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

TL;DR

This paper investigates how a non-zero dilaton charge in Einstein-Maxwell dilaton-axion gravity affects black hole reflection spectra, providing tighter constraints on the dilaton charge from X-ray observations.

Contribution

It presents the first detailed analysis of the impact of dilaton charge on reflection spectra and tightens the observational constraints using NuSTAR data.

Findings

Dilaton charge affects black hole reflection spectra.

Constraint on dilaton charge improved to r_2 < 0.011.

General Relativity is recovered at r_2=0.

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 $r_2 < 0.1$, 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.