Novel inner shadows of the Kerr black hole with a tilted thin accretion disk

Abstract

The inner shadow of a black hole, as a projection of the event horizon, is regarded as a potential tool for testing gravitational theories and constraining system parameters. Whether this holds in the case of a tilted accretion disk warrants further investigation. In this paper, we employ a ray-tracing algorithm to simulate images of the Kerr black hole with a tilted thin accretion disk, with particular attention to the relationship between the inner shadow and system parameters. Our findings reveal that in the case of an equatorial accretion disk, the Kerr black hole exhibits a minimum inner shadow size of Smin = 13.075 M2, where M denotes the black hole mass. This minimum is achieved when the viewing angle is 0° and the spin parameter approaches 1. However, with a non-zero disk tilt, the inner shadow exhibits novel configurations — taking on petal, crescent, or eyebrow shapes — significantly smaller than Smin across various parameter spaces. This indicates that the inner shadow is highly sensitive to the accretion environment, suggesting that caution is needed when using it as a diagnostic tool for black holes. Notably, an observed inner shadow smaller than Smin would either indicate the presence of a tilted accretion disk or support the viability of modified gravity. Moreover, in certain parameter spaces, we identify the emergence of a dual-shadow structure, which could also serve as a probe for the tilted accretion disk.