Nature caught ‘recycling’ a star

MCGILL (Canada)—For the first time, researchers have observed the rare rebirth of an ordinary, slow-rotating pulsar into a superfast millisecond pulsar with an almost infinite lifespan.

The discovery was made during a large radio sky survey by an international team of astrophysicists at McGill University, the University of British Columbia (UBC), West Virginia University, the U.S. National Radio Astronomy Observatory (NRAO), and several other institutions in the United States, the Netherlands, and Australia.

The sky survey used the Robert C. Byrd radio telescope at Green Bank, West Virginia, to observe nearly a third of the celestial sphere. The discovery was made by graduate student Anne Archibald and her supervisor, Victoria Kaspi of the McGill Pulsar Group. The team’s results were published online by the journal Science on May 21.

“This survey has found many new pulsars, but this one is truly special—it is a very freshly ‘recycled’ pulsar that is emerging straight from the recycling plant,” says Archibald.

Pulsars are rapidly rotating, highly magnetized neutron stars, the remnants left after massive stars have exploded as supernovae. Pulsars emit lighthouse-like beams of radio waves that sweep around as the star rotates. Most rotate relatively slowly, 10 times a second or less, and their magnetic fields ordinarily slow them down even further over the course of millennia. Millisecond pulsars, however, rotate hundreds of times a second.

“We know normal pulsars typically pulsate in the radio spectrum for one million to 10 million years, but eventually they slow down enough to die out,” says Kaspi. “But a few of these old pulsars get ‘recycled’ into millisecond pulsars. They end up spinning extremely fast, and then they can pulsate forever. How does nature manage to be so green?”

It has long been theorized that millisecond pulsars are created in double-star systems when matter from the companion star falls into the pulsar’s gravity well and increases the rotation speed, but until now the process has never been observed directly.

“Imagine a ping-pong ball in the bathtub, and then you take the plug out of the drain,” explains Archibald. “All the water swirling around the ping-pong ball suddenly makes it spin a lot faster than when it was just bobbing on the surface.

“We’ve seen systems that are undergoing spin-up, because when the matter is falling in, the stars get really bright in X-rays and they’re easy to see,” she adds. “But we’ve never seen radio pulsations from these stars during the process of spin-up. At last we’ve found a true radio pulsar that shows direct evidence for having just been recycled.”

The pulsar found by the survey team was fortuitously observed by an independent, optical research group to have had swirling matter surrounding it roughly a decade ago—the blink of an eye  in astronomical time. That group recorded the observation as puzzling, never dreaming that a full-fledged radio pulsar would emerge.

“In other words, for the first time, we have caught a glimpse at an actual cosmic recycling factory in action,” says Ingrid Stairs of UBC. “This system gives us an unparalleled cosmic laboratory for studying how millisecond pulsars evolve and  get reborn.”

The McGill researchers worked with Ingrid Stairs of UBC and Scott Ransom of NRAO as well as others from the collaboration to carry out more observations of this unusual pulsar. Other major contributors to this study include Maura McLaughlin and  Duncan Lorimer of West Virginia University and Scott Ransom of NRAO. In addition to the GBT, the scientists also used the Westerbork radio telescope in the Netherlands, the Arecibo radio telescope in Puerto Rico, and the Parkes radio telescope in Australia during their study.

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