AO Observations of a Binary Pulsar Test the Theory of General Relativity in New Science Publication

Image Credit: World Science Festival

Two tests of general relativity were achieved using observations of a binary pulsar from the Arecibo Observatory spanning 2012 - 2018 and archival data recorded between 2005 - 2009 at AO and the Nançay radio telescope, the September publication in Science revealed.
The study’s authors, led by Dr. Desvignes of the Max Planck Institute, used polarization measurements of the pulsar J1906+0746 to show the relativistic spin precession of strongly self-gravitating bodies, likely caused by the misalignment of the pulsar’s spin vector with respect to the binary system’s total angular momentum due to an asymmetric supernova explosion.

“This paper was only made possible thanks to the continuous monitoring with the Arecibo telescope over the last decade,” Dr. Desvignes conveyed. “Arecibo allowed us to probe the pulsar's beam up to the edge where its radio luminosity is so low that any detection would be impossible with other radio telescopes.”

Arecibo allowed us to probe the pulsar's beam up to the edge where its radio luminosity is so low that any detection would be impossible with other radio telescopes. - Dr. Desvignes (Max Planck Institute)

J1906+0746 is a young pulsar with a spin period of ~144 ms that orbits around another neutron star in ~4 hours. When it was discovered in 2004, polarized emission from both magnetic poles were detected. This fortuitous orthogonal alignment of the pulsar to Earth in 2004 was compared with data from 1998 that revealed only one, stronger pulse was detectable in that era, suggesting the effects of relativistic spin precession.
Previous timing analyses of J1906+0746 measured the spin precession rate, the orbital inclination, and the orbital decay of the binary pulsar, which were consistent with predictions from general relativity. The new analysis, which modeled polarization observations (rather than pulsar timing observations) from AO’s Puerto Rico Ultimate Pulsar Processing Instrument (PUPPI) at a frequency of 1.38 GHz, independently confirmed the spin precession rate and determine the inclination without the directional ambiguity of the timing analyses.

The study predicts that Earth-based measurements of the emission from one pole will no longer be detectable between 2028 and 2070, while the opposite pole’s emission will be visible again between 2085 - 2105.
The observed pulse profiles during different epochs were used to construct sky-projected beam maps of radio emission from both magnetic poles and the polarization properties. The maps show that radio emission is not restricted to one side of a magnetic pole and match theoretical predictions of the current density in the polar cap for an orthogonal rotator and the theoretical emission heights from the surface of the pulsar.

These measurements independently determined the portion of the sky illuminated by the pulsar beam (the beaming fraction). The study revealed a large beaming fraction of 0.52 with substantial luminosity variations. Typically, pulsar J1906+0746 is often used to determine the predicted number of Galactic double neutron stars (from which the rate of gravitational wave detections for neutron star mergers in derived); however, the luminosity conal beam used for these predictions is small and more uniform than these observations reveal. The study concludes that the merger rate of double neutron stars cannot be reliably constrained using the simple conal beam model with uniform luminosity as applied for pulsar J1906+0746.

Keywords:observatory, arecibo, J1906+0746, puppi, Desvignes, relativistic, Nançay, Planck, max, Galactic, Earth