Massive scalar fields in eccentric regime: Detectability and constraints from LISA observations of extreme mass-ratio inspirals

Abstract

Extreme mass-ratio inspirals (EMRIs) are among the prime sources for future space-borne gravitational wave (GW) observatories and provide a useful setting for testing the presence of fundamental fields and possible deviations from general relativity (GR) in both strong and weak gravity regimes. In this work, we study the effect of a massive scalar field on eccentric equatorial EMRI dynamics around Kerr black holes. Considering that the inspiralling stellar-mass object carries a scalar charge and emits scalar radiation together with tensor GWs, we compute the relevant relativistic fluxes within the adiabatic treatment of the inspiral. With the solution of the scalar perturbation equation in the frequency domain, the resulting fluxes are presented through the Chebyshev interpolants in order to have the efficient inspiral evolution across the parameter space considered. We quantify the impact of scalar field mass and scalar charge on the orbital evolution and GW signal through phase shifts and waveform mismatches relative to both GR and the massless-scalar scenario. We find that massive scalar radiation can generate significant GW dephasing that increases with orbital eccentricity; however, the scalar flux is suppressed as the scalar field mass is becoming larger. Using a Fisher information matrix (FIM) analysis, we estimate the ability of Laser Interferometer Space Antenna (LISA) to measure or constrain the scalar charge and scalar field mass. Our results indicate that eccentric EMRIs can place meaningful constraints on massive scalar fields and provide a promising as well as important avenue for testing scalar-tensor extensions of gravity in the region of a strong gravitational field.