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 鈥� 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 鈥減ulses鈥� as the star spins. The stars are then known as pulsars.
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Ordinary pulsars spin just a few times per second, but some spin hundreds of times faster. Astronomers suspect that many of these 鈥渕illisecond pulsars鈥� are essentially no different from ordinary pulsars, except that their rotation has sped up as they have stripped material from nearby stars.
Stolen girdle
The stripped material accretes into a disc around the neutron star, transferring to the star 鈥� which makes it rotate more quickly 鈥� 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 of McGill University in Montreal and colleagues say they鈥檝e 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 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鈥檚 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.
Terminal pulsar
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鈥檚 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.
鈥淲hat we found is a system that鈥檚 right on the end, that鈥檚 turning from an accreting system into a millisecond pulsar,鈥� Archibald says.
鈥淚t鈥檚 another piece in the puzzle of how you get from a neutron star to essentially the fastest rotating objects in the universe,鈥� says of MIT. 鈥淭he fact that this object is apparently making the transition from one state to the other could be a big step forward.鈥�
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 to look out for another accretion event.
Journal reference: , (in press)
(Animation: Bill Saxton, NRAO / AUI / NSF)
