Scalar emission from binary neutron stars in scalar-tensor theories with kinetic screening

We investigate the scalar emission from binary neutron stars in shift-symmetric scalar-tensor theories with kinetic screening (K-essence), using 3 + 1 numerical simulations in the decoupling limit. To construct static binary initial data in the regime where the screening radius r* greatly exceeds the orbital separation, we introduce a hyperbolization of the static field equations that bypasses the Keldysh-type breakdown affecting direct time evolutions. For equal-mass binaries, where the scalar emission is dominated by the l = m = 2 mode, kinetic screening acts nonmonotonically on the scalar radiation, suppressing or enhancing the quadrupolar amplitude depending on the relative size of r* and lambda 22 (with lambda 22 the wavelength): for lambda 22 << r* it is suppressed relative to the Fierz-Jordan-Brans-Dicke (FJBD) case, while for lambda 22 greater than or similar to r* it is amplified above FJBD. For unequal-mass binaries a scalar dipole reemerges, growing linearly with the mass asymmetry, while the quadrupolar screening remains close to the equal-mass case down to mass ratios similar to 0.6. The nonmonotonic behavior of kinetic screening that we uncover has potential implications for gravitational-wave-based tests of gravity. The relativistic double pulsar, in particular, requires r* >> 109 km to efficiently suppress the scalar quadrupole; for cosmologically-motivated Lambda, r* similar to 1011 km (for a solar-mass source), giving only moderate suppression.