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After 14 years of staring at a dead star, astronomers once again confirmed Einstein's theory of general relativity. PSR J1906 + 0746, a pulsar 25,000 light-years away, wobbles slightly when spinning – an effect that could see its pulses disappear from our sky in less than a decade.
This is called precession, a phenomenon predicted by general relativity that has been observed only in very few pulsars. The new discoveries could help us set a limit on the number of binary pulsars in the galaxy, allowing us to determine the expected rate of binary neutron star collisions.
Pulsars are perhaps the most useful stars in the sky. They are fast rotating neutron stars with bright radio waves emitted by their magnetic poles. When they turn, these beams can sweep the Earth, depending on the orientation of the star: a bit like a lighthouse.
They are also incredibly accurate, with predictable rotations up to a millisecond. These pulsars in milliseconds can keep time so precise that they could guide future navigation in space.
But even the majority of pulsars – those that do not have this level of accuracy in milliseconds – are still useful, especially for general relativity tests. Indeed, according to general relativity, the pulsars in the binary systems should present a slight axial ripple (think of a slowing turntable). This is the axial precession.
As neutron stars are so dense – 1.4 times the mass of the Sun, clustered in a stellar nucleus 20 kilometers in diameter – their gravitational intensity should deform space-time.
When the spin orientation is not correctly aligned with the orientation of the binary orbit, this should cause the pulsar spin in an axial precession. It is thought that such misalignment is due, for example, to an asymmetric supernova explosion.
Thus, while the pulsar is wobbling on its axis, we should be able to detect changes in its pulse profile.
When PSR J1906 + 0746 was discovered in 2004, it showed two separate bent or polarized (beam) transmissions per rotation. However, when a team of astronomers led by Gregory Desvignes from the Max Planck Institute of Radioastronomy went to search the archival data collected by the Parkes Observatory radio telescope. she found only one beam.
To understand what was happening with their subject of study, between 2005 and 2009, with the radio telescopes of Nançay and Arecibo and between 2012 and 2018 with Arecibo, the team monitored the PSR J1906 + 0746.
When they started observing the star in 2005, they saw the two beams per rotation detected in 2004. Gradually, the beam of the North Pole of the star was weakened; in 2016, he had completely disappeared.
The team predicted that the polarization data contained information on pulsar precession. They modeled these data, extending them for 50 years, and then comparing them to the pulsar observation data.
It corresponded, with a level of uncertainty of only 5%, perfectly corresponding to the predictions of general relativity – as well as to predictions concerning the polarization properties of pulsars published 50 years ago by Venkatraman Radhakrishnan and David Cooke.
The team also realized that the line of sight of the Earth had crossed the magnetic pole of the pulsar in a north-south direction, which allowed it to map the pulsar beam – which allowed it to determine the proportion of illuminated sky by the beam. .
This helps to estimate the number of neutron stars in the galaxy, which can help determine the number of binaries that should collide, producing gravitational waves.
And their model did not work in reverse. Seeing how this fit the observation data meant that they could also predict. The team believes that the southern beam will also disappear around 2028.
It should reappear between 2070 and 2090, while the north beam should reappear between 2085 and 2105.
"Pulsars can provide gravity tests that can not be performed in any other way," said astronomer Ingrid Stairs of the University of British Columbia. "This is another example of such a test."
The search was published in Science.
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