An international team of astronomers using ESO’s Very Large Telescope (VLT) has discovered and studied a strange stellar pair about 7,000 light-years away that consists of the most massive neutron star confirmed so far, orbited every 2.46 hours by a white dwarf star. According to the team, this binary proves that Albert Einstein’s theory of relativity is still right.

The neutron star is a pulsar labeled PSR J0348+0432. It is twice as heavy as the Sun, but just 20 km across. The gravity at its surface is more than 300 billion times stronger than that on Earth and at its center every sugar-cubed-sized volume has more than one billion tones of matter squeezed into it. Its companion white dwarf star is only slightly less exotic – it is the glowing remains of a much lighter star that has lost its atmosphere and is slowly cooling.

John Antoniadis of the Max Planck Institute for Radio Astronomy in Bonn, lead author of a paper reporting the discovery in the journal Science (arXiv.org version), said: “I was observing the system with ESO’s Very Large Telescope, looking for changes in the light emitted from the white dwarf caused by its motion around the pulsar. A quick on-the-spot analysis made me realize that the pulsar was quite a heavyweight. It is twice the mass of the Sun, making it the most massive neutron star that we know of and also an excellent laboratory for fundamental physics.”

Einstein’s general theory of relativity, which explains gravity as a consequence of the curvature of space-time created by the presence of mass and energy, has withstood all tests since it was first published almost a century ago. But it cannot be the final explanation and must ultimately break down.

Scientists have devised other theories of gravity that make different predictions from general relativity. For some of these alternatives, these differences would only show up in extremely strong gravitational fields that cannot be found in the Solar System.

In terms of gravity, PSR J0348+0432 is a truly extreme object, even compared to the other pulsars that have been used in high precision tests of Einstein’s general relativity. In such strong gravitational fields small increases in the mass can lead to large changes in the space-time around such objects. Up to now astronomers had no idea what would happen in the presence of such a massive neutron star as PSR J0348+0432. It offers the unique opportunity to push tests into new territory.

The team combined VLT observations of the white dwarf with very precise timing of the pulsar from radio telescopes. Such a close binary radiates gravitational waves and loses energy. This causes the orbital period to change very slightly and the predictions for this change from general relativity and other competing theories are different.

“Our radio observations were so precise that we have already been able to measure a change in the orbital period of 8 millionths of a second per year, exactly what Einstein’s theory predicts,” said co-author Dr Paulo Freire, also from the Max Planck Institute for Radio Astronomy.

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Bibliographic information: John Antoniadis et al. 2013. A Massive Pulsar in a Compact Relativistic Binary. Science, vol. 340, no. 6131; doi: 10.1126/science.1233232