General relativistic aberration equation and measurable angle of light ray in Kerr spacetime
We will mainly discuss the measurable angle (local angle) of the light ray [Formula: see text] at the position of the observer [Formula: see text] instead of the total deflection angle (global angle) [Formula: see text] in Kerr spacetime. We will investigate not only the effect of the gravito-magnetic field or frame dragging due to the spin of the central object but also the contribution of the motion of the observer with a coordinate radial velocity [Formula: see text] and a coordinate transverse velocity [Formula: see text] where [Formula: see text] is the impact parameter ([Formula: see text] and [Formula: see text] are the angular momentum and the energy of the light ray, respectively) and [Formula: see text] is a coordinate angular velocity. [Formula: see text] and [Formula: see text] are computed from the components of the four-velocity of the observer [Formula: see text] and [Formula: see text], respectively. Because the motion of observer causes an aberration, we will employ the general relativistic aberration equation to obtain the measurable angle [Formula: see text] which is determined by the four-momentum of the light ray [Formula: see text] and the four-momentum of the radial null geodesic [Formula: see text] as well as the four-velocity of the observer [Formula: see text]. The measurable angle [Formula: see text] given in this paper can be applied not only to the case of the observer located in an asymptotically flat region but also to the case of the observer placed within the curved and finite-distance region. Moreover, when the observer is in radial motion, the total deflection angle [Formula: see text] can be expressed by [Formula: see text]; this is consistent with the overall scaling factor [Formula: see text] instead of [Formula: see text] with respect to the total deflection angle [Formula: see text] in the static case ([Formula: see text] is the velocity of the lens object). On the other hand, when the observer is in transverse motion, the total deflection angle is given by the form [Formula: see text] if we define the transverse velocity as having the form [Formula: see text].