Astronomers spotted a star dragging space-time around it

Mark Myers, OzGrav ARC Centre of Excellence.

In Albert Einstein’s General Relativity theory gravity is not a force. The presence of a mass doubles the space around it, which means that objects that move near that mass deviate from a straight path.

The deviation makes it appear as the object is being dragged towards the mass by a force we call gravity. When a large mass is spinning, space also rotates slightly in its direction. This effect is known as dragging.

Years after Einstein broke down his theory, an international team of astrophysicists led by Australian professor Matthew Bailes, member of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), has shown new and exciting tests of space-time dragging, observing how the spin of a white dwarf twists space and time.

The effect is so subtle that it was first measured a decade ago and the data, which provide additional evidence to Albert Einstein's theory of general relativity, has been published by the journal Science.

For the last 20 years, researchers have been using radio telescopes to track the movement of a pulsar - which consists of dense remains of a massive star that became a supernova - and a rotating white dwarf. The white dwarf is about the size of the Earth, but around 300,000 times heavier, and has a pulse radius of just the size of a city, but 400,000 times denser.

The pulsar, called PSR J1141–6545, emits a steady rhythm of radio waves as it rotates, and by recording the arrival times of those pulses, researchers are able to find out when the pulsar moves to and from Earth. This exotic star pair is 12,000 light-years away from Earth, in the Musca constellation.

Pulsars are incredibly precise clocks, which emit radio waves from their magnetic poles at precise intervals. The scientist’s team used twenty years of radio data to measure subtle changes in the orbit of 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,” explained Vivek Venkatraman Krishnan, member of the Max Planck Institute for Radio Astronomy.

According to general relativity, any rotating mass drags space-time with it. A turn in the structure of space-time, predicted by the theory, is causing the orbit of a stellar corpse to wobble around another stellar corpse. This event is helping astronomers rebuild the last days of these two dead stars a long time ago.

The white dwarf and the pulsar orbit each other every five hours, so the dragging is quite weak, despite the size of the star. Even so, over the course of 20 years, astrophysicists estimated that the orbit of the pulsar would have drifted about 150 kilometres.

“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 Willem van Straten, co-author of the study.

They also discovered that the white dwarf rotates on its axis every 100 seconds, confirming the hypothesis of the moment of its formation.

It would be normal for the supernova to explode and then the parent of the white dwarf to throw gas to the pulsar after the explosion, aligning the turn to orbit.

However, the opposite has happened in this case, as the progenitor of the pulsar was the one who threw gas on the white dwarf and then the supernova took place.

Reference: V. Venkatraman Krishnan el al., "Lense – Thirring frame dragging induced by a fast-rotating white dwarf in a binary pulsar system," Science (2020).… 1126 / science.aax7007

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