Pulsars spin neutron stars – compact remnants of supernova explosions that, among other things, emit beams of radio radiation. This radiation is observed on Earth in the form of very fast and regular pulses. This allows them to be used as astronomical clocks, and this, in turn, makes it possible to measure their orbital motions very precisely.
The “double pulsar” made by Kramer and his team consists of two radio pulsars that orbit each other at speeds of about one million kilometers per hour in just under two hours. One pulsar rotates about 44 times per second, while the other rotates about three seconds. Both pulsars have a mass about thirty percent more than our sun, but only 15 miles in size.
Using seven radio telescopes, including the Westerbork Synthesis Radio Telescope, astronomers have been able to measure the amount of energy pulsars lose more accurately than ever before because they emit so-called gravitational waves. This effect has been measured before with another double pulsar, but not nearly as accurately.
The measurement results are not only fully consistent with general relativity, but also show expected effects that we have not seen before in a pulsar system: “Shapiro lag” and the diffraction of light under the influence of a strong gravitational field.
Astronomers have also been able to measure – with an accuracy of 1 in a million – how the direction of the orbit of the fastest spinning pulsar changes. This “preemptive motion” can also be observed on Mercury, the innermost planet of our solar system, but 140,000 times weaker. The accuracy of the measurement achieved was so great that the effect of the pulsar on the surrounding spacetime, which is dragged along, had to be taken as it were: the so-called Lense-Thirring cycle.
Image: Artist’s impression of the PSR J0737-3039 A/B double pulsar system, which consists of two energetic pulsars orbiting each other in just two hours. (Michael Kramer/MPIfR)
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