Constraining possible variations of the fine structure constant in strong gravitational fields with the Kα iron line

Abstract

In extensions of general relativity and in theories aiming at unifying gravitywith the forces of the Standard Model, the value of the ``fundamental constants''is often determined by the vacuum expectation value of new fields, which may thuschange in different backgrounds. Variations of fundamental constants with respectto the values measured today in laboratories on Earth are expected to be moreevident on cosmological timescales and/or in strong gravitational fields. In thispaper, I show that the analysis of the Kα iron line observed in the X-rayspectrum of black holes can potentially be used to probe the fine structure constantα in gravitational potentials relative to Earth of Δϕ ≈ 0.1.At present, systematic effects not fully under control prevent to get robust andstringent bounds on possible variations of the value of α withthis technique, but the fact that current data can be fitted with models basedon standard physics already rules out variations of the fine structure constantlarger than some percent.