Cosmic Lighthouse Disappears From View

Joeri Van Leeuwen. A pulsar's axis changes direction as it orbits a close companion

A pulsar has disappeared from our sight, but its vanishing act is pleasing scientists since it is consistent with predictions made using Einstein's general theory of relativity.

PSR J1906+0746 is a pulsar—a neutron star that emits beams of intense radiation. Like other pulsars, it rotates rapidly, causing these beams to sweep across the sky in a manner commonly compared to a lighthouse. The galaxy is almost certainly littered with pulsars that we know nothing about because their beams pass above or below the Earth.

The most scientifically valuable pulsars are those in binary systems, locked in mutual orbits with other stars. The pulsar B1913+16 is so important, it won its discoverers a Nobel Prize for providing a natural laboratory to confirm the theory of general relativity; the finding showed that the orbital distance between the two stars decays through the loss of energy.

PSR J1906+0746's orbit around its companion lasts just 3.98 hours, the second shortest of any known pulsar. A paper in The Astrophyiscal Journal estimates its mass as 1.29 times that of the sun, with the companion slightly larger at 1.32 solar masses.

The closeness of the orbits, and the similarity in mass of the two stars, distorts spacetime so strongly that it unbalances PSR J1906+0746, swinging its axis across the sky. This process, known as geodetic precession, shifts the direction of the beams as well. Astronomers realized that with time the beams would cease to sweep across the Earth.

Credit Joeri Van Leeuwen. The precession of PSR J1906+746 as it and its companion distort spacetime.

Since neither PSR J1906+0746 nor its companion is detectable through other means, this caused an urgent race to collect as much information as possible before we lost the signal. Five of the largest radio telescopes in the world were pressed into service, producing a wealth of data that is being analyzed after the beam largely disappeared from view in 2009.

The authors note that it is rare to witness such shifts in pulsar beams' planes. “In the vast majority of observed binary pulsar systems, the pulsar is the first-born compact object,” they say. These objects have “very stable rotation and evolve only very slowly.” The changes observed in PSR J1906+0746 result from the fact that the companion probably evolved into a neutron star or white dwarf before the pulsar, an astronomical pup at 112,000 years old, reached its current state.

The unusual circumstances gave astronomers a chance to use general relativity to predict the rate of precession and test this against what was observed. The results fitted the predictions well, providing yet another confirmation of general relativity against competing theories of gravity. Dr. Joeri van Leeuwen of the Netherlands Institute for Radio Astronomy says, “This is the first time such a young pulsar has disappeared through precession. Fortunately this cosmic spinning top is expected to wobble back into view, but it might take as long as 160 years.”

The authors also raise the possibility that the companion is a pulsar as well, and precession could eventually cause its beams to sweep across the Earth, prompting a recommendation for regular check-ups on the now invisible system.



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