A team of astrophysicists headed by Dr Joeri van Leeuwen of the Netherlands Institute for Radio Astronomy and the University of Amsterdam has successfully measured the space-time warp in the gravity of a relativistic binary system.
This image shows the ancient globular cluster Terzan 5, home to PSR J1906+0746. Image credit: ESA / NASA/ Hubble.
The system is located about 25,000 light-years away from our Sun. It contains a young pulsar, called PSR J1906+0746, with the second-shortest known orbital period.
Discovered in 2004 with the Arecibo Observatory, J1906+0746 spins and emits a lighthouse-like beam of radio waves every 144 milliseconds.
It orbits its companion star – another neutron star or white dwarf – in about 4 hours.
As the supernova explosion that formed the pulsar 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+0746 is a neutron star, it is likely recycled, although it appears to not be shining our way.
“By precisely tracking the motion of the pulsar, we were able to measure the gravitational interaction between the two highly compact stars with extreme precision,” said Prof Ingrid Stairs of the University of British Columbia, who is a co-author of the paper published Astrophysical Journal (arXiv.org version).
“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 scientists now measured this geodetic precession in J1906+0746. Because of the curved space time, one 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.”
Dr van Leeuwen, Prof Stairs and their colleagues also determined the masses of J1906+0746 and its companion.
“Our result is important because weighing stars while they freely float through space is exceedingly difficult,” Dr van Leeuwen said.
“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.”
He added: “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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J. van Leeuwen et al. 2015. The Binary Companion of Young, Relativistic Pulsar J1906+0746. ApJ 798, 118; doi: 10.1088/0004-637X/798/2/118
