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WVU scientists help discover star system

CHARLESTON, W.Va.-- An international team of astronomers, including researchers from West Virginia University, have used the Green Bank Telescope to discover a unique triple star system that will be used to challenge Einstein's theory of general relativity.

The star system, located about 4,200 light years from Earth, consists of a super-dense pulsar being orbited by a hot white dwarf star, both of which are being orbited by a second, more distant, white dwarf.

White dwarfs are stars that have burned off their nuclear fuel, expelling most of their outer material and leaving behind extremely hot cores. Pulsars are neutron stars that emit lighthouse-like beams of radio waves that rapidly sweep through space as they spin on their axes -- in this case, at nearly 366 times per second.  Such rapidly spinning pulsars, called millisecond pulsars, can be used by astronomers as precision tools for studying a variety of phenomena, including searches for elusive gravity waves.

WVU graduate student Jason Boyles, now a faculty member at Western Kentucky University, discovered the millisecond pulsar in 2012 during a large-scale search for pulsars using the Green Bank Telescope. Follow-up observations showed that two white dwarfs are orbiting the pulsar in a space smaller than Earth's orbit around the sun.

Boyle and his research advisers, Duncan Lorimer and Maura McLaughlin, associate professors in WVU's Department of Physics and Astronomy, collaborated with a team of researchers led by Scott Ransom of the National Radio Astronomy Observatory. Others on the team are Anne Archibald of the Netherlands Institute for Radio Astronomy and Ingrid Stairs of Canada's University of British Columbia.

The results of the team's research were published in the Jan. 5 online edition of the journal Nature.

"This is the first millisecond pulsar found in such a system, and we immediately recognized that it provides us a tremendous opportunity to study the effects and nature of gravity," Ransom said

"This triple system gives us a natural cosmic laboratory far better than anything found before for learning exactly how such three-body systems work," said Lorimer. The system offers the potential for the research team to detect problems with Einstein's theory of general relativity, that "physicists expect to see under extreme conditions," he added.

While Einstein's theory of general relativity has been confirmed by every experiment to date, it is not compatible with quantum theory -- the theoretical basis of modern physics that explains the nature and behavior of matter and energy on the atomic and subatomic level. Physicists believe Einstein's theory will break down in extreme situations like the one occurring in the newly discovered star system.

The new star system gives the researchers the best opportunity yet to test an Einsteinian concept called the Equivalence Principle, which holds that the effect of gravity on a body does not depend on the nature or the internal structure of that body. One of the most famous experiments illustrating the Equivalence Principle is Galileo's reputed dropping of two balls of different weights from the top of the Leaning Tower of Pisa, and seeing them strike the ground at the same time.

"The particular theory that this system will be a good test for is something called the Strong Equivalence Principle, which means that the orbital motions of the low-mass, low-gravity white dwarf stars are the same as those of the high-mass, high-gravity neutron stars," or pulsars, said McLaughlin.

"Such tests have been done on the Earth-Moon and Sun-planet systems, and no violations have been found," she said. "However, this pulsar system is far more sensitive. With another year of so of continued timing, we should have the most precise test ever of this principle. This is important as other theories of gravity, aside from general relativity, predict violations of the Strong Equivalence Principle. If we find no violations, then we will have further proof that Einstein is correct."

The research team expects the planned "pulsar timing" experiment to determine "whether the pulsar and the white dwarfs are falling at different rates in the mutual gravitational field," Lorimer said. "Einstein predicts no, but other theories of gravity predict small deviations which might be detectable."

By recording the time of arrival of the pulsar's pulses, the researchers were able to calculate the geometry of the system and the masses of the stars with unparalleled precision. Although the star system is 4,200 light years distant, "Some of our measurements of the relative positions of the stars in this system are accurate to within hundreds of meters," Archibald said.

In addition to observations made using the Green Bank Telescope in Pocahontas County, observations from the Arecibo Radio Telescope in Puerto Rico and the Westerbork Synthesis Radio Telescope in the Netherlands were used to study the new star system. The researchers also made use of data from the Sloan Digital Sky Survey, the GALEX satellite, the WIYN telescope at Kitt Peak, Ariz., and the Spitzer Space Telescope.

In addition to providing observations for use in the pulsar timing experiment for the new star system, the Green Bank Telescope will continue searching for undiscovered pulsars for the next several years as part of an all-sky survey.

The GBT's all-sky survey will involve observations taken at the same radio frequency as the search that turned up the new triple-star system, McLaughlin said. "We have already found several more binary pulsars (pulsars that orbit another star) and will undoubtedly find other objects we can't explain," she said.

"Without the survey using the GBT, there would be no discovery," Lorimer said. "It's vital to keep the GBT open so that we can do more of these experiments in the future. Without a doubt, there are more weird and exotic objects out there waiting to be found."

Reach Rick Steelhammer at rsteelhammer@wvgazette.com or 304-348-5169.

 


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