Astrophysical and Theoretical Physics Implications from Multimessenger Neutron Star Observations

The Neutron Star Interior Composition Explorer (NICER) recently measured the mass and equatorial radius of the isolated neutron star PSR J0030+0451. We use these measurements to infer the moment of inertia, the quadrupole moment, and the surface eccentricity of an isolated neutron star for the first time, using relations between these quantities that are insensitive to the unknown equation of state of supranuclear matter. We also use these results to forecast the moment of inertia of neutron star A in the double pulsar binary J0737-3039, a quantity anticipated to be directly measured in the coming decade with radio observations. Combining this information with the measurement of the tidal Love number with LIGO/Virgo observations, we propose and implement the first theory-agnostic and equation-of-state-insensitive test of general relativity. Specializing these constraints to a particular modified theory, we find that consistency with general relativity places the most stringent constraint on gravitational parity violation to date, surpassing all other previously reported bounds by 7 orders of magnitude and opens the path for a future test of general relativity with multimessenger neutron star observations.

Automated summary

NICER recently measured the mass and equatorial radius of an isolated neutron star, inferring information about its moment of inertia, quadrupole moment, and surface eccentricity. By combining various observations, including radio observations, the first theory-agnostic and equation-of-state-insensitive test of general relativity was implemented, setting a new stringent constraint on gravitational parity violation. This paves the way for future tests of general relativity using multimessenger neutron star observations.