Triple-Star System Can Give Clues to True Nature of Gravity

Astronomers have discovered a unique stellar system of the superdense pulsar PSR J0337+1715 and two white dwarfs, all packed within a space smaller than the Earth’s orbit around the Sun. The closeness of the stars, combined with their nature, is enabling astronomers to probe one of the principal outstanding problems of fundamental physics – the true nature of gravity.

This image shows the pulsar PSR J0337+1715. Image credit: Ransom SM et al.

This image shows the pulsar PSR J0337+1715. Image credit: Ransom SM et al.

“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 Dr Scott Ransom of the National Radio Astronomy Observatory, who is the lead author of the paper published in the journal Nature (arXiv.org).

PSR J0337+1715 is an unusual neutron star located about 4,200 light-years from Earth, spinning nearly 366 times per second. It was first observed by Jason Boyles of Western Kentucky University using the NSF’s Green Bank Telescope.

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 the elusive gravitational waves.

Subsequent observations showed that PSR J0337+1715 is in a close orbit with a white dwarf star, and that pair is in orbit with another, more-distant white dwarf star.

The astronomers then began intensive observations using the Green Bank Telescope, the Arecibo Radio Telescope in Puerto Rico, 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 millisecond pulsar PSR J0337+1715, 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. Illustration by Bill Saxton / NRAO / AUI / NSF.

The millisecond pulsar PSR J0337+1715, 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. Illustration by Bill Saxton / NRAO / AUI / NSF.

By very accurately recording the time of arrival of the pulsar’s pulses, the team was 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. Some of our measurements of the relative positions of the stars in the system are accurate to hundreds of meters,” added study co-author Dr Anne Archibald of the Netherlands Institute for Radio Astronomy.

“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,” Dr Ransom said.

“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.”

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,” Dr 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 PSR J0337+1715. If the 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.

“By doing very high-precision timing of the pulses coming from the pulsar, we can test for such a deviation from the strong equivalence principle at a sensitivity several orders of magnitude greater than ever before available,” explained study co-author Dr Ingrid Stairs of the University of British Columbia.

“Finding a deviation from the strong equivalence principle would indicate a breakdown of General Relativity and would point us toward a new, revised theory of gravity.”

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Ransom SM et al. A millisecond pulsar in a stellar triple system. Nature, published online January 05, 2014; 10.1038/nature12917