Appendix B: The light surface

The light surface is defined by (28) which limits the region defined by

$$\xi^2(x) a^2 m^2\omega' - 1 + 2\frac{m}{x}\left(1-a m\sqrt{\omega'}\right)^2 + \omega' x^2$$ =

$$\le$$ 0. (B.2)

In the general case (with $\omega'\le\omega'_{\rm max}$, this corresponds to a domain $x_{e} \le x^{-}_{\rm lc} \le x \le x^{+}_{\rm lc}$ including the Alfvén point (x_a=1). In the Minkowski case, $x^{-}_{\rm lc}=0$ and $x^{+}_{\rm lc}=\frac{1}{\sqrt{\omega'}}$.