Assume that the inner event horizon r_ = m - Sqrt[m^2-a^2] is greater than the Ring singularity radius a. That is, a^2 < (m - Sqrt[m^2-a^2])^2 which for positive mass leads to imaginary a. We therefore must have the event horizon inside the Ring, and a < m or J < m^2. 

Note that if we further consider the inner ergosphere radius with respect to the Ring and upon demanding cos^2[theta] positive definite we get at the boundary a restriction on the angular momentum: J = m Sqrt[2m-cos^2]

This defines a band for J and M : m Sqrt[2m-1] < J < m Sqrt[2m].

See attached plot 

http://arxiv.org/pdf/0706.0622.pdf

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