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Critical Emission from a High-Spin Black Hole


We consider a rapidly spinning black hole surrounded by an equatorial, geometrically thin, slowly accreting disk that is stationary and axisymmetric. We analytically compute the broadening of electromagnetic line emissions from the innermost part of the disk, which resides in the near-horizon region. The result is independent of the details of the disk model and therefore universal. This is an example of critical behavior in astronomy that is potentially observable by current or future telescopes.