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Unique Double Pulsar Tests Einstein's Theory

In this World Year of Physics, celebrating 100 years since Albert Einstein published his three epoch-making papers, astronomers are using a unique pair of orbiting pulsars to make extremely precise measurements of gravitational effects predicted 90 years ago by Einstein, and so far, the famous physicist's predictions are proving exactly correct.

"This pair of orbiting pulsars, discovered only two years ago, is one of the best laboratories in the Universe for studying the effects of Einstein's theory of General Relativity," said Dr. Ingrid Stairs, an assistant professor at the University of British Columbia (UBC) in Vancouver, Canada. Stairs presented a report on behalf of an international research collaboration to the American Astronomical Society's meeting in Minneapolis, Minnesota.

The double pulsar system, PSR J0737-3039A and B, is 2000 light-years away in the direction of the constellation Puppis. It was discovered in 2003 by an international team of astronomers and consists of two massive, highly compact neutron stars orbiting each other once every 2.4 hours. Both neutron stars emit lighthouse-like beams of radio waves that are seen as radio "pulses" every time the beams sweep past the Earth.

The Double
An artist's impression of the orbits of the two pulsars (not to scale), showing the "lighthouse" emission beams. The underlying grid illustrates the general relativistic warping of space near the two massive compact stars

While several other systems contain two neutron stars, this is the only one in which both stars are observed to produce radio pulses. Pulsar A spins once every 23 milliseconds, while Pulsar B is much slower, rotating once every 2.8 seconds.

The astronomers have been closely tracking this exciting duo with the Australia Telescope National Facility's (ATNF) 64-meter Parkes Telescope, the National Science Foundation's (NSF) 100-meter Robert C. Byrd Green Bank Telescope (GBT) in West Virginia, and the 76-meter Lovell telescope at Jodrell Bank Observatory (JBO) in England.

Because the pulsars orbit each other so quickly, a number of effects of General Relativity (GR) are predicted to be quite large. In fact, four such effects were measured within a few months of discovery.

Now, the astronomers say they have measured another very significant phenomenon -- the stars' orbit is shrinking because of a loss of energy caused by the emission of gravity waves. The orbit is currently shrinking by 7 millimeters per day, and this decay will accelerate in the future. This means, the scientists say, that the two superdense neutron stars will collide in 85 million years.

"The measured decay of the orbit is exactly what is predicted by GR, so this is another important victory for Einstein's theory," says Stairs. A similar measurement of the orbital decay in another double-neutron-star system earned that system's discoverers, Russell Hulse and Joseph Taylor, the 1993 Nobel Prize in Physics.

Meanwhile, other GR effects are also starting to crop up. One important prediction is that the stars should wobble like tops as they move in the curved space-time of their orbit. This should result in changes in the observed radio pulse shapes, as telescopes on Earth see slightly different parts of the irregularly shaped "lighthouse" beams.

The team now has good evidence for this wobble in the slower B pulsar. Its radio-wave-emitting region is buffeted by the strong particle winds from the A pulsar, meaning that B is only observable at certain times of the orbit. Now the times when it is visible have started to change, and the pulse shapes also have changed. "While we thought these variations were likely due to the wobble, there was a lingering possibility that they could have been due to other GR-induced changes in the orientation of the orbit," says team member Dr. Marta Burgay of Cagliari Astronomical Observatory (INAF-OAC). Now, however, the sensitive GBT observations have revealed small profile variations in the powerful A pulsar as well.

"The A pulse changes can only be explained by the predicted wobble, which gives us more confidence that the observed changes in B are also due to a wobble," says team member Dr. Michael Kramer of JBO.

More observations of this unique pair will be needed to use the observed wobbles to figure out the full geometry of the system, including the exact directions of the pulsar spin axes and the sizes of the lighthouse beams. These ongoing observations will also, in the long term, provide the most precise test of the predictions of Einstein's theory.

"There is a lot more to be learned from this system," says Dr. Richard Manchester of the ATNF.

The full team consists of:

Dr. Ingrid Stairs and Mr. Robert Ferdman, UBC
Dr. Michael Kramer, Prof. Andrew Lyne, Dr. Maura McLaughlin, and Dr. Duncan Lorimer, JBO
Dr. Richard Manchester, ATNF
Drs. Marta Burgay, Andrea Possenti and Nichi D'Amico, INAF-OAC
Dr. Paulo Freire, Arecibo Observatory
Dr. B. C. Joshi, National Centre for Radio Astrophysics, India
Dr. Fernando Camilo, Columbia University.

Relevant published papers include:

        M. Burgay, A. Possenti, R. N. Manchester, M. Kramer, M. A. McLaughlin,
        D. R. Lorimer, I. H. Stairs, B. C. Joshi, A. G. Lyne, F. Camilo,
        N. D'Amico, P. C. C. Freire, J. M. Sarkissian, A. W. Hotan,
        G. B. Hobbs,
        "Long-term Variations in the Pulse Emission from PSR J0737-3039B,"

        Astrophys. J. Lett., 624, L113 (2005).

        R. N. Manchester, M. Kramer, A. Possenti, A. G. Lyne, M. Burgay,
        I. H. Stairs, A. W. Hotan, M. A. McLaughlin, D. R. Lorimer,
        G. B. Hobbs, J. M. Sarkissian, N. D'Amico, F. Camilo, B. C. Joshi,
        P. C. C. Freire,
        "The Mean Pulse Profile of PSR J0737-3039A,"
        Astrophys. J. Lett., 621, L49 (2005).

        A. G. Lyne, M. Burgay, M. Kramer, A. Possenti, R. N . Manchester,
        F. Camilo, M. A. McLaughlin, D. R. Lorimer, N. D'Amico, B. C. Joshi,
        J. Reynolds and P. C. C. Freire,
        "A Double-Pulsar System -- A Rare Laboratory for Relativistic Gravity and Plasma Physics,"
        Science 303, 1153 (2004)

        M. Burgay, N. D'Amico, A. Possenti, R. N. Manchester A. G. Lyne,
        B. C. Joshi, M. A. McLaughlin, M. Kramer, J. M. Sarkissian, F. Camilo,
        V. Kalogera, C. Kim, D. R. Lorimer,
        "An increased estimate of the merger rate of double neutron stars from observations of a highly relativistic system,"
        Nature 426, 531 (2003).