Sen— Astronomers have discovered an exotic double object, the most massive neutron star confirmed so far, closely orbited by a white dwarf star, that confirms Albert Einstein’s theory of general relativity.
An international team, using the European Southern Observatory's (ESO) Very Large Telescope, along with radio telescopes around the world, have found a tiny, but unusually heavy neutron star that spins 25 times each second, orbited every two and a half hours by a white dwarf star. The neutron star is a pulsar that is giving off radio waves that can be picked up on Earth by radio telescopes.
The pulsar, named PSR J0348+0432, is the remains of a supernova explosion. It is twice as heavy as the Sun, but only 20 kilometres across. The gravity at its surface is more than 300 billion times stronger than on Earth. At its centre each cubic centimetre contains more than one billion tonnes of matter. The companion white dwarf star is the glowing remains of a much lighter star that has lost its atmosphere and is slowly cooling.
Artist's impression of PSR J0348+0432 (right) and a white dwarf star (left). The relative sizes of the two objects are not drawn to scale. Image credit: ESO/L. Calçada
"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," says John Antoniadis, a PhD student at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn and lead author of the paper. "A quick on-the-spot analysis made me realise 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 explains gravity as a consequence of the curvature of spacetime created by the presence of mass and energy. It 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 under extreme conditions. It is not consistent with quantum mechanics or other theories of gravity that make different predictions from general relativity. These differences would only show up in extremely strong gravitational fields.
In terms of gravity, PSR J0348+0432 is a truly extreme object. Such a close binary radiates gravitational waves and loses energy, causing the orbital period to change. General relativity predicts the 2.46-hour orbital period of the two stars should decrease by eight-millionths of a second each year. The effect is too small to be detectable unless the objects that are extremely massive and in tight orbits as in this case.
The team combined the Very Large Telescope and William Herschel Telescope observations of the white dwarf with very precise timing of the pulsar from the Effelsberg, Arecibo and Green Bank radio telescopes.
"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," states Paulo Freire, another team member.
Astronomers will be using this unique object to test general relativity to ever greater precision as time goes on.