Astronomers catch a star being revved-up

The work, which is published online in Science Express on 21 May, was undertaken by an international team led by PhD student Anne Archibald and her supervisor, Professor Victoria Kaspi of McGill University in Montreal, Canada.  “This object, called PSR J1023+0038, is a millisecond pulsar, a condensed star about the size of a major city,” said CSIRO’s Australia Telescope National Facility researcher Dr David Champion.

“It’s now revolving at 592 times a second, which means it’s one of the fastest spinning objects we know.”  PSR J1023+0038 lies 4000 light-years from Earth in the constellation Sextans.

Pulsars are superdense neutron stars, the squashed-in cores of massive stars that have exploded. We detect them by the beams of radio waves they emit, which sweep over Earth like a lighthouse beam as the pulsar rotates.

A teaspoon of neutron-star material has a mass of about a billion tonnes.

Most pulsars spin relatively slowly, a few to tens of times a second, but the “millisecond pulsars” scream along, rotating hundreds of times a second.

Astronomers have long thought that millisecond pulsars are ordinary pulsars “spun up” with the help of an orbiting companion star. The companion dumps matter onto the pulsar, causing it to spin far faster.

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.

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.

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

When astronomers looked at earlier records, they found that a Sun-like star – the pulsar’s companion – had been seen by the Sloan Digital Sky Survey at the same location in 1999, and the system had been observed in the radio by the NRAO’s Very Large Array telescope in 1998.

When observed again with optical telescopes in 2000, the system had shown clear evidence of an “accretion disk” around 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 University of British Columbia team member Ingrid Stairs, who is on sabbatical in Australia visiting CSIRO and Swinburne University of Technology.

“This system gives us an unparalleled ‘cosmic laboratory’ for studying how millisecond pulsars evolve.”

But now, after observations with several radio telescopes, the pieces of the puzzle have fallen into place.

“No other millisecond pulsar has ever shown evidence of an accretion disk,” said Ms Archibald.

Other major contributors to this study include Dr Maura McLaughlin and Dr Duncan Lorimer of West Virginia University and Dr Scott Ransom of NRAO.  In addition to the Parkes telescope and the GBT, the scientists also used the Westerbork radio telescope in the Netherlands and the Arecibo radio telescope in Puerto Rico.

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