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 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.

Testing the Kerr black hole hypothesis with GRS 1716-249 by combining the continuum-fitting and the iron-line methods

“…This is simply because, for the moment, efforts have been devoted to improving the model and understanding the systematic uncertainties, while less importance has been given to test specific scenarios of new physics. However, nkbb and relxill nk can be easily modified to test any black hole solution known in analytic form (see, e.g., Zhu et al 2020;Zhou et al 2021;Tripathi et al 2021c).…”

Testing General Relativity with black hole X-ray data: recent progress and future developments

“…relxill nk has been employed even to constrain specific gravity models, and we have tested conformal gravity 23 , Kaluza-Klein gravity 24 , asymptotically safe gravity 25 , and Einstein-Maxwell dilaton-axion gravity 26 . Our measurements of the spacetime around black holes are all consistent with the Kerr metric and we do not see any indication of possible new physics.…”

Testing General Relativity with black hole X-ray data: a progress report

“…[57,58], we presented the first version of the relativistic reflection model relxill nk, which is an extension of the relxill package developed by Thomas Dauser and Javier Garcia [59][60][61]. As nkbb, the default version of relxill nk employs the Johannsen metric with the possible non-vanishing deformation parameter α 13 [46], but modified versions of relxill nk have been used to test black hole solutions in conformal gravity [62,63], in Kaluza-Klein models [64], in asymptotically safe quantum gravity [65], and in Einstein-Maxwell dilaton-axion gravity [66], as in all these theories we know the analytic expression of the metric of rotating black holes. relxill nk has also been used for testing the Weak Equivalence Principle [67], while we have not used our model for testing the atomic physics in the strong gravitational fields of black holes.…”