Probing bumblebee gravity with black hole X-ray data

TL;DR

This study tests bumblebee gravity, a Lorentz-violating theory, using black hole X-ray reflection spectra, but finds current data cannot constrain the Lorentz-violating parameter due to parameter degeneracy.

Contribution

Constructed a reflection model in bumblebee gravity and applied it to real X-ray data to test Lorentz symmetry breaking in black holes.

Findings

Analysis could not constrain the Lorentz-violating parameter $\, ext{ extbackslash}ell$

Degeneracy between $\, ext{ extbackslash}ell$ and black hole spin parameter $a_*$

Potential to break degeneracy with additional observations

Abstract

Bumblebee gravity is one of the simplest gravity theories with spontaneous Lorentz symmetry breaking. Since we know a rotating black hole solution in bumblebee gravity, we can potentially test this model with the available astrophysical observations of black holes. In this work, we construct a reflection model in bumblebee gravity and we use our model to analyze the reflection features of a NuSTAR spectrum of the Galactic black hole EXO 1846-031 in order to constrain the Lorentz-violating parameter $\ell$. We find that the analysis of the reflection features in the spectrum of EXO 1846-031 cannot constrain the parameter $\ell$ because of a very strong degeneracy between the estimates of $\ell$ and of the black hole spin parameter $a_*$. Such a degeneracy may be broken by combining other observations.