# Waveform systematics for binary neutron star gravitational wave signals: Effects of spin, precession, and the observation of electromagnetic counterparts

## TL;DR

This study investigates biases in current waveform models for spinning and precessing binary neutron star systems, emphasizing the importance of including neutron star quadrupolar deformation for accurate parameter estimation, and explores the benefits of combined gravitational wave and electromagnetic observations.

## Contribution

The paper provides a Bayesian analysis highlighting the necessity of modeling neutron star quadrupolar deformation for spins above 0.20 and assesses the detectability of precession effects and electromagnetic counterparts.

## Key findings

- Quadrupolar deformation must be included for spins > 0.20 to avoid bias.
- Precession effects are unlikely to be detected in GW170817-like signals with current models.
- Electromagnetic counterparts help constrain parameters but minimally improve intrinsic property estimates.

## Abstract

Extracting the properties of a binary system emitting gravitational waves relies on models describing the last stages of the compact binary coalescence. In this article, we study potential biases inherent to current tidal waveform approximants for spinning and precessing systems. We perform a Bayesian study to estimate intrinsic parameters of highly spinning binary neutron star systems. Our analysis shows that one has to include the quadrupolar deformation of the neutron stars due to their rotation once dimensionless spins above $\chi \sim 0.20$ are reached, otherwise the extracted intrinsic parameters are systematically biased. We find that at design sensitivity of Advanced LIGO and Virgo, it seems unlikely that for GW170817-like sources a clear imprint of precession will be visible in the analysis of the signal employing current waveform models. However, precession effects might be detectable for unequal mass configurations with spins larger than $\chi>0.2$. We finalize our study by investigating possible benefits of a combined gravitational wave and electromagnetic detection. The presence of electromagnetic counterparts help in reducing the dimensionality of the parameter space with constraints on the sky location, source distance, and inclination. However, we note that although a small improvement in the estimation of the tidal deformability parameter is seen in these cases, changes in the intrinsic parameters are overall very small.