The Challenge of Explaining the Nonlinear Features in the Light Curve of the ZZ Ceti Star G117-B15A

We present our response to a challenge raised publicly during the Barcelona EU-ROWD08 workshop concerning the modeling of nonlinearities observed in the light curves of pulsating white dwarfs. We have been able to explain quite successfully the nonlinear structure observed in the ZZ Ceti star G117-B15A, which was chosen at the outset because it was supposed to be an easy case. This was done on the basis of the nonlinear approach developed in Brassard, Fontaine, & Wesemael (1995), which includes explicitly the nonlinear response of the emergent flux to temperature variations, unlike the method used by our respected opponent, Dr. Mike Montgomery. The latter did not provide a response in Tubingen, so we must presume that he has not pursued this any further. Anticipating that this could happen, we did test on our own the method proposed by our opponent, only to find out that it is impossible, on its basis, to reproduce quantitatively the nonlinear structure observed in the "easy star" G117-B15A. Our point is that the nonlinear response of the flux should never be neglected in modeling of this kind.