Today we have quite stringent constraints on possible violations of the weak-equivalence principle from the comparison of the acceleration of test-bodies of different composition in Earth’s gravitational field. In the present paper, we propose a test of the weak-equivalence principle in the strong gravitational field of black holes. We construct a relativistic reflection model in which either the massive particles of the gas of the accretion disk or the photons emitted by the disk may not follow the geodesics of the spacetime. We employ our model to analyze the reflection features of a NuSTAR spectrum of the black hole binary EXO 1846–031 and we constrain two parameters that quantify a possible violation of the weak-equivalence principle by massive particles and x-ray photons, respectively.

• Received 17 March 2021
• Accepted 9 July 2021

DOI:https://doi.org/10.1103/PhysRevD.104.044001