Photon rings and shadows of black holes with non-minimal couplings between curvature and electromagnetic field
Year: 2026
Type: article
Abstract: We investigate black holes with non-minimal couplings between the electromagnetic field and spacetime curvature tensors, focusing on their event horizons, shadows, and photon rings. Such couplings can naturally arise from effective field theories of gravity and quantum corrections in curved spacetime. Starting from a general action with three independent coupling terms, we derive static and spherically symmetric black hole solutions using a series expansion method. We find that all couplings enlarge the event horizon and photon sphere, while their observational consequences differ. The coupling F ν μ F μ ρ R ν ρ slightly increases the shadow radius and the separation between the zeroth- and first-order photon rings, leaving higher-order spacings nearly unchanged. The coupling F μν F σρ R μνσρ significantly enlarges the shadow and the separation between the zeroth- and first-order photon rings, but rapidly suppresses the spacing between higher-order rings. In contrast, the F 2 R coupling reduces the shadow radius and causes the zeroth- and first-order rings to nearly coincide, leading to an enhanced photon-ring brightness, while increasing the separation of higher-order rings and rendering them easier to resolve observationally. Backward ray-tracing simulations further confirm that these characteristic signatures persist in synthetic black-hole images. These findings provide potential observational signatures of non-minimal couplings between the electromagnetic field and spacetime curvature tensors.
Source: Physics Letters B
Authors: Zhixiang Yin, Changjun Gao, Yun-Long Zhang
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: diamond
OpenAlex ID: W7155083331