# Improving the NRTidal model for binary neutron star systems

## TL;DR

This paper enhances the NRTidal gravitational waveform model for binary neutron star systems by incorporating improved analytical expressions, tidal amplitude corrections, and extended spin effects, resulting in more accurate and computationally efficient waveforms.

## Contribution

The paper introduces a new closed-form tidal phase expression, tidal amplitude correction, and extended spin effects into the NRTidal model, and integrates these into existing waveform models for better BNS GW analysis.

## Key findings

- The improved model matches numerical relativity waveforms more accurately.
- The new phase and amplitude corrections enhance waveform fidelity.
- Model validation shows consistency across a wide parameter space.

## Abstract

Accurate and fast gravitational waveform (GW) models are essential to extract information about the properties of compact binary systems that generate GWs. Building on previous work, we present an extension of the NRTidal model for binary neutron star (BNS) waveforms. The upgrades are: (i) a new closed-form expression for the tidal contribution to the GW phase which includes further analytical knowledge and is calibrated to more accurate numerical relativity data than previously available; (ii) a tidal correction to the GW amplitude; (iii) an extension of the spin-sector incorporating equation-of-state-dependent finite size effects at quadrupolar and octupolar order; these appear in the spin-spin tail terms and cubic-in-spin terms, both at 3.5PN. We add the new description to the precessing binary black hole waveform model IMRPhenomPv2 to obtain a frequency-domain precessing binary neutron star model. In addition, we extend the SEOBNRv4_ROM and IMRPhenomD aligned-spin binary black hole waveform models with the improved tidal phase corrections. Focusing on the new IMRPhenomPv2_NRTidalv2 approximant, we test the model by comparing with numerical relativity waveforms as well as hybrid waveforms combining tidal effective-one-body and numerical relativity data. We also check consistency against a tidal effective-one-body model across large regions of the BNS parameter space.