In the News this month... evidence for the origins of millisecond pulsars
Artists impression of a neutron star with accretion disk (left) drawing material from companion star (right). CREDIT: Bill Saxton, NRAO/AUI/NSF
Fast-spinning radio pulsars with millisecond rotation periods are thought to be the result of a process involving the transfer of material from a companion low-mass star onto a normal pulsar. This accretion process adds mass and angular momentum to the pulsar resulting in its rotation rate speeding up and the emission of X-rays. Using telescopes around the world, a team of astronomers has for the first time discovered evidence of this process taking place.
Pulsars are extremely dense neutron stars, left over after massive stars explode as supernovae. They have strong magnetic fields which generate beams of light and radio waves which sweep across the sky as the pulsar spins. Most pulsars rotate a few times a second, but some, known as millisecond pulsars, rotate hundreds of times a second. Ordinary pulsars in a binary system with a low-mass companion can start to accumulate material in an accretion disk - a flat spinning ring of material around the pulsar. While this disk exists, it is thought that the radio waves characteristic of a pulsar would be quenched and the object would not appear as a pulsar. When the rate of infalling material slows down and stops, the pulsar's emission would be able to disrupt the accretion disk, blowing material out of the system and allowing the radio emission to resume. Now, a team led by Anne Archibald at McGill University in Montreal, Canada, have found evidence of this process taking place in a binary star system 4000 light years away.
A millisecond pulsar was discovered in the system in 2007, so the team looked back through archive data from several telescopes. What they discovered was a dramatic change in the system over the last decade. Optical observations in 1999 showed a Sun-like star, while observations a year later showed evidence of an accretion disk around the neutron star. By 2002, the evidence for this disk had disappeared. The observations in 2007, made with the Green Bank Telescope in West Virginia, found a millisecond pulsar spinning 592 times per second. The researchers say that this system appears to be the missing link between millisecond pulsars and accreting binary systems known as Low Mass X-ray Binaries. The results were published in the journal Science during May.