calc_Z1
=======

.. py:function:: luminet.black_hole_math.calc_Z1(bh_mass, a)

   Calculate :math:`Z1` for Kerr black holes.

   The variable :math:`Z1` is used to calculate the innermost orbit for Kerr black holes.

   .. math::

      Z_1 \equiv 1 + \sqrt[3]{1-a_*^2}\left[ \sqrt[3]{1+a_*} + \sqrt[3]{1-a_*} \right]

   :Parameters: * **bh_mass** (*float*) -- Mass of the black hole.
                * **a** (*float*) -- Specific angular momentum of the black hole. Should always be between :math:`-1` and :math:`1`. :math:`a > 0` if the accretion disk orbits in the same direction as the hole rotates; :math:`a < 0` if it orbits in the opposite direction.

   .. seealso:: :cite:t:`Page_1974` Equation 15l

   .. seealso:: :meth:`calc_innermost_orbit` for the calculation of the innermost orbit of Kerr black holes.