Effective-one-body model for coalescing binary neutron stars: Incorporating tidal spin and enhanced radiation from dynamical tides

Tidal interactions in a coalescing binary neutron star (BNS) or neutron star-black hole (NSBH) system driven by gravitational wave (GW) radiation contain precious information about physics both at extreme density and in the highly relativistic regime. In the late inspiral stage, where the tidal effects are the strongest, dynamical corrections to the tidal response become significant. Previous analyses model the finite-frequency correction through the effective Love number approach, which only accounts for the correction in the radial interaction but ignores the lag in the tidal bulge behind the companion due to the continuous orbital shrinkage. The lag provides a torque, causing the star's spin to change over time. We dub the evolving component of the spin the tidal spin, whose dimensionless value can reach 0.03-0.4 depending on how rapidly the background star rotates. We present an effective-one-body (EOB) waveform model for BNSs and NSBHs incorporating the tidal spin, particularly its back reaction to the orbit due to the Newtonian tidal torque and the relativistic orbital hang-up. Beyond the conservative dynamics, we also derive the corrections to the dissipative radiation due to finite-frequency effects to the first post-Newtonian order. Depending on the star's background spin, the phase error in the time-domain waveform due to ignoring the tidal spin ranges from 0.3 to 4 radians at the waveform's peak amplitude. The difference in the waveforms with and without the tidal spin remarkably resembles the difference between previous effective Love number models and numerical relativity simulations, underscoring the significance of tidal spin in the construction of faithful models. Our model further extends the description of dynamics in the high-background spin regions of the parameter space that are yet to be covered by numerical simulations.