a) χ2 significance contours for the emissivity index, ϵ, and the spin, a. Contours are based on Δχ2 = 2.30, 4.61, and 9.21, i.e., 68%, 90%, and 99% confidence for two parameters of interest, with respect to the best fit case with ϵ let free. Two minima are apparent, one indicating a low-spin black hole at an unphysically large ϵ, and one indicating an almost maximally spinning black hole consistent with emission from a thin accretion disk (ϵ = 3). b) χ2 as a function of a for ϵ = 3.
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