Testing the Weak Equivalence Principle near black holes

TL;DR

This paper proposes a novel test of the Weak Equivalence Principle in the strong gravitational field of black holes by analyzing X-ray reflection spectra, constraining potential violations for particles and photons.

Contribution

It introduces a relativistic reflection model to test WEP violations near black holes, extending previous Earth-based constraints into strong gravity regimes.

Findings

Constraints on WEP violations for particles and photons near black holes

Analysis of NuSTAR spectrum of EXO 1846-031

No significant violation detected within current measurement limits

Abstract

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.