Towards New Tests of Strong-Field Gravity with Measurements of Surface Atomic Line Redshifts from Neutron Stars

In contrast to gravity in the weak-field regime, which has been subject to numerous experimental tests, gravity in the strong-field regime is largely unconstrained by observations. We show that gravity theories that pass solar system tests, but that diverge from general relativity in the strong-field regime, predict neutron stars with significantly different properties than their general relativistic counterparts. The range of redshfits of surface atomic lines predicted by such theories is significantly wider than the uncertainty introduced by our lack of knowledge of the equation of state of ultradense matter. Measurements of such lines with x-ray observatories can thus put new constraints on strong-field gravity.

Figure 1

Mass-radius relations for neutron stars in the GR, bimetric, and scalar-tensor theories. For the scalar-tensor theory, we plot the relation for only one value of the parameter $\beta$. The shaded regions show the allowed values of $M$ and $R$ for equations of state with stiffness between EOS A and EOS UU.

Figure 2

Contours of constant redshift in the scalar-tensor theory, as a function of $M_{\mathrm{A}\mathrm{D}\mathrm{M}}$ and $\beta$, for EOS A. The dashed line shows the value of $M_{\mathrm{A}\mathrm{D}\mathrm{M}}$, as a function of $\beta$, for which the baryonic mass is equal to $1.4M_{\odot }$. The heavy lines on the plot show the mass limits for neutron stars and the bounding region where spontaneously scalarized stars are produced. No stable neutron stars in the crosshatched region exist.

Figure 3

Contours of constant redshift in the scalar-tensor theory, as a function of $M_{\mathrm{A}\mathrm{D}\mathrm{M}}$ and $\beta$, for EOS UU. See Fig. 2 for details.

Figure 4

Mass-redshift relations for the GR and bimetric theories. The dashed line shows $z=0.35$, the value reported by Cottam et al. [1].