Pulsar PSR J0337+1715 is a neutron star - a stellar core 1.44 times the mass of the Sun that has collapsed into a ball of 25km in diameter - orbiting with two white-dwarf stars. One of them is very close to the pulsar, 1/10th of the Sun-Mercury distance only, and the other one is located at a distance comparable to the Earth-Sun distance. While spinning with a period of only 3ms, the pulsar emits a beam of radio waves which, such as a galactic beacon, sweeps across space. At each turn this creates a flash of radio light which is recorded with high accuracy by Nançay’s radiotelescope. As the pulsar moves on its orbit, the light arrival time at Earth is shifted. It is the accurate measurement and mathematical modeling (down to a nanosecond accuracy) of these times of arrival that allows scientists to infer with exquisite precision the motion of the star.
Above all, it is the unique configuration of that system, with the presence of a second companion towards which the two other stars "fall" (orbit) that has allowed to perform a stellar version of Galileo’s famous experiment from Pisa’s tower : two bodies of different compositions fall with the same acceleration in the gravitational field of a third one (the Earth for Galileo, the second companion in the present case). Thus, the team has demonstrated that the extreme gravity field of the pulsar cannot differ by more than 1.8 part per million (with a confidence level of 95%) from the prediction of general relativity. This result is the most accurate confirmation of Einstein’s theory ever obtained for highly self-gravitating objects.
Reference
- An improved test of the strong equivalence principle with the pulsar in a triple star system , G. Voisin, I. Cognard, P. C. C. Freire, N. Wex, G. Guillemot, G. Desvignes, M. Kramer, G. Theureau Astronomy & Astrophysics, May 2020