Image of the thin accretion disk around compact objects in the Einstein\u2013Gauss\u2013Bonnet gravity

Galin Gyulchev,Petya Nedkova,Stoytcho Yazadjiev,Tsvetan Vetsov

doi:10.1140/epjc/s10052-021-09624-5

Abstract

We study the optical appearance of a thin accretion disk around compact objects within the Einstein–Gauss–Bonnet gravity. Considering static spherically symmetric black holes and naked singularities we search for characteristic signatures which can arise in the observable images due to the modification of general relativity. While the images of the Gauss–Bonnet black holes closely resemble the Schwarzschild black hole, naked singularities possess a distinctive feature. A series of bright rings are formed in the central part of the images with observable radiation 10^3 times larger than the rest of the flux making them observationally significant. We elucidate the physical mechanism, which causes the appearance of the central rings, showing that the image is determined by the light ring structure of the spacetime. In a certain region of the parametric space the Gauss–Bonnet naked singularities possess a stable and an unstable light ring. In addition the gravitational field becomes repulsive in a certain neighbourhood of the singularity. This combination of features leads to the formation of the central rings implying that the effect is not specific for the Einstein–Gauss–Bonnet gravity but would also appear for any other compact object with the same characteristics of the photon dynamics.

Highlights

The solution was obtained in a different context considering the classical four-dimensional (4D) Einstein– Guass–Bonnet equations [4]
We study the optical appearance of a thin accretion disk around compact objects within the Einstein–Gauss– Bonnet gravity
Considering static spherically symmetric black holes and naked singularities we search for characteristic signatures which can arise in the observable images due to the modification of general relativity

Summary

Introduction

The solution was obtained in a different context considering the classical four-dimensional (4D) Einstein– Guass–Bonnet equations [4]. While the proposed regularization procedure is not consistent for general gravitational fields [5,6], it leads to correct predictions in a number of cases with high symmetries, such as spherically or cylindrically symmetric spacetimes, and homogeneous and isotropic cosmologies These geometries arise independently as solutions to a subclass of the Horndeski theory following from a welldefined action principle in four dimensions [7–12]. Irrespective of the theoretical framework in which the spherically symmetric solutions are interpreted, the nontrivial contribution of the Gauss–Bonnet term is expected to have phenomenological impact This triggered a range of works investigating different observational features such as the shadow and the innermost stable orbit [14,15], the particle dynamics [16,17], the gravitational lensing [18,19], and the radiation from the accretion disk [20].

Properties of the exact solution

Image formation for the weakly naked singularities

Findings

Conclusion