SYDNEY: Astronomers have discovered a unique binary star system that represents a ‘missing link’ stage in the birth process of the most rapidly spinning stars in the universe – millisecond pulsars.
“We’ve thought for some time that we knew how these pulsars get ‘spun up’ to rotate so swiftly, and this system looks like it’s showing us the process in action,” said Anne Archibald, of McGill University in Montreal, Canada.
Archibald is lead author of a study detailing the find today in the U.S. journal Science.
All spun up
Pulsars are superdense neutron stars, the remnants left after massive stars have exploded as supernovae. Their powerful magnetic fields generate lighthouse-like beams of light and radio waves that sweep around as the star rotates. Most rotate a few to tens of times a second, slowing down over thousands of years.
However, some, dubbed millisecond pulsars, rotate hundreds of times a second. Astronomers believe the fast rotation is caused by a companion star dumping material onto the neutron star and ‘spinning it up’.
The material from the companion would form a flat, spinning disk around the neutron star, and during this period, the radio waves characteristic of a pulsar would not be seen coming from the system. As the amount of matter falling onto the neutron star decreased and stopped, the radio waves could emerge, and the object would be recognised as a pulsar.
This sequence of events is apparently what happened with a binary-star system some 4,000 light-years from Earth. The millisecond pulsar in this system, called J1023, was discovered in 2007 in a survey led by astronomers at West Virginia University and the U.S. National Radio Astronomy Observatory (NRAO).
592 times a second
The astronomers then found that the object had also been detected by the Very Large Array (VLA) radio telescope during a large sky survey in 1998, and again in visible light by the Sloan Digital Sky Survey in 1999, revealing it to be a Sun-like star. A whole sequence of observations meant experts could observe how it had changed over time.
In observations from 2000, the object had changed dramatically from 1998, showing evidence for a rotating disk of material, called an accretion disk, surrounding the neutron star. But by May 2002, the evidence for this disk had disappeared.
“This strange behaviour puzzled astronomers, and there were several different theories for what the object could be,” said co-author Ingrid Stairs of the University of British Columbia, in Canada, who is temporarily based at the Australia Telescope National Facility and Swinburne University in Melbourne.
Observations taken in 2007 with the U.S. National Science Foundation’s Green Bank Telescope (GBT) in West Virginia finally showed that the object has become a millisecond pulsar, spinning an astounding 592 times per second.
“No other millisecond pulsar has ever shown evidence for an accretion
disk,” said Archibald. “We know that another type of binary-star system, called a low-mass X-ray binary (LMXB), also contains a fast-spinning neutron star and an accretion disk, but these don’t emit radio waves.”
Experts thought that LMXBs were probably stars in the process of getting spun up, and that they would later emit radio waves as a pulsar – but before now there was no proof. “This object appears to be the ‘missing link’ connecting the two types of systems,” said Archibald.
“It appears this thing has flipped from looking like an LMXB to looking like a pulsar, as it experienced an episode during which material pulled from the companion star formed an accretion disk around the neutron star. Later, that mass transfer stopped, the disk disappeared, and the pulsar emerged,” said Scott Ransom of the NRAO.
The scientists have now studied J1023 in detail, and their results indicate that the neutron star’s companion has less than half the Sun’s mass, and orbits it once every four hours and 45 minutes. “This system gives us an unparalleled ‘cosmic laboratory’ for studying how millisecond pulsars evolve,” said Stairs.
Co-author Maura McLaughlin, of West Virginia University, agrees: “Future observations of this system at radio and other wavelengths are sure to hold many surprises.”
The study in Science