http://arxiv.org/abs/1401.1282
General relativity makes clear predictions about the spacetime geometry around black holes. In the near future, very long baseline interferometry facilities will be able to directly image the accretion flow around SgrA$^*$, the supermassive black hole candidate at the Center of our Galaxy, and open a new window to test gravity in the strong field regime. In this paper, we compute light curves and images associated with compact emission regions (hot spots) orbiting around Kerr and non-Kerr black holes. We study how the analysis of the properties of the radiation emitted by a hot spot can be used to test the Kerr nature of SgrA$^*$. Even in the most favorable case of hot spots orbiting at the innermost stable circular orbit, the sole observation of the hot spot light curve can more likely only constrain a combination of the black hole spin and of possible deviations from the Kerr solution. This happens because the same orbital frequency around a Kerr black hole can be found for a non-Kerr object with different spin parameter. Second order corrections to the light curve due to the background geometry are typically tiny and they are probably too difficult to identify. The degeneracy between the spin and the deformation parameters might be broken by accurate observations capable of identifying the radiation from the secondary image of the hot spot. Otherwise, the light curve information should be added to some other observable quantity in order to test the Kerr nature of SgrA$^*$.
Wed, 8 Jan 14
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