def coreBurkAlpha(self, R, Rs, rho0, r_core, ax_x, ax_y):
        """
        deflection angle

        :param R:
        :param Rs:
        :param rho0:
        :param r_core:
        :param ax_x:
        :param ax_y:
        :return:
        """
        x = R * Rs ** -1
        p = Rs * r_core ** -1

        gx = self._G(x, p)

        a = 2 * rho0 * Rs ** 2 * gx / x

        return a * ax_x / R, a * ax_y / R