Shapes and Positions of Black Hole Shadows in Accretion Disks and Spin Parameters of Black Holes

Details Shapes and Positions of Black Hole Shadows in Accretion Disks and Spin Parameters of Black Holes Shapes and Positions of Black Hole Shadows in Accretion Disks and Spin Parameters of Black Holes

2004-05-06

arxiv.org/abs/astro-ph/0405099v1

J.Korean Phys.Soc.45:S1808-S1812,2004; Astrophys.J.611:996-1004,2004

Astronomy and Astrophysics

Astrophysics

13 pages, 6 figures, accepted for publication in ApJ

Scientific paper

10.1086/422403

Can we determine a spin parameter of a black hole by observation of a black hole shadow in an accretion disk? In order to answer this question, we make a qualitative analysis and a quantitative analysis of a shape and a position of a black hole shadow casted by a rotating black hole on an optically thick accretion disk and its dependence on an angular momentum of a black hole. We have found black hole shadows with a quite similar size and a shape for largely different black hole spin parameters and a same black hole mass. Thus, it is practically difficult to determine a spin parameter of a black hole from a size and a shape of a black hole shadow in an accretion disk. We newly introduce a bisector axis of a black hole shadow named a shadow axis. For a rotating black hole a shape and a position of a black hole shadow are not symmetric with respect to a rotation axis of a black hole shadow. So, in this case the minimum interval between a mass center of a black hole and a shadow axis is finite. An extent of this minimum interval is roughly proportional to a spin parameter of a black hole for a fixed inclination angle between a rotation axis of a black hole and a direction of an observer. In order to measure a spin parameter of a black hole, if a shadow axis is determined observationally, it is crucially important to determine a position of a mass center of a black hole in a region of a black hole shadow.

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