A heavyweight for Einstein

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

April 25, 2013

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.

An artist’s impression of the PSR J0348+0432 binary system. The pulsar (with radio beams) is extremely compact, leading to a strong distortion of space-time (illustrated by the green mesh). The white-dwarf companion is shown in light-blue.

Imagine half a million Earths packed into a sphere 20 kilometres in diameter, spinning faster than an industrial kitchen blender. These extreme conditions, almost unimaginable by human standards, are met in a neutron star – a type of stellar remnant formed in the aftermath of a supernova explosion. Neutron stars often catch the attention of astronomers because they offer the opportunity to test physics under unique conditions. They were first discovered almost half a century ago as pulsars which emit radio pulses like a lighthouse. Pulsar research has been honoured with two Nobel prizes, one for their discovery (1974) and one for the first indirect detection of gravitational waves (1993) – a consequence of Einstein’s theory of General Relativity.

PSR J0348+0432 is a pulsar in orbit with a white-dwarf, recently discovered using the Green-Bank radio telescope in an ongoing global effort to find more of these exciting pulsars. With a separation of just 830,000 km, the pulsar and the white dwarf in this system are close enough to emit a significant amount of gravitational waves. This should make the orbital size and period shrink, as predicted by General Relativity. To verify this prediction, one needs to know both the mass of the pulsar and its companion.

“I was observing the system with ESO’s Very Large Telescope in Chile, trying to detect changes in the light emitted from the white dwarf caused by its two million km/h motion around the pulsar.” says John Antoniadis, IMPRS student at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn and leading author of the paper. “This allows us to weigh both, the white dwarf and the pulsar. After a quick on-the-spot analysis I realized that the pulsar was quite a heavyweight: a mass twice that of the Sun, making it the most massive neutron star we know of.”

With these masses at hand, one can calculate the amount of energy taken away from the system by gravitational waves, causing the orbital period to shrink. The team immediately realized that this change in the orbital period should be visible in the radio signals of the pulsar and turned its full attention to PSR J0348+0432, using the three largest single-dish radio telescopes on Earth. “Our radio observations with the Effelsberg and Arecibo telescopes were so precise that by the end of 2012 we could already measure a change in the orbital period of 8 microseconds per year, exactly what Einstein’s theory predicts”, states Paulo Freire, scientist at MPIfR. “Such measurements are so important that the European Research Council has recently funded BEACON, a new state-of-the-art system for the Effelsberg radio telescope.”

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