In 1999, the Parkes Radio Telescope in Australia discovered a binary system made up of a fast-spinning white dwarf and a pulsar, a rapidly rotating neutron star created in a supernova blast. The pulsar – PSR J1141-6545 – completes one orbit every 4.8 hours at a velocity of nearly 1 million kilometres per hour (620,000 mph).
“This pulsar’s orbit is very special,” said Vivek Venkatraman Krishnan, a scientist at the Max Planck Institute for Radio Astronomy who has been studying the unusual binary since he was a Ph.D. student. “It hurtles through space with a maximum speed of almost a million km/h in its orbit as the maximum separation between the stars is barely larger than the size of our Sun.”
The white dwarf is the slowly cooling, roughly Earth-size remnant of a Sun-like star that exhausted its nuclear fuel. Theoretical models suggest the white dwarf formed before the supernova that created the pulsar. The models predict that before that titanic blast happened about 1.5 million years ago, the white dwarf stripped mass away from its larger companion, speeding up its rotation.
Measuring that rotation is a key test of such models, but that posed a problem in this case because PSR J1141-6545 and its white dwarf companion are too far away for the spectral analysis that would normally be used. Instead, the researchers took advantage of the clock-like radio emissions from the pulsar.
“With the help of atomic clocks, we were able to perform highly accurate measurements of the arrival times of the pulsar signals at the Parkes and UTMOST radio telescopes,” said Vivek Venkatraman Krishnan, lead author of a paper in the journal Science. “We could track the pulsar in its orbit with an average ranging precision of 30 kilometres (19 miles) per measurement, over a period of almost twenty years. This led to a precise determination of the size and orientation of the orbit.”
Albert Einstein showed that the rotation of a massive body will distort the space around it in a sort of swirl now known as “frame dragging.” Frame dragging caused by Earth’s rotation was measured by satellite and found to match the predictions of relativity theory. One of those experiments – LAGEOS 1 and 2 – used laser ranging to measure a slow precession of the satellites’ orbital plane in the direction of the planet’s rotation, a phenomenon known as Lense-Thirring precession.
Over two decades of observation, the orbital plane of PSR J1141-6545 precessed by about 150 kilometres (19 miles), in line with what was predicted and a direct confirmation of frame dragging around the white dwarf.
A detailed analysis, taking into account the Lense-Thirring effect, showed the white dwarf is rotating every 100 seconds or so, confirming the theorised mass transfer before the pulsar’s progenitor exploded.
“Here Albert Einstein gave us a tool, which we can now use to find out more about pulsars and their companions in the future,” said Matthew Bailes, a researcher at Swinburne University in Australia an co-author of the Science paper.