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

Understand

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 $r_2 \ge 0$ 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.