Dynamics of null particles and shadow for general rotating black hole

TL;DR

This paper investigates how deviations from the Kerr metric in Johannsen black holes affect null particle dynamics and shadow images, constraining deviation parameters using M87* and SgrA* observations and exploring accretion disk deformations.

Contribution

It provides the first constraints on Johannsen black hole deviation parameters using shadow observations and analyzes their effects on accretion disk images.

Findings

Deviation parameters are constrained within specific ranges for M87* and SgrA*.

Deviations influence the shape and size of the black hole shadow.

Accretion disk images are deformed by deviation parameters, testable with future observations.

Abstract

The Johannsen black hole (BH) is a generic rotating BH admitting three constants of motions (energy, angular momentum, and Carter constant) and is characterized by four deviation parameters besides mass and spin, which could be a model-independent probe of the no-hair theorem. We systematically study the dynamics of null particles around Johannsen BH, revealing the effects of the deviation parameters on the BH shadow as well as the effects of spin. By using the shadow boundaries of M87* and SgrA*, for the first time, the deviation parameters of those BHs are constrained. The detailed results depend on the spin $a$ and inclination angle $ \theta_0$. Assuming $a=0.2$ and $\theta_0=15^{\circ}$, the deviation parameter $\alpha_{13}$ are constained within $\sim $ [-3.5, 6] for M87* observation and [-3, 0.5] for SgrA* observation. We also show the images of a Johannsen BH surrounded by a Page-Thorne thin accretion disk observed by a remote observer with a ray-tracing method and discuss the effects of the deviation parameters on deforming the accretion disk image, which could be tested by observations with higher sensitivities in the future.