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Pulsar in a Stellar Triple System Makes Unique Gravitational Laboratory

Astronomers using the National Science Foundation's Green Bank Telescope (GBT) have discovered a unique stellar system of two white dwarf stars and a superdense neutron star spinning 365 times per second, all packed within a space smaller than Earth's orbit around the Sun. Using the National Science Foundation's Arecibo telescope in Puerto Rico, the arrival time at the Earth of the radio pulses from the neutron star are measured clock-like with a precision better than 1 microsecond. Such precise measurements of the close stars, combined with their nature, have allowed scientists to make the best measurements yet of ordinarily unobservable complex gravitational interactions.

In addition, detailed studies of this system may provide a key clue for resolving one of the principal outstanding problems of fundamental physics -- the true nature of gravity.

"This triple system gives us a natural cosmic laboratory far better than anything found before for learning exactly how such three-body systems work and potentially for detecting problems with General Relativity that physicists expect to see under extreme conditions," said Scott Ransom of the National Radio Astronomy Observatory (NRAO), lead author of the international team presenting their results this week in the journal Nature .

The pulsar, 4200 light-years from Earth, was discovered during a large-scale search for pulsars using the GBT. Pulsars are neutron stars that emit lighthouse-like beams of radio waves that rapidly sweep through space as the object spins on its axis.

Such rapidly-spinning pulsars are called millisecond pulsars, and can be used by astronomers as precision tools for studying a variety of phenomena, including searches for elusive gravitational waves. Subsequent observations showed that the pulsar is in a close orbit with a white dwarf star, and that pair is in orbit with another, more-distant white dwarf.

"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.

The scientists began an intensive observational program using the GBT, the Arecibo telescope, and the Westerbork Synthesis Radio Telescope in the Netherlands. They also studied the system using data from the Sloan Digital Sky Survey, the GALEX satellite, the WIYN telescope on Kitt Peak, Arizona, and the Spitzer Space Telescope.

"The gravitational perturbations imposed on each member of this system by the others are incredibly pure and strong," Ransom said. "The millisecond pulsar serves as an extremely powerful tool for measuring those perturbations incredibly well," he added.

By very accurately recording the time of arrival of the pulsar's pulses, the scientists were able to calculate the geometry of the system and the masses of the stars with unparalleled precision.

"We have made some of the most accurate measurements of masses in astrophysics," said Anne Archibald, of the Netherlands Institute for Radio Astronomy. "Some of our measurements of the relative positions of the stars in the system are accurate to hundreds of meters," she said. Archibald led the effort to use the measurements to build a computer simulation of the system that can predict its motions.

The research on this system used techniques dating back to those used by Isaac Newton to study the Earth-Moon-Sun system, combined with the "new" gravity of Albert Einstein, which was required to make the precise measurements. In turn, the scientists said, the system promises a chance to point the way to the next theory of gravity.

The system gives the scientists the best opportunity yet to discover a violation of a concept called the Equivalence Principle. This principle states that the effect of gravity on a body does not depend on the nature or internal structure of that body.

The most famous experiments illustrating the equivalence principle are Galileo's reputed dropping of two balls of different weights from the Leaning Tower of Pisa and Apollo 15 Commander Dave Scott's dropping of a hammer and a falcon feather while standing on the airless surface of the Moon in 1971. (While there is no confirmation that Galileo actually performed the experiment from the Leaning Tower, he did demonstrate the principle by rolling balls down inclined planes, an experiment that often is repeated in introductory physics laboratories.)

"While Einstein's Theory of General Relativity has so far been confirmed by every experiment, it is not compatible with quantum theory. Because of that, physicists expect that it will break down under extreme conditions," Ransom explained. "This triple system of compact stars gives us a great opportunity to look for a violation of a specific form of the equivalence principle called the Strong Equivalence Principle," he added.

When a massive star explodes as a supernova and its remains collapse into a superdense neutron star, some of its mass is converted into gravitational binding energy that holds the dense star together. The Strong Equivalence Principle says that this binding energy still will react gravitationally as if it were mass. Virtually all alternatives to General Relativity hold that it will not.

"This system offers the best test yet of which is the case," Ransom said.

Under the Strong Equivalence Principle, the gravitational effect of the outer white dwarf would be identical for both the inner white dwarf and the neutron star. If the strong equivalence principle is invalid under the conditions in this system, the outer star's gravitational effect on the inner white dwarf and the neutron star would be slightly different and the high-precision pulsar timing observations could easily show that. These tests will now be carried out with high-precision timing observations of the radio pulses detected from the pulsar.

"This is a fascinating system in many ways, including what must have been a completely crazy formation history, and we have much work to do to fully understand it," Ransom said.

"This is a remarkable and so far unique system", said Fernando Camilo, Director of Astronomy at Arecibo. "This international team of researchers had to use a variety of the best astronomical instruments in the world, supported by multiple funding agencies - GBT to discover the pulsar, Arecibo to obtain the most precise radio measurements, Westerbork to observe it almost daily for months on end, optical telescopes to study the white dwarf companions - in order to uncover the scientific gems hidden within such a beautiful triple stellar system."

Ransom and Archibald were on an international team of researchers that reported their findings in the online edition of the journal Nature on January 5.

Millisecond Pulsar

Figure: Millisecond pulsar, left foreground, is orbited by a hot white dwarf star, center, both of which are orbited by another, more-distant and cooler white dwarf, top right. Credit: Bill Saxton, NRAO/AUI/NSF.

A video animation of the triple-star system is available here .

Original Publication
Results are published as "A millisecond pulsar in a stellar triple system" (S. M. Ransom et al.) in the online January 5, 2014, issue of Nature.
Dr. Scott Ransom
National Radio Astronomy Observatory, Charlottesville, Virginia
Phone: 434-284-2604

Dr. Fernando Camilo
Arecibo Observatory, Arecibo, Puerto Rico
Phone: 410-302-0299

About Arecibo Observatory

The Arecibo Observatory is operated by SRI International in alliance with Ana G. MĂ©ndez-Universidad Metropolitana and the Universities Space Research Association, under a cooperative agreement with the National Science Foundation (AST-1100968). The Arecibo Planetary Radar program is supported by NASA's Near Earth Object Observation program. The Arecibo Observatory is sponsored by the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

About USRA

Universities Space Research Association (USRA) is an independent, nonprofit research corporation where the combined efforts of in-house talent and university-based expertise merge to advance space science and technology. USRA works across disciplines including biomedicine, astrophysics, and engineering and integrates those competencies into applications ranging from fundamental research to facility management and operations. USRA engages the creativity and authoritative expertise of the research community to develop and deliver sophisticated, forward-looking solutions to Federal agencies and other customers - on schedule and within budget.