Constraining Scalar Charge in Neutron Stars through Gravitational Wave Signals from NSBH Mergers

Abstract

Neutron stars (NS) offer an exceptional opportunity to investigate gravitational physics in extreme environments. In the context of alternative theories of General Relativity (GR), a scalar field non-minimally coupled to gravity can lead to "hairy" NS solutions with a non-zero scalar charge. As a result, binary systems containing at least one "hairy" NS can serve as probes for testing GR through observations. In this work, we analyze the gravitational wave signals from four neutron-star-black-hole (NS-BH) merger events: GW190814, GW200115, GW200105, and GW230529 examine the "hairy" NS hypothesis predicted by the luminal Horndeski theories. Our analysis constrains the scalar charge to be on the order of $10^{-3}$ to $10^{-4}$, with the most stringent limit arising from the recent GW230529 event. In addition, we discuss the implications of our results in relation to the scalarized NS solutions found in the literature.