Harmonic-resolved fluctuation spectrum for the same observations as in Fig. 3 (main panel, per the colour scale at the right). Rather dramatically, this shows that HW's three features are all phase-modulated. The low-frequency response, by contrast, is a mixture of phase and amplitude modulation, and the 12P₁ response appears to represent mostly amplitude modulation. This distinction can also be seen in the main panel where the low-frequency modulation is carried by smaller numbered Fourier harmonics than HW's drift modulation, which seem to peak about number 18 or so. In the central panel, the amplitude scale has been compressed by a factor of 3 to accentuate the three phase-modulated periodicities.