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Double pulsar find to test relativity


An international team of scientists working in the UK, Australia, Italy and the USA has made an astronomical discovery that has major implications for testing Einstein’s general theory of relativity.

Using the 64-m CSIRO Parkes radio telescope in New South Wales, Australia, the team recently detected the first system of two pulsars orbiting each other - the only system of its kind found so far among the 1400-plus pulsars discovered in the last 35 years.

Team member Dr. Richard Manchester of CSIRO’s Australia Telescope National Facility described the pulsar pair - PSR J0737-3039A and PSR J0737-3039B - as a "fantastic natural laboratory" for testing Albert Einstein’s famous hypothesis.

A radio pulsar is a special type of neutron star - a city-sized ball of extremely dense matter - which spins and emits radio waves. All radio pulsars are neutron stars, but not all neutron stars are radio pulsars.

The researchers originally believed the new-found duo consisted of a pulsar with a period of 23 milliseconds and a non-pulsing companion neutron star.

They announced the discovery of this system in December [Nature 4 December, 2003] but follow-up observations with the Parkes telescope and the 76-m Lovell Telescope at the University of Manchester in Cheshire, UK, revealed the occasional presence of radio pulses with a period of 2.8 seconds from the companion.

"While experiments on one pulsar in such an extreme system as this are exciting enough, the discovery of two pulsars orbiting one another opens up new precision tests of general relativity," said Dr. Andrew Lyne, Director of the University’s Jodrell Bank Observatory.

By chance, the orbit of the two stars is nearly edge-on to us, and one pulsar’s radio signal periodically eclipses the other’s.

"This provides us with a wonderful opportunity to probe the physical conditions of a pulsar’s outer atmosphere, something we’ve never been able to do before," said Dr. Andrea Possenti of Cagliari Astronomical Observatory.

The two pulsars lie 1600-2000 light-years (500-600 pc) away in our Galaxy and are separated by 800,000 km, about twice the distance between the Earth and Moon. They orbit each other in 2.4 hours, which makes them some of the fastest-moving stars known.

The two stars will gradually draw closer together, with the orbital energy being lost from the system in the form of gravitational radiation.

This effect, which provided strong evidence for the existence of gravitational waves, was first measured by Russell Hulse and Joseph Taylor in the first-known ’binary pulsar’ system ? a pulsar, PSR 1913+16, and its neutron star companion. (For their discovery of this system in 1974, Hulse and Taylor won the 1993 Nobel Prize for Physics.)

The PSR J0737-3039 system is 10-times closer to Earth than is PSR 1913+16, which makes it easier to study.

The two pulsars in the new system coalesce in about 85 million years, sending a ripple of gravity waves across the Universe. The characteristics of the system suggest that such coalescences occur more often than previously thought. "The news has been welcomed by gravitational wave hunters, since it boosts their hopes for detecting the gravitational waves," said Professor Nichi D’Amico of Cagliari University.

The surveys designed by the team to discover new pulsars at the Parkes Telescope have been extraordinarily successful. They have discovered over 700 pulsars in the last five years, nearly as many as were discovered in the preceding 30 years. The discovery of the double pulsar system is the major jewel in the crown.

The discovery was announced online in ’Science Express’ on 8 January and will be presented at the Binary Radio Pulsars meeting at the Aspen Center for Physics in Aspen, Colorado, from 4:30 pm Monday, 12 January (Aspen time).


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 Express, 8 January,
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Background information

A pulsar is the collapsed core of a massive star that has ended its life in a supernova explosion. Weighing more than our Sun, yet only 20 kilometres across, these incredibly dense objects produce beams of radio waves which sweep round the sky like a lighthouse, often hundreds of times a second. Radio telescopes receive a regular train of pulses as the beam repeatedly crosses the Earth so the objects are observed as a pulsating radio signal.

Pulsars make exceptional clocks, which enable a number of unique astronomical experiments. Some very old pulsars, which have been ’spun-up’ to speeds of over 600 rotations per second by material flowing onto them from a companion star, appear to be rotating so smoothly that they may even ’keep time’ more accurately than the best atomic clocks here on Earth. Very precise timing observations of systems in which a pulsar is in orbit around another neutron star proved the existence of gravitational radiation as predicted by Albert Einstein and have provided very sensitive tests of his theory of General Relativity ? the theory of gravitation which supplanted that of Isaac Newton. The neutron star binary system reported in this paper is one of these systems, with an orbit that is decaying more rapidly than any previously discovered.

The Parkes survey using a multi-beam system that led to the discovery of the double-pulsar system is an international collaboration of a team of astronomers from the UK, Australia, Italy and the USA. The researchers have been surveying our Galaxy, the Milky Way, for new radio pulsars using the 64-metre Parkes Radio Telescope in New South Wales, Australia. Following initial detection at Parkes, confirmation and follow-up observations for many of the new pulsars are made with the 76-metre Lovell Radio Telescope at Jodrell Bank. The main processing of the survey in which the PSR J0737-3039 system was discovered was conducted on a cluster of computers at Cagliari Astronomical Observatory.

Bill Stephens | CSIRO
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