MORGANTOWN , W.Va.Astronomers have discovered a unique double-star system that represents amissing linkstage in what they believe is the birth process of the most rapidly-spinning stars in the universemillisecond pulsars.
For some time, astronomers have theorized that millisecond pulsars get spun-up by accretion of matter and angular momentum from a companion. For the first time, we have observed a pulsar which is undergoing this â~recyclingprocess,said Duncan Lorimer, assistant professor of physics at West Virginia University and principle investigator of the Green Bank Telescope survey which discovered this object.
Lorimer and Maura McLaughlin, also an assistant professor of physics at WVU , are members of an international scientific team with representatives from McGill University, the University of British Columbia, the NRAO and others. The scientists announced their discovery in the May 21 issue of the scientific journal, Science .
Pulsars are super dense neutron starsthe 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 per second and slowing down over millions of years.
However, some pulsars, dubbed millisecond pulsars, rotate hundreds of times per second. Astronomers believe the fast rotation is caused by a companion star dumping material onto the neutron star and spinning it up, or â~recyclingit. 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 recognized as a pulsar. These recycled pulsars have very short spin periods and much lower magnetic fields than the rest of the population. As a result, they will shine in the radio for an almost infinitely long time.
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 , was discovered by the National Science Foundations (NSF) Robert C. Byrd Green Bank Telescope (GBT) in West Virginia in 2007. The data for this survey were taken by Lorimer, McLaughlin and collaborators during the summer of 2007 in â~drift-scanmode while the telescope was undergoing repairs.
The astronomers then found that the object had been detected by NSF ’s Very Large Array (VLA) radio telescope during a large sky survey in 1998, and had been observed in visible light by the Sloan Digital Sky Survey in 1999.
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 the end of 2001, the evidence for this disk had disappeared. The 2007 Green Bank Telescope observations showed that the object is a millisecond pulsar, spinning 592 times per second.
No other millisecond pulsar has ever shown evidence for an accretion disk.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 dont emit radio waves,said Anne Archibald of McGill University and lead author of the research paper.Weve thought that LMXBs are probably in the process of getting spun up, and will later emit radio waves as a pulsar. This object appears to be the â~missing linkconnecting the two types of systems.
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 team member Scott Ransom of the National Radio Astronomy Observatory.
The scientists have studied J1023 in detail with the GBT , with the Westerbork radio telescope in the Netherlands, with the Arecibo radio telescope in Puerto Rico, and with the Parkes radio telescope in Australia. Their results show that the neutron star is between one and three times the mass of our Sun, and its companion has less than half the Sun’s mass. The companion orbits the neutron star once every four hours and 45 minutes. Unlike all other millisecond pulsars seen before, the companion is a”normal”star, rather than a white dwarf or another neutron star.
This system gives us an unparalleled â~cosmic laboratoryfor studying how millisecond pulsars evolve,”said Ingrid Stairs of the University of British Columbia.
Maura McLaughlin, assistant professor at West Virginia University, said,Future observations of this system at radio and other wavelengths are sure to hold many surprises.
Content provided by Dave Finely, NRAO and Maura McLaughlin
To view animations of millisecond pulsars, visit http://www.nrao.edu/pr/2009/mspulsarbirth/