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On the Approximation of the Black Hole Shadow with a Simple Polar Curve

The Astrophysical journal, 2020-09, Vol.900 (1), p.77 [Peer Reviewed Journal]

2020. The American Astronomical Society. All rights reserved. ;Copyright IOP Publishing Sep 01, 2020 ;ISSN: 0004-637X ;EISSN: 1538-4357 ;DOI: 10.3847/1538-4357/aba59a

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  • Title:
    On the Approximation of the Black Hole Shadow with a Simple Polar Curve
  • Author: Farah, Joseph R. ; Pesce, Dominic W. ; Johnson, Michael D. ; Blackburn, Lindy
  • Subjects: Black hole physics ; Image resolution ; Shadows
  • Is Part Of: The Astrophysical journal, 2020-09, Vol.900 (1), p.77
  • Description: A black hole embedded within a bright, optically thin emitting region imprints a nearly circular "shadow" on its image, corresponding to the observer's line of sight into the black hole. The shadow boundary depends on the black hole's mass and spin, providing an observable signature of both properties via high-resolution images. However, standard expressions for the shadow boundary are most naturally parametrized by Boyer-Lindquist radii rather than by image coordinates. We explore simple, approximate parameterizations for the shadow boundary using ellipses and a family of curves known as limaçons. We demonstrate that these curves provide excellent and efficient approximations for all black hole spins and inclinations. In particular, we show that the two parameters of the limaçon naturally account for the three primary shadow deformations resulting from mass and spin: size, displacement, and asymmetry. These curves are convenient for parametric model fitting directly to interferometric data, they reveal the degeneracies expected when estimating black hole properties from images with practical measurement limitations, and they provide a natural framework for parametric tests of the Kerr metric using black hole images.
  • Publisher: Philadelphia: The American Astronomical Society
  • Language: English
  • Identifier: ISSN: 0004-637X
    EISSN: 1538-4357
    DOI: 10.3847/1538-4357/aba59a
  • Source: Alma/SFX Local Collection
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