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Friday, 22 May 2009 18:35

Astronomers see star turn into pulsar

An international team of astronomers has observed for the first time a star being turned into a very fast rotating neutron star, what is called a millisecond pulsar. This discovery helps to find answers to how such massive stars evolve later in their lives.

The paper “A Radio Pulsar/X-ray Binary Link” was published online on May 21, 2009 in Science Express (DOI: 10.1126/science.1172740), part of Science magazine. Team members are from Canada, Australia, Russia, the Netherlands, and the United States.

Its authors are Anne M. Archibald, Ingrid H. Stairs, Scott M. Ransom, Victoria M. Kaspi, Vladislav I. Kondratiev, Duncan R. Lorimer, Maura A. McLaughlin, Jason Boyles, Jason W. T. Hessels, Ryan Lynch, Joeri van Leeuwen, Mallory S. E. Roberts, Frederick Jenet, David J. Champion, Rachel Rosen, Brad N. Barlow, Bart H. Dunlap, and Ronald A. Remillard.

According to the CSIRO media release, “Astronomers catch a star being revved-up,” an international team of astronomers observed a star being transformed into a millisecond pulsar, which spins at nearly 600 revolutions per second.

Check out the CSIRO article for animation of the millisecond pulsar and its companion star.

Such pulsars are super-dense neutron stars; that is, the leftover of a massive star that has exploded and then condensed into a very dense core. In fact, the CSIRO article states, “A teaspoon of neutron-star material has a mass of about a billion tonnes.”

And, this pulsar is a special type of pulsar called a millisecond pulsar because it rotates at hundreds of times per second. Most pulsars rotate at only tens of times per second.

In the past, astronomers thought millisecond pulsars were just ordinary pulsars that had been forced to spin faster by the addition of material from a companion star, while it orbits close by.

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The CSIRO article states, “The material from the companion would form a flat, spinning ‘accretion disk’ around the neutron star, blocking the pulsar’s radio waves. As the flow of material from the companion decreased and then stopped, the radio waves would reappear, and the object could be recognised as a pulsar.”

However, astronomers have never before found an accretion disk around a millisecond pulsar. Anne Archibald, one of the lead researchers, states, “No other millisecond pulsar has ever shown evidence of an accretion disk.” [CSIRO]

However, they found an accretion disk (as early as 2001) around this one, called PSR J1023+0038, but it disappeared over time. Its companion star is a 17th-magnitude star.

They conclude, from their research, that material from a companion star is not always necessary to form a millisecond pulsar.

In fact, the researchers state in the abstract to their paper, “Radio pulsars with millisecond spin periods are thought to have been spun up by transfer of matter and angular momentum from a low-mass companion star during an x-ray-emitting phase. The spin periods of the neutron stars in several such low-mass x-ray binary (LMXB) systems have been shown to be in the millisecond regime, but no radio pulsations have been detected.”

“Here, we report on detection and follow-up observations of a nearby radio millisecond pulsar (MSP) in a circular binary orbit with an optically identified companion star. Optical observations indicate that an accretion disk was present in this system within the last decade. Our optical data show no evidence that one exists today, suggesting that the radio MSP has turned on after a recent LMXB phase.”

Astronomers observe pulsars by the radio wave radiation they emit as the pulsar rotates. Scientists often compare its radio wave signal to a beam of light coming from a lighthouse, as seen on Earth.

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The team, headed by Anne Archibald and Victoria Kaspi, of the McGill Pulsar Group at McGill University in Montreal, Canada, made their discovery (pdf file showing initial request for telescope time) with telescopes located in the United States, Australia, and the Netherlands.

CSIRO states, “PSR J1023+0038 was found in a 2007 observation with the US National Radio Astronomy Observatory (NRAO) Robert C. Byrd Green Bank Telescope (GBT) during a survey led by NRAO and West Virginia University.”

And, “CSIRO’s Parkes telescope was used to get the first full-orbit observations of the pulsar, which helped to characterise the pulsar’s properties.”

The scientists also used the Westerbork radio telescope in the Netherlands and the Arecibo radio telescope in Puerto Rico.

One of the researchers, David Champion, of CSIRO Australia Telescope National Facility, stated, “This object, called PSR J1023+0038, is a millisecond pulsar, a condensed star about the size of a major city.” He added, “It’s now revolving at 592 times a second, which means it’s one of the fastest spinning objects we know.” [CSIRO]

The Archibald-Kaspi team states that PSR J1023+0038 is about 4,000 light-years from Earth, within the constellation Sextans.

This discovery provides a first chance for astronomers to see how a millisecond pulsar evolves, and it important to finding more answers to the continuing evolution of the Universe.


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