July 9, 2008

Stars aligned for Purdue physicist and international research group

The McGill University-led team included Maxim Lyutikov, assistant professor of physics at Purdue, and colleagues in Canada, the United Kingdom, the United States, France and Italy. The findings were published in the July 3 issue of the journal Science.

"A binary pulsar creates ideal conditions for testing general relativity's predictions because the larger and closer the masses are to one another, the more important relativistic effects are," said Rene Breton, the McGill astrophysics doctoral candidate who led the research team. "Differences between general relativity and alternative theories of gravity might only shake out in extremely powerful gravity fields such as those near pulsars."

More than 1,700 pulsars have been discovered in our galaxy, but this system of two pulsars orbiting one another is the only known double-pulsar system, Lyutikov said. It was discovered in 2003.

Pulsars, formed when massive stars explode as supernovae, are extremely dense stellar objects. They have a mass greater than that of the sun, but are compressed to the size of a mid-sized city.

As a pulsar spins, it emits a radio wave like a rotating lighthouse beacon. If the pulsar beam happens to swing in the direction of Earth, scientists can capture the waves using radiotelescopes and study the system, Lyutikov said.

"For more than 30 years scientists have hoped to find a double-pulsar system," Lyutikov said. "This system is exceptional because our line of sight to it is almost even with the plane of orbit. This allows us to take measurements that would otherwise be impossible."

Pulsars are too small and too distant for direct observations of their orientation, but measurements can be taken using the eclipses visible when one of the two pulsars passes in front of the other.

Einstein's theory predicts that in a strong gravitational field an object's spin axis should slowly change direction as it orbits around the other object.

"It is similar to a spinning top," Lyutikov said. "As it tilts slightly to one side, the spin axis slowly changes direction or precesses."

Around each pulsar is a magnetosphere shaped like a doughnut that spins with the pulsar. When the magnetosphere of the first pulsar partly absorbs the radio "light" being emitted from the other, an eclipse occurs. This eclipse allows researchers to determine the spatial orientation of the pulsar.

"The precession of the pulsar affects how the eclipse occurs, and the shape of the eclipse changes with time," Lyutikov said. "A comparison of how quickly the precession occurs and the predicted precession speed allowed for a completely new test of general relativity that was not possible by any other means."

After four years of observations, the research team determined that the pulsar's spin axis precesses just as Einstein predicted.

The researchers used a detailed geometric model of the double-pulsar system, based on a model created by Lyutikov, to predict the rate of precession. They found that the observations matched the predictions and confirmed the accuracy of the model, which could be used for a broad range of future studies.

"It is surprising how well the model worked because it is very simple but reproduces the observations very well," Lyutikov said. "It is very difficult for astrophysicists to test models. We have to wait for a system like the double pulsar to come along to see if it agrees with observations."

In addition to the strong gravitational field, pulsars have the strongest magnetic fields found anywhere in the universe, he said. The unit of measurement for magnetic fields is a Tesla. The strength of a typical refrigerator magnet is about one-millionth of a Tesla, and a medical magnetic resonance imaging machine produces a field of about 1 Tesla.

"The strongest magnetic fields we can achieve on Earth are about 50 Teslas," Lyutikov said. "Pulsars have magnetic fields as strong as 10 billion Teslas. Having the ability to take very detailed measurements of this system is exceptional and allows us both to test theories of gravity, as well as study plasma physics in conditions unattainable on Earth."

In addition to Breton and Lyutikov, co-authors of the paper are Victoria M. Kaspi of McGill Univeristy, Michael Kramer of the University of Manchester (England), Maura A. McLaughlin of West Virginia University and the National Radio Astronomy Observatory in Green Bank, W. Va., Scott M. Ransom of the National Radio Observatory in Charlottesville, Va., Ingrid H. Stairs and Robert D. Ferdman of the University of British Columbia, Fernando Camilo of Columbia University, and Andrea Possenti at the Osservatorio Astronomico di Cagliari, Poggio dei Pini in Capoterra, Italy.

The researchers studied the twin pulsar using the 100-metre Robert C. Byrd Green Bank Radio Telescope at the National Radio Astronomy Observatory in Green Bank, W. Va.

A series of animations of the double pulsar is available online at www.physics.mcgill.ca/~bretonr/doublepulsar/

Writer: Elizabeth Gardner, (765) 494-2081, ekgardner@purdue.edu

Source: Maxim Lyutikov, (765) 494-5396, lyutikov@purdue.edu

Purdue News Service: (765) 494-2096; purduenews@purdue.edu

Note to Journalists: This news release includes portions of a release issued by McGill University, available online at https://www.mcgill.ca/newsroom/news/item/?item_id=10082  

Photo caption: Illustration of the double pulsar PSR J0737-3039A/B (Image courtesy of René Breton Laboratory/McGill University) 

Double pulsar image: https://www.physics.mcgill.ca/~bretonr/doublepulsar/doublepulsar_eclipse.jpg

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