10^{22}$cm$^{-2}$ is still consistent with the galactic N$_H$ as determined from radio measurements (Dickey & Lockman 1990). In this scenario, a large intrinsic absorption is present in the system and would be variable on a time scale longer than that of the long quiet intervals examined here.

Class $\phi$: state only
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Class $\phi$ is the second of the two classes where no variability is observed. An example of light curve and CD is shown in Fig. 12 (upper panels). Compared to class $\chi$, there is less white noise in the light curve (although in some cases red-noise variability is observed), and the points in the CD are much softer and more concentrated. The net count rate is $<$10 kcts/s. To determine whether this class corresponds to state or state , we need to compare the CD with that of an observation which shows both states. In Fig. 11 we show the average colors of all observations in class $\phi$ (see Table 1) together with the state points from Fig. 4. By comparing Fig. 4c and Fig. 11, it is evident that observations in class $\phi$ have to be identified with state . Note that the quietness of the light 