Tidal torque induced by orbital decay in compact object binaries

TL;DR

This paper demonstrates that orbital decay in compact binaries can induce a persistent tidal torque even without viscosity, leading to differential rotation and magnetic field amplification in neutron stars prior to merger.

Contribution

It reveals a new mechanism for tidal torque in inviscid bodies during binary coalescence, impacting neutron star magnetization and dynamics.

Findings

Orbital decay causes a persistent torque in inviscid binaries.

The torque induces differential rotation and stores significant free energy.

Potential for magnetic field amplification up to 10^15 Gauss.

Abstract

As we observe in the moon-earth system, tidal interactions in binary systems can lead to angular momentum exchange. The presence of viscosity is generally regarded as the condition for such transfer to happen. In this paper, we show how the orbital evolution can cause a persistent torque between the binary components, even for inviscid bodies. This preferentially occurs at the final stage of coalescence of compact binaries, when the orbit shrinks successively by gravitational waves and plunging on a timescale shorter than the viscous timescale. The total orbital energy transferred to the secondary by this torque is ~0.01 of its binding energy. We further show that this persistent torque induces a differentially rotating quadrupole perturbation. Specializing to the case of a secondary neutron star, we find that this non equilibrium state has an associated free energy of 10^47-10^48 erg, just prior to coalescence. This energy is likely stored in internal fluid motions, with a sizable amount of differential rotation. By tapping this free energy reservoir, a preexisting weak magnetic field could be amplified up to a strength of ~10^15 Gauss. Such a dynamically driven tidal torque can thus recycle an old neutron star into a highly magnetized neutron star, with possible observational consequences at merger.