A heavyweight for Einstein

Observations of the most massive neutron star confirm Einstein's Relativity Theory

An international research team led by astronomers from the Max Planck Institute for Radio Astronomy) used a collection of large radio and optical telescopes in Bonn to investigated in detail a pulsar and its white dwarf companion. The observations revealed a binary stellar system with unusual properties. The pulsar weighs twice as much as the sun, making it the most massive neutron star measure to date. This, in combination with its short orbital period of only 2.5 hours, provides new insights into the emission of gravitational radiation. The system thus serves as a laboratory for General Relativity in extreme conditions.

In terms of gravity, PSR J0348+0432 is a truly extreme object, even compared to other pulsars which have been used in high precision tests of Einstein’s general relativity. At its surface, for example, it has a gravitational strength that is more than 300 billion times stronger than that on Earth. In the centre of that pulsar, more than one billion tons of matter is squeezed into a volume of a sugar cube. These numbers nearly double the ones found in other ‘pulsar gravity labs’. In the language of general relativity, astronomers were able for the first time to precisely investigate the motion of an object with such a strong space-time curvature.

“The most exciting result for us was, that general relativity still holds true for such an extreme object”, says Norbert Wex, a theoretical astrophysicist in MPIfR’s fundamental physics research group. In fact, there are alternative theories that make different predictions, and therefore are now ruled out. In this sense, PSR J0348+0432 is taking our understanding of gravity even beyond the famous ‘Double Pulsar’, J0737-3039A/B, which was voted as one of the top ten scientific breakthroughs of 2004 by the ‘Science’ journal.

“Such extreme physical conditions are impossible to replicate in laboratories on Earth,” says Thomas Tauris, a member of the Stellar Physics group at the Argelander Institute for Astronomy at the University of Bonn. “We would certainly like to learn how nature built such systems for us.  For the J0348+0432 system, however, our formation theories are stretched to the limit. The system has a peculiar combination of properties: the tight orbital period and the pulsar’s high mass, relatively slow rotation and strong magnetic field. It therefore poses an interesting challenge to the understanding of binary evolution.”

Last but not least, these findings are also important for scientists who search for gravitational waves. On Earth, they are using large detectors, like the laser interferometers GEO600, LIGO and VIRGO. One of the key signals they are looking for in their data are the gravitational waves emitted by two neutron stars during those last few minutes when they quickly spiral towards each other and finally collide. Decades of mathematical research in general relativity were necessary to calculate the expected gravitational waves from such a collision. Those equations are needed to identify them in the detectors’ recordings. The first such identification is expected within the next five years.

“Our results on J0348+0432 provide added confidence in these equations for the whole range of neutron star masses observed in nature”, says Michael Kramer, director at MPIfR and head of its fundamental physics research group. “Given the great effort involved in deriving these equations, Einstein’s theory passing this test is good news for our colleagues in gravitational wave astronomy.” 

NJ/HOR