calc_x0
=======

.. py:function:: luminet.black_hole_math.calc_x0(bh_mass, a)

   Calculcate :math:`x_0` for Kerr black holes.

   .. math::

      x_0 = \sqrt{\frac{r_{ms}}{M}}

   :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 15k

   .. seealso:: :meth:`calc_innermost_orbit` for the calculation of :math:`r_{ms}`