Video: Pulsar steals from neighbour
A neutron star with a cosmic case of indigestion could help to explain why some of these ultra-dense stellar embers spin much more quickly than others.
Astronomers have found that the gluttonous star stole material from a companion star before blasting it away again – apparently confirming their ideas about how such rapidly-spinning pulsars might develop.
Neutron stars are the remains of massive stars that have exploded, leaving only city-sized balls of neutrons. They often emit intense beams of radio waves from their poles, which are detected as a signal that "pulses" as the star spins. The stars are then known as pulsars.
Ordinary pulsars spin just a few times per second, but some spin hundreds of times faster. Astronomers suspect that many of these "millisecond pulsars" are essentially no different from ordinary pulsars, except that their rotation has sped up as they have stripped material from nearby stars.
The stripped material accretes into a disc around the neutron star, transferring angular momentum to the star – which makes it rotate more quickly – and shorting out its production of radio waves. Eventually, however, the intense magnetic fields around the star blast the accreted material away and the neutron star once again becomes detectable as a pulsar, spinning much more quickly than before.
Until recently, however, there was no direct evidence for this process. Now Anne Archibald of McGill University in Montreal and colleagues say they've identified a millisecond pulsar that was once surrounded by an accretion disc of material from its companion star.
The neutron star, part of a pair of stellar objects called J1023, sits in the Milky Way some 4000 light years from Earth and spins 592 times per second. It was discovered in 2007 in a survey of the sky conducted with the Green Bank Telescope in West Virginia. Its companion is a star with about half the mass of the sun; the two objects orbit each other roughly every five hours.
Archibald's team looked back through archived data from other telescopes. They found that, in 2000, the neutron star was encircled by a disc of material stolen from its companion. The intense gravity of the pulsar made this disc dense and hot, glowing four times more brightly than the companion star.
However, by 2002 the disc had disappeared, presumably cast out by the intense magnetic fields around the star. Now the neutron star looks like a pulsar again, beaming intense radio waves from its poles.
This is the first time a fast-spinning pulsar has been shown to have gathered matter from another star. But the process is probably coming to an end in J1023's case: the neutron star is now spinning so fast that any matter torn off the companion star in the future might simply be flung out of the system altogether.
"What we found is a system that's right on the end, that's turning from an accreting system into a millisecond pulsar," Archibald says.
"It's another piece in the puzzle of how you get from a neutron star to essentially the fastest rotating objects in the universe," says Deepto Chakrabarty of MIT. "The fact that this object is apparently making the transition from one state to the other could be a big step forward."
Nonetheless, astronomers are hoping the J1023 pulsar will grab more material from its companion soon. That would be detectable as a shorting-out of radio waves around the star. The team has already enlisted the help of amateur astronomers to look out for another accretion event.
(Animation: Bill Saxton, NRAO / AUI / NSF)
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