Constraints on Einstein-Maxwell dilaton-axion gravity from X-ray reflection spectroscopy

Abstract

Einstein-Maxwell dilaton-axion gravity is a string-inspired model arising from the low energy effective action of heterotic string theory and an important candidate as alternative to General Relativity. Recently, some authors have explored its astrophysical implications in the spectra of accreting black holes and inferred the constraint r2 < 0.1, where r2 ≥ 0 is the black hole dilaton charge and General Relativity is recovered for r2 = 0. In the present paper, we study the impact of a non-vanishing black hole dilaton charge on the reflection spectrum of the disk. From the analysis of a NuSTAR spectrum of the black hole binary EXO 1846-031, we find the constraint r2 < 0.011 (90% CL), which is an order of magnitude more stringent.