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.

Figure 1

Predictions for the moment of inertia of PSR J0737-3039A. We compare our predicted $I_{1.3381}$ using both the MCMC samples from Miller et al. [44] and Riley et al. [45] against: (i) Landry and Kumar [52] (LK18), which used binary Love [54] and I-Love relations with the tidal-deformability constraints from binary neutron-star merger GW170817 [55], and (ii) Lim et al. [53] (LHS19) which carried out Bayesian modeling of a number of equations of state. The larger moment of inertia that we predict is due to the larger radii favored by an $M\approx 1.4M_{\odot }$ neutron star by NICER’s observation relative what is inferred by the two other methods, as $I\sim M{R}_e^2$.

Figure 2

Multimessenger test of general relativity using the parametrized I-Love relation. The vertical (horizontal) lines delimit the 90% confidence region (shaded) for ${\overline{\lambda }}_{1.4}$ [57] (${\overline{I}}_{1.4}$, this work), while the circle marks the median (190, 14.6). The solid black line corresponds to the I-Love relation in general relativity [Eq. (1)] and is consistent with the inferred values of ${\overline{I}}_{1.4}$, ${\overline{\lambda }}_{1.4}$ at 90% confidence. Starting from $b=-2$ and moving clockwise, we show the parametrized I-Love curves $(b,{\beta }_{\mathrm{crit}})$, where $b\in [-2,5]$ and ${\beta }_{\mathrm{crit}}$ is the critical value of $\beta$ above which the parametrized I-Love relation [Eq. (2)] fails to pass by the 90% confidence region in the plane. Here we used the value of ${\overline{I}}_{1.4}$ inferred using the results by Miller et al. [8, 44]. We found similar results using the results by Riley et al. [7, 45] (see Supplemental Material [23]).