Eccentric extreme-mass-ratio inspirals: a new window into ultra-light vector fields

Abstract Space-based gravitational-wave detectors, such as the Laser Interferometer Space Antenna (LISA), provide a platform to probe new fundamental fields through extreme-mass-ratio inspirals (EMRIs), where a compact secondary object carrying scalar or vector charges inspirals into a massive primary. In a theory-agnostic framework, we compute the ultra-light vector and gravitational radiation emitted by eccentric EMRIs and determine the corresponding inspiral trajectories. We evaluate the impact of a massive vector (Proca) field on EMRIs waveform through dephasing and mismatches with predictions by general relativity. Using a Fisher information matrix analysis, we further assess LISA’s capability to constrain the Proca mass from future EMRIs observations. We find that orbital eccentricity can improve estimation accuracy of parameters, making the vector mass $$\mu $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> become detectable for the case of $$\mu =0.02$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>μ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.02</mml:mn> </mml:mrow> </mml:math> . Correlation analysis further reveals strong positive dependencies between the Proca mass and intrinsic source parameters, indicating that improved measurement of these parameters directly tightens constraints on vector mass. These results demonstrate that high-eccentricity EMRIs observed by LISA offer a powerful channel to detect or constrain massive vector-field extensions of GR in the strong-field regime.