An international team of scientists from the Jodrell Bank Observatory in the UK and from Australia, Italy, India and the USA have announced in today's issue of the journal Science the first discovery of a double pulsar system.
They have shown that the compact object orbiting the 23-millisecond pulsar PSR J0737-3039A, which they reported previously in Nature is not only, as suspected, another neutron star, but is also a detectable pulsar. The companion, PSR J0737-3039B, is rotating once every 2.8 seconds and orbits PSR J0737-3039A in only 2.4 hours. Both stars in this remarkable binary system have masses greater than that of our Sun, but are only 20km across and have an orbital separation which is less than the size of the Sun.
Professor Andrew Lyne of the University of Manchester points out that "While experiments on one pulsar in such an extreme system as this are exciting enough, the discovery of two pulsars orbiting one another will allow us to undertake precise new tests of general relativity and to probe energetic physical processes that occur in the magnetosphere, or `extended atmosphere', of a neutron star."
The 23-millisecond pulsar was first detected in April 2003 using the 64-m Parkes radio telescope in New South Wales, Australia. Subsequent observations were made both at Parkes and with the 76-m Lovell Telescope of the University of Manchester's Jodrell Bank Observatory in Cheshire.
Already, four different effects beyond those predicted by Newton's laws of gravity have been measured and are completely consistent with Einstein's general theory of relativity. Dr. Richard Manchester of the Australia Telescope National Facility says "The fact that both objects are detectable as pulsars enables completely new high-precision tests of gravitational theories. This system is really extreme." Future observations of the two stars will measure their slow spiral in towards each other as they emit gravitational waves - a dance of death leading to their ultimate fusion into what may become a black hole.
The stars will coalesce in only approximately 85 million years, sending a ripple of gravity waves across the Universe. The discovery of the system shows that such coalescences will occur more frequently than previously thought. "The news has been welcomed by gravitational wave hunters, since it boosts their hopes for detecting these elusive signals." says Prof. Nichi D'Amico of Cagliari University.
General relativity also predicts that the two stars will slowly wobble like spinning tops. Future observations of this "precession" will allow new tests of Einstein's theory. "This effect will ultimately shift the pulsars' beams out of our line of sight, making them undetectable", explains Dr. Michael Kramer of the University of Manchester.
Another unique aspect of the new system is the strong interaction between the radiation from the two stars. By chance, the orbit is seen nearly edge on to us, and the signal from one pulsar is eclipsed as it passes behind the other. This provides a unique opportunity to probe the physical conditions of a pulsar's magnetosphere, something has never been possible before.
The pulsar surveys responsible for this discovery have used "multibeam" receivers built at the Jodrell Bank Observatory and have used the Parkes Telescope and Jodrell Bank's Lovell Telescope for observations. They have been extraordinarily successful, discovering over 700 pulsars in the last 5 years, nearly as many as in the preceding 30 years. The discovery of this new binary system has been one of the "holy grails" of pulsar astronomy and will provide a wealth of astrophysical information for years to come.