The wobbling orbit of a pulsar proves Einstein right, yet again

New observations of ‘frame dragging’ help reveal details of the final days of a pair of stars

white dwarf illustration

A white dwarf (illustrated, center) twists spacetime as it spins, forcing the orbit (pink) of a neighboring pulsar (illustrated with radio jets) to wobble.

Mark Myers/ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)

Chalk up yet another win for Einstein.

A twist in the fabric of spacetime — predicted by the physicist’s theory of general relativity (SN: 10/7/15) — is causing the orbit of one stellar corpse to teeter around another stellar corpse, researchers report. And the relativistic corkscrew is helping astronomers reconstruct the final days of these two long-dead stars.

According to general relativity, any spinning mass drags spacetime around with it, like a hand mixer in molasses. One way to see this “frame dragging” is to keep a careful eye on anything circling the spinning object on a tilted orbit — the spacetime maelstrom will make the orbit wobble, or precess.

For the last 20 years, researchers have been using radio telescopes to track the motion of a pulsar, the dense remains of a massive star that went supernova, as it orbits a spinning white dwarf, the core of a lighter star that died less violently. The pulsar, dubbed PSR J1141–6545, emits a steady beat of radio waves as it spins, and by recording the arrival times of those pulses, researchers can tell when the pulsar is moving toward and away from Earth.

Over those two decades, the orbit of the pulsar has been slowly precessing, astronomers report. The precession isn’t much — the orbit’s tilt drifts by just 0.0004 degrees per year. But it matches what researchers expect if the neighboring white dwarf whips up spacetime as it spins. Vivek Venkatraman Krishnan, an astrophysicist at the Max Planck Institute for Radio Astronomy in Bonn, Germany, and colleagues report the results in the Jan. 31 Science.

This finding isn’t the first time that researchers have observed frame dragging. Satellites in Earth’s orbit have captured the relatively puny effect around our planet (SN: 11/24/15). And astronomers also have observed fluctuations in the frequency of X-ray light coming from a black hole, where frame dragging should be quite intense, suggesting that gas may be precessing around it (SN: 12/17/15).

The new observation “is much more direct than mine,” says Adam Ingram, an astrophysicist at the University of Oxford who studied the black hole. “I can only infer that something is precessing in black hole systems, whereas the precision radio observations presented here leave little room for ambiguity.”

The pulsar precession helps researchers piece together the final moments in the lives of both stars. Relativistic wobbling occurs only if the orbit of the pulsar and the spin of the white dwarf are misaligned, something which is usually smoothed over by an exchange of mass between the dying stars. “This immediately tells us that the orbit was tilted due to the supernova explosion that produced the pulsar,” Venkatraman Krishnan says.

Normally, the supernova would go off and then the progenitor of the white dwarf would dump gas on the pulsar after the explosion, aligning spin to orbit. But in this case, the opposite happened: The pulsar’s progenitor dumped gas on the white dwarf and then the supernova occurred.

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

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