Does the shape of the shadow of a black hole depend on motional status of an observer?

Abstract

In a recent work on rotating black hole shadows [Phys. Rev. D{\bf 101}, 084029 (2020)], we proposed a new approach for calculating size and shape of the shadows in terms of astrometrical observables with respect to finite-distance observers. In this paper, we introduce a distortion parameter for the shadow shapes and discuss the appearance of the shadows of static spherical black holes and Kerr black holes in a uniform framework. We show that the shape of the shadow of a spherical black hole is circular in the view of arbitrary observers, and the size of the shadows tends to be shrunk in the view of a moving observer. The diameter of the shadows is contracted even in the direction perpendicular to the observers' motion. This seems not to be understood as length contraction effect in special relativity. The shape of Kerr black holes is dependent on motional status of observers located at finite distance. In spite of this, it is found that there is not a surrounding observer who could view the shape of the Kerr black hole shadows as circularity. These results could be helpful for observation of the Sagittarius A* in the centre of the Milky Way, as our solar system is moving around the centre black hole.

20 pages, 10 figures; v2: published in PRD

References in corpus (8)

• First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
• Measurement of the Kerr Spin Parameter by Observation of a Compact Object's Shadow
• Shadow of a black hole surrounded by dark matter
• Magnetically charged black holes from non-linear electrodynamics and the Event Horizon Telescope
• Photon Rings around Kerr and Kerr-like Black Holes
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• Constraints on Barrow entropy from M87* and S2 star observations
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• Testing Born-Infeld $f(T)$ teleparallel gravity through Sgr A${}^{\star }$ observations
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• Geodesic analysis and black hole shadows on a general non--extremal rotating black hole in five--dimensional gauged supergravity
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• Aberration effect on lower-order images of thin accretion disk in the astrometric approach
• Determining parameters of Kerr black holes at finite distance by shadow observation
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• Optical and Thermodynamic Properties of a Rotating Dyonic Black Hole Spacetime in $\mathcal{N}=2,U(1{)}^2$ gauged supergravity