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

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 r(2) < 0.1, where r(2) >= 0 is the black hole dilaton charge and General Relativity is recovered for r(2) = 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 r(2) < 0.011 (90% CL), which is an order of magnitude more stringent.

Automated summary

The Einstein-Maxwell dilaton-axion gravity theory is a string-inspired model with potential as an alternative to General Relativity. Recent studies have explored its astrophysical implications in the spectra of accreting black holes and found a constraint verified by analyzing a NuSTAR spectrum of the black hole binary EXO 1846-031. The analysis revealed a constraint on the black hole dilaton charge that is an order of magnitude more stringent than previous constraints.