a rotation once every 2.8 seconds. Pulsar B passes in front of its partner every 2.4 hours, and its strong magnetic field blocks the light from Pulsar A for about 30 seconds.

The fact that each pulsar's light has to pass the other on its way to Earth provided an exquisite test of general relativity's dictum that mass bends spacetime. If spacetime is like a sheet, a massive object like a pulsar is similar to a heavy ball sitting on that sheet and creating a well. That means the pulses from Pulsar A should arrive microseconds later than usual when they travel past Pulsar B ? and they do. The data agrees with Einstein's predictions to within 99.99 per cent, McLaughlin says.

"We can see the light from one pulsar being bent as it travels through the gravitational well of the other pulsar," she says. "It's really neat. We have proof that one of these objects is distorting spacetime."

The eclipsing pulsars also provided a test of "spin precession", the idea that the pulsars' axes should wobble around like a top as they spin. Rene Breton, then a graduate student at McGill University in Montreal, Canada, and colleagues measured the light from 63 eclipses collected over four years at the Green Bank Telescope in West Virginia. They combined those measurements with a model to infer the shape of Pulsar B's magnetic field. From there they figured out where its spin axis was pointed.

The study was among the first to observe spin precession in a real astrophysical system ? another triumph for general relativity. It showed that, over four years, the axis rotated at a rate of a bit less than 5 degrees per year. Over 75 years, the beam will wobble in a full circle.

"It proves that the precession is a real effect ? it's really happening," McLaughlin says.

But sadly, Pulsar B's precession means it rotates in and out of view from Earth.

"The pulsar has been getting fainter and fainter and fainter since we found it. Then a few years ago we just weren't able to detect it at all," McLaughlin says.

Astronomers at Green Bank still check once a month to see if the pulsar has returned yet. Because it's always rotating, the beam should come back. Depending on the beam's shape, it could be as early as 2014 or as late as 2030.

But there's a silver lining. The fact that at least one double pulsar system is currently masquerading as an ordinary pulsar-neutron star system means that there are probably many more double pulsars out there ? thousands in our galaxy alone, according to a new study led by McLaughlin's group.

"We expect there to be lots of other B pulsars out there, whose beams are precessing out of and into our line of sight," she says.

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