As part of an international team of astronomers, ASTRON astronomers Jason Hessels and Joeri van Leeuwen have discovered a unique binary star system. This new system is a "missing link" in the process that creates the fastest-spinning stars in our Universe: millisecond pulsars. The team has published their results in the May 21st online edition of Science (Science Express, Archibald et al. 2009).
"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 Canada, who is lead author on the discovery paper.
Pulsars are superdense neutron stars, the remnants left after massive stars have exploded as supernovae. Their powerful magnetic fields focus 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.
"Some pulsars, dubbed millisecond pulsars, rotate hundreds of times a second. We believe that this fast rotation is caused by a companion star dumping material onto the neutron star and spinning it up", said van Leeuwen, who made observations required to determine the characteristics of the pulsar's orbit. The material from the companion forms a flat, spinning disk around the neutron star, and during this period, the radio waves characteristic of a pulsar cannot be observed. As the amount of matter falling onto the neutron star decreases and eventually stops, the radio waves can emerge, and the object is observable as a radio pulsar.
This sequence of events is apparently what happened with a binary-star system some 4200 light-years from Earth. The millisecond pulsar in this system, called J1023, rotates 592 times a second, and was discovered with the Green Bank Telescope in West Virginia in 2007 in a survey led by astronomers at West Virginia University and the National Radio Astronomy Observatory (NRAO).
The astronomers then found that the object had also been detected by the Very Large Array radio telescope during a large sky survey in 1998, and had been observed in visible light by the Sloan Digital Sky Survey in 1999, showing a Sun-like star.
When observed again in 2000, the object had changed dramatically, showing evidence for a rotating disk of material, called an accretion disk, surrounding the neutron star. By May of 2002, the evidence for this disk had disappeared.
"No other radio millisecond pulsar has ever shown evidence for an accretion disk," says Hessels, who used low-frequency observations with the Westerbork Synthesis Radio Telescope to study the gas still orbiting in the system. "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. We've thought that LMXBs probably are in the process of getting spun up, and will later emit radio waves as a pulsar. This object appears to be the 'missing link' connecting the two types of systems," he explained.
The team studied J1023 in detail with four of the largest radio telescopes on Earth, including ASTRON's Westerbork Synthesis Radio Telescope (WSRT). The WSRT can make very sensitive low-frequency radio observations, and gave the most complete view of the interaction between the pulsar's wind and gas from the companion star. It will be possible to study the regular eclipses visible in this system in even better detail using the LOFAR radio telescope, which is currently under construction.
Caption: Impression of the radio millisecond pulsar. Credit: Archibald/van Leeuwen (McGill/ASTRON).