The Disappearance of the Cosmic Spinning Top

Scientists measured the space-time warp in the gravity of a binary star and determined the mass of a neutron star — just before it disappeared.

"Our result is important because weighing stars while they freely float through space is exceedingly difficult," said Joeri van Leeuwen, an astrophysicist at The Netherlands Institute for Radio Astronomy ASTRON, and University of Amsterdam, The Netherlands, who led the study. "That is a problem because such mass measurements are required for precisely understanding gravity, the force that is intimately linked to the behavior of space and time on all scales in our Universe."

An international team of astronomers has now measured both masses in a relativistic binary pulsar system known as PSR J1906+0746 or J1906 for short. The pulsar spins and emits a lighthouse-like beam of radio waves every 144 milliseconds. It orbits another neutron star — or maybe a white dwarf — in a little under 4 hours.

Only in a handful of other double neutron stars have masses been measured, and J1906 is by far the youngest. As the supernova explosion that formed J1906 occurred only 100,000 years ago, the binary is in a remarkably pristine and unevolved state. Normal pulsars live to be around 10 million years old; they can then be recycled by binary companion to live for yet another 1 billion years. If the companion to J1906 is a neutron star, it is likely recycled, although it appears to not be shining our way.

The results are published in The Astrophysical Journal, and presented at the 225th meeting of the American Astronomical Society in Seattle, on January 8.

The pulsar was discovered in 2004 with the Arecibo Observatory, the world’s most sensitive radio telescope due to its large, 305-meter dish. From that moment on, the team monitored the pulsar almost daily with the 5 largest radio telescopes on Earth: the Arecibo Telescope (USA), the Green Bank Telescope (USA), Nançay Telescope (France), the Lovell Telescope (UK) and the Westerbork Synthesis Radio Telescope (The Netherlands). Over 5 years, that campaign kept exact score of all rotations of the pulsar — an astounding one billion in total.

"By precisely tracking the motion of the pulsar, we were able to measure the gravitational interaction between the two highly compact stars with extreme accuracy," notes co-author Ingrid Stairs, professor of physics and astronomy at The University of British Columbia, Canada. "These two stars each weigh more than the Sun, but are still over 100 times closer together than the Earth is to the Sun. The resulting extreme gravity causes many remarkable effects."

One of these is geodetic precession. When you start a spinning top, it doesn’t only rotate – it also wobbles. According to general relativity, neutron stars, too, should start to wobble as they move through the gravitational well of a massive, nearby companion star. Orbit after orbit the pulsar travels through a space-time that is curved, which leaves an imprint on the spin axis.

The team now measured this geodetic precession in J1906. Because of the curved space time, 1 part in about a million of the pulsar’s orbit is “missing”, compared to a flat space time. Over the course of an Earth year of observations, this adds up to a change of 2.2 degrees in the orientation of the pulsar rotation axis.

"Through the effects of the immense mutual gravitational pull, the spin axis of the pulsar has now wobbled so much that the beams no longer hit Earth," said van Leeuwen. "The pulsar is now all but invisible to even the largest telescopes on Earth. This is the first time such a young pulsar has disappeared through precession. Fortunately this cosmic spinning top is expected to wobble back into view but it might take as long as 160 years."

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