Pulsars are fast-spinning neutron stars that concentrate 40 percent more mass than the Sun – or more! – into a small sphere of only about 20 kilometres (12 miles) diameter. They have extremely strong magnetic fields and emit a beam of radio waves along their magnetic axes above each of their opposite magnetic poles.
Due to their stable rotation, a lighthouse effect produces pulsed signals that arrive on Earth with the accuracy of an atomic clock. The large mass, the compactness of the source, and the clock-like properties allows astronomers to use them as laboratories to test Einstein’s general theory of relativity.
The theory predicts that spacetime is curved by massive bodies such as pulsars. One expected consequence is the effect of relativistic spin precession in binary pulsars. The effect arises from a misalignment of the spin vector of each pulsar with respect to the total angular momentum vector of the binary system, and is most likely caused by an asymmetric supernova explosion. This precession causes the viewing geometry to vary, which can be tested observationally by monitoring systematic changes in the observed pulse profile.
Evidence for a variable pulse profile attributed to changes in the viewing geometry caused by spin precession have been observed and modelled in the Nobel-prize winning Hulse-Taylor binary pulsar B1913+16. Other binary pulsars also show the effect, but none of them has allowed studies at the precision and level of detail obtainable with PSR J1906+0746.
The target is a young pulsar with a spin period of 144 milliseconds in a 4-hour orbit around another neutron star in the direction of the constellation Aquila (the Eagle), pretty close to the plane of our Galaxy, the Milky Way.
“PSR J1906+0746 is a unique laboratory in which we can simultaneously constrain the radio pulsar emission physics and test Einstein’s general theory of relativity”, says Gregory Desvignes from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, the first author of the study.
The research team monitored the pulsar from 2012 to 2018 with the 305-m Arecibo radio telescope at a frequency of 1.4 GHz. Those observations were supplemented with archival data from the Nançay and Arecibo radio telescopes recorded between 2005 and 2009. In total, the available dataset comprises 47 epochs spanning from July 2005 to June 2018.
The team noticed that initially it was possible to observe the pulsar’s opposite magnetic poles, when both “Northern” and “Southern” beams (referred to as “main pulse” and “interpulse” in the study) were pointed to Earth once per rotation. With time, the Northern beam disappeared and only the Southern beam remained visible.
Based on a detailed study of the polarisation information of the received emission, it was possible to apply a 50-year old model, predicting that the polarisation properties encoded information about the geometry of the pulsar.
The pulsar data validated the model and also allowed the team to measure the rate of precession with only 5 percent uncertainty level, tighter than the precession rate measurement in the Double Pulsar system, a reference system for such tests so far. The measured value agrees perfectly with the prediction of Einstein’s theory.