Equation Icon  { [ ]} 2 −1 photon rph = 2rG 1 + cos 3-cos (a∗) , (30 ) ( √ ------) bound rmb = 2rG 1 − a∗-+ 1 − a∗ , (31 ) { 2 } stable r = r 3 + Z − [(3 − Z )(3 + Z + 2Z )]1∕2 , (32 ) ms G ( 2 ) 1 1 2 ∘ -----2 horizon rH = rG 1 + 1 − a∗ , (33 ) ( ∘ -----2---2--) ergosphere r0 = rG 1 + 1 − a∗cos 𝜃 , (34 )