Pulsars are rapidly rotating neutron stars that concentrate 1.4 times the mass of the Sun - or more ! - in a sphere of 20 km in diameter. They have an extremely strong magnetic field and emit a beam of radio waves above each of their two magnetic poles.
Like a lighthouse at the seaside, the pulsar emission is perceived on Earth as impulses with a regularity that rivals the precision of the best atomic clocks. These massive and compact objects are thus used by astronomers as cosmic clocks to test Einstein’s theory of general relativity.
Einstein’s theory predicts that space-time is deformed by massive objects like pulsars. One of the predictions of this theory is the precession of the axis of rotation of a pulsar when it belongs to a two-star system.
This precession is a slow change of orientation of its axis in the manner of a spinning top at the end of its launching. This effect is due to the misalignment between the axis of rotation of the pulsar and the total angular momentum of the system caused by a supernova. This precession varies the geometry of observation of the pulsar which can be studied thanks to the received radio pulses.
PSR J1906 + 0746 is located near the plane of the Milky Way at a distance of about 20 000 light-years in the direction of the Eagle constellation. This pulsar turns on itself in 144 ms and orbits around another neutron star in 4 hours.
Researchers have been observing this pulsar since 2012 with the Arecibo radio telescope in Puerto Rico (USA). To complete the study, the team also reanalyzed archives of the Arecibo radio telescope and those of the Nançay radio telescope located in the Cher (France). In total, the observations cover a period from July 2005 to June 2018.
The team was able to determine that the radio emission detected between 2005 and 2016 came from the two magnetic poles of the pulsar, when the two radio beams illuminated the Earth. In 2016, the radio emission from one of the two poles was no longer detected and since then only the radio emission from the second south pole remains detectable.
Using a theory dating back to 1969 which predicts that the polarization of the radio emission provides information on the geometrical orientation of the pulsar, the team was able to validate this model and measure the precession of the axis of rotation of the pulsar with an uncertainty of 5%. This result is in perfect agreement with the prediction of Einstein’s general relativity.
The study also makes it possible to predict the disappearance and reappearance of the emission coming from the two magnetic poles of the pulsar. The emission from the last visible pole should disappear from our line of sight around 2028 and reappear between 2070 and 2090. The emission of the first pole should reappear between 2085 and 2105.
These observations also make advances in our understanding of radio pulsar emission by studying the radio emission properties above a magnetic pole. The reconstruction of the radio emission beam finally allows to determine the fraction of the sky which is illuminated by this pulsar. This parameter affects the estimated number of two-neutron star systems in our Galaxy and therefore the coalescence rate of these systems.
Reference
- Radio emission from a pulsar’s magnetic pole revealed by general relativity, Desvignes, G. et al 2019, Science