The observer-dependent shadow of the Kerr black hole

Abstract

Motivated by inclination of the Earth's orbit that is not located at galactic plane for observing the shadow of Sgr A*, we consider the black hole shadow for arbitrary inclinations and different velocities of observers. A surprising finding of the study is that rotation axis of a black hole might not be extracted from its shadow, since the ways of the shadow getting distorted depend not only on the spin of the black hole, but also velocities of observers. Namely, appearance of the shadow could be rotated by an angle in observers' celestial sphere due to the observer in motion. In order to further confirm this result, a formalism is presented for calculating the shadow in terms of the velocity perturbations. Besides, we also consider the Earth's orbit for the shadow of Sgr A* by making use of this formalism.