properties in detail and related them to the behavior of the X-ray flux and the X-ray pulsations. Any suitable model for the 1 Hz modulation has to explain the following key properties that we reported in § \[1Hz:results\]:

-   [In 2002 and 2005, it appears at the end of the fast decay, after the pulse phases have drifted by 0.2 cycles (Fig \[decay\])]{}

-   [It appears at a flux level that is consistent in the 2002 and 2005 outbursts (§ \[fastdecayjump\])]{}

-   [It appears sporadically only in a narrow range of luminosities ($\simless 15$mCrab, § \[qporelations\])]{}.

-   [It is very coherent in some observations while in some others it is a broad incoherent feature (Fig. \[modulation\] and \[QPO\])]{}

-   [In a few cases, its amplitude might be connected with the 401 Hz pulse phases (§ \[phasejump\], Fig. \[jumps\]).]{}

-   [Its fractional rms amplitude can be as high as 125% and then $10\%$ for the same X-ray luminosity (Fig. \[jumps\])]{}

-   [Its amplitude is energy dependent, rising with energy up to $\sim 17$keV (Fig. \[energy\])]{}

-   [Its rms amplitude is very high, up to 170% in the 10-16 keV band (Fig. \[energy\])]{}

-   [Its centroid frequency is quite stable and slightly 