Constraints on Brans-Dicke gravity from neutron star-black hole merger events using higher harmonics

TL;DR

This study derives new constraints on Brans-Dicke and scalar-tensor theories using neutron star-black hole merger data, emphasizing the importance of higher harmonic modes for accurate bounds.

Contribution

It introduces the inclusion of higher harmonic modes in gravitational wave analysis to tighten constraints on scalar-tensor theories, especially for asymmetric binary systems.

Findings

Higher harmonic modes improve constraints on scalar-tensor parameters.

Including higher modes yields a 6.5 ext{%} better bound on bd.

Combining multiple GW events significantly strengthens the bounds.

Abstract

In this paper, we derive a 90\% credible lower bound on the modified parameter of scalar-tensor theories as $\varphi_{-2}>-7.94\times10^{-4}$ by using dominant-mode correction. Specific to BD theory, we have the constraint $\omega_{\rm BD}>4.75$. Asymmetric binary systems usually have a significant mass ratio; in such cases, higher harmonic modes cannot be neglected. Our work considers higher harmonic corrections from scalar-tensor theories and provides a tighter constraint of $\varphi_{-2}>-7.59\times10^{-4}$. Transitioning to the BD theory, the constraint is $\omega_{\rm BD}>5.06$, with a 6.5\% improvement. We also consider a plausible NSBH event, GW190814, which is a highly unequal mass ratio source and exhibits strong evidence for higher-order multipoles. We obtain poorly converged results when using the dominant mode while getting a constraint of $\varphi_{-2}>-6.60\times10^{-4}$ on scalar-tensor theories when including the higher harmonic modes. This suggests that the difference between the dominant mode and higher modes has a significant impact on our analysis. Furthermore, treating this suspected event as an NSBH event, we find $\omega_{\rm BD}>6.12$ when including the higher harmonic modes. Combining GW200115 and GW190814 and including higher modes, the constraint is improved to $\omega_{\rm BD}>110.55$. This is currently the strongest constraint utilizing GWs, contingent upon GW190814 being an NSBH event. Additionally, we take into account a BD-like theory, known as screened modified gravity (SMG), and investigate the coupling constant constraints, both with and without higher-mode corrections, by using data from both GW200115 and GW190814.