Discovered by astronomers using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), PSR J1856–0039 is a newly identified double neutron star system in a remarkably compact 2.36-hour orbit, providing a fresh test of Einstein’s theory of general relativity according to a study published in the Physical Review Journal.
Inside the 2.36-Hour Compact Orbit
Compact binaries containing neutron stars remain prime laboratories for testing gravitational physics. When one of those stellar remnants acts as a pulsar, it sweeps radio emission across our line of sight at regular intervals, functioning as an extraordinarily precise natural clock. Tracking tiny variations in pulse arrival times lets researchers measure orbital motion. The newly found system, designated PSR J1856–0039, exhibits an orbital period of just 2.36 hours alongside an eccentricity of $e=0.106$, denoting a slightly non-circular path.
Z. L. Yang and a team of researchers from the National Astronomical Observatories at the Chinese Academy of Sciences and the University of Chinese Academy of Sciences uncovered the system. Their analysis shows that one of these neutron stars is the lowest mass neutron star known to date.
Testing General Relativity with Post-Keplerian Parameters
General relativity dictates that massive objects orbiting each other must shed energy through gravitational waves, causing the orbit to shrink over time. To verify whether Einstein’s framework holds under these extreme conditions, researchers look past standard Keplerian metrics to examine post-Keplerian parameters. For PSR J1856–0039, the authors measured the orbital period derivative, the periastron advance, and the Einstein delay.
When calculated against the masses of the neutron stars, each parameter yields an independent prediction for the system’s components. If general relativity is sound, all three measurements must converge on the same mass constraints. The data demonstrates that they do. When comparing the observed orbital period derivative directly with the rate predicted by gravitational-wave emission under general relativity, the team found a ratio of 1.009 $pm$ 0.014. The observed decay matches Einstein’s predictions to a margin of about 1.4%.
The Road to Lense-Thirring Precession and 2076
While the system aligns with standard theoretical models, further observational campaigns could isolate even subtler relativistic mechanics. General relativity predicts that massive, rotating bodies drag the very fabric of spacetime along with them, an effect known as frame dragging or Lense-Thirring precession. This phenomenon alters the orientation of nearby orbits over extended periods.
Calculations indicate that PSR J1856–0039 should produce some of the largest observable changes in nearby objects. Capturing the effect reliably requires sustained precision timing over half a century. Astronomers will need to continue timing observations until the year 2076 before the data can confirm whether frame dragging manifests in this compact cosmic laboratory.
