Testing Einstein-dilaton-Gauss-Bonnet gravity with the reflection spectrum of accreting black holes

Abstract

Einstein-dilaton-Gauss-Bonnet gravity is a theoretically well-motivated alternative theory of gravity emerging as a low-energy 4-dimensional model from heterotic string theory. Its rotating black hole solutions are known numerically and can have macroscopic deviations from the Kerr black holes of Einstein's gravity. Einstein-dilaton-Gauss-Bonnet gravity can thus be tested with observations of astrophysical black holes. In the present paper, we simulate observations of the reflection spectrum of thin accretion disks with present and future X-ray facilities to understand whether X-ray reflection spectroscopy can distinguish the black holes in Einstein-dilaton-Gauss-Bonnet gravity from those in Einstein's gravity. We find that this is definitively out of reach for present X-ray missions, but it may be achieved with the next generation of facilities.

relxill_nk: A Relativistic Reflection Model for Testing Einstein’s Gravity

“…The method was originally proposed and developed for measuring black hole spins under the assumption that the metric around astrophysical black holes is described by the Kerr solution [36,37]. More recently, the technique has been proposed for testing Einstein's gravity in the strong field regime [38][39][40][41][42][43][44][45][46][47][48][49][50][51]. Note that spin measurements (if we assume the Kerr metric) or tests of the Kerr metric require fitting the whole reflection spectrum, not just the iron line, even if often (but not always) the iron line is the feature that primarily determines the measurement of the parameters of the background metric in the strong gravity region.…”

Exterior spacetime of relativistic stars in scalar-Gauss-Bonnet gravity

“…While there are many modified theories that attempt to explain anomalies between observations and the theoretical predictions of GR [13,14], a particularly interesting one is Einsteindilaton-Gauss-Bonnet (EdGB) gravity. This theory is interesting because it emerges in the low energy limit of heterotic string theory [15], and it agrees well with GR in the weak field region [16]. EdGB gravity modifies the Einstein-Hilbert action through the coupling of the Gauss-Bonnet invariant and a dynamical (dilaton) scalar field [17].…”

Electromagnetic fields of slowly rotating magnetized compact stars in conformal gravity

“…Thus the strong gravitational field regime near relativistic compact stars can play a role of laboratory to test general relativity versus other modified or alternative theories of gravity. Testing gravity theories using the strong field regime has been performed for X-ray sources from the stellar black hole candidates [27][28][29][30][31][32][33][34][35][36][37][38]. The comparison of the electromagnetic field and radiation of the compact star with the pulsar spin down can also be used to constrain the alternative theories of gravity [38,39].…”