Distinguishing black holes with and without spontaneous scalarization in Einstein-scalar-Gauss-Bonnet theories via optical features

Abstract: Spontaneous scalarization in Einstein-scalar-Gauss-Bonnet theory admits both vacuum-general relativity (GR) and scalarized hairy black holes as valid solutions, which provides a distinctive signature of new physics in strong gravity regime. In this paper, we shall examine the optical features of Gauss-Bonnet black holes with spontaneous scalarization, which is governed by the coupling parameter $\lambda$. We find that the photon sphere, critical impact parameter and innermost stable circular orbit all decrease as the increasing of $\lambda$. Using observable data from Event Horizon Telescope, we establish the upper limit for $\lambda$. Then we construct the optical appearances of the scalarized black holes illuminated by various thin accretions. Our findings reveal that the scalarized black holes consistently exhibit smaller shadow sizes and reduced brightness compared to Schwarzschild black holes. Notably, in the case of thin spherical accretion, the shadow of the scalarized black hole is smaller, but the surrounding bright ring is more pronounced. Our results highlight the observable features of the scalarized black holes, providing a distinguishable probe from their counterpart in GR in strong gravity regime.