New gravitational waveform model for precessing binary neutron stars with double-spin effects

We present two new frequency-domain gravitational waveform models for the analysis of signals emitted by binary neutron star coalescences: IMRPhenomXAS_NRTidalv2 and IMRPhenomXP_NRTidalv2. Both models are available through the public algorithm library lalsuite and represent the first extensions of IMRPhenomX models including matter effects. We show here that these two models represent a significant advancement in efficiency and accuracy with respect to their phenomenological predecessors, IMRPhenomD_NRTidalv2 and IMRPhenomPv2_NRTidalv2. The computational efficiency of the new models is achieved through the application of the same multibanding technique previously applied to binary black hole models. Furthermore, IMRPhenomXP_NRTidalv2 implements a more accurate description of the precession dynamics, including double-spin effects and, optionally, matter effects in the twisting-up construction. The latter are available through an option to use a numerical integration of the post-Newtonian precession equations. We show that the new precession descriptions allow the model to better reproduce the phenomenology observed in numerical-relativity simulations of precessing binary neutron stars. Finally, we present some applications of the new models to Bayesian parameter estimation studies, including a reanalysis of GW170817 and a study of simulated observations using numerical relativity waveforms for nonprecessing binary neutron stars with highly spinning components. We find that in these cases the new models make a negligible difference in the results. Nevertheless, by virtue of the aforementioned improvements, the new models represent valuable tools for the study of future detections of coalescing binary neutron stars.