In the southern constellation of Musca, the Fly, there is a truly peculiar binary star system. A pulsar, a type of pulsating neutron star, and a massive white dwarf orbit each other every five hours. Unlike other pulsar-white dwarf systems, all models suggested that in this case, the white dwarf formed first and stole material from the star, which eventually exploded leaving the pulsar behind. The issue was proving that this was the case.
If the white dwarf stole material then this would cause it to start rotating faster. This star is too faint to see, but scientists have now been able to do just that using one of the effects predicted in general relativity: frame-dragging. According to the theory, any massive object that's rotating drags a bit of space-time along with it. This is known as the Lense-Thirring effect. This has been demonstrated around Earth, with the rotation of our planet subtly influencing the orbit of satellites around it.
We can’t put a satellite around this white dwarf but as demonstrated in Science, we do not need to. Pulsars are incredibly precise clocks, emitting radio waves from their magnetic poles at precise intervals. The team used 20 years' worth of radio data to measure subtle changes in the orbit of the pulsar, which is known as pulsar J1141-6545.
“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," lead author Dr Vivek Venkatraman Krishnan, from the Max Planck Institute for Radio Astronomy, said in a statement. "We could track the pulsar in its orbit with an average ranging precision of 30 km per measurement, over a period of almost 20 years. This led to a precise determination of the size and orientation of the orbit.”
The white dwarf and pulsar orbit each other every five hours, so the frame-dragging is quite weak, despite the size of the star. Still, over the course of 20 years, it becomes sizable and the team estimated that the orbit of the pulsar would have drifted by about 150 kilometers (93 miles). And that's what they've observed.
“Observations of pulsar J1141-6545 indeed show such a deviation which, after detailed calculations and ruling out a range of potential experimental errors, were confirmed to be caused by a change in its orbital orientation”, explained co-author Dr Willem van Straten, from Auckland University of Technology in New Zealand.
Taking into consideration the full relativistic description and combining it with models, the astronomers were able to determine the masses of the two degenerate stars as well as orbital parameters like inclination. They also showed that the white dwarf spins on its axis every 100 seconds, confirming the hypothesis of when it formed.