All Figures

	Figure 1: Background subtracted, 2.4-10 keV PN light curves of Mrk 421. If no temporal gaps are encountered the time binning is 80 s; times are counted from the beginning of the actual exposure. The curves are labeled by the orbit of the observation.
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In the text

	Figure 2: Ratios of the raw spectra of individual observations divided by the spectrum of observation 165.
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In the text

	Figure 3: Continuously curved model fit to the PN Small Window mode data of Mrk 421during orbit 165 in the energy range 0.2-12 keV. The lower panel show the residual in units of  per energy bin.
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In the text

	Figure 4: Broken power law model fitted to the 1.5-12 keV data during orbit 165. The middle panel shows the ratio between model and data when the fitted model is extrapolated to lower energies. The lower panel shows the same ratio for the continuously curved best fit model.
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In the text

Figure 5: In the upper and lower panels we plot the normalized hard (black) and soft (grey) light curves for orbit 084 (timing mode) and the hardness ratios (HR) as a function of the total count rate, respectively. In the HR plot, points plotted as crosses correspond to the first 13 ksec part of the observation, when the total count rate was increasing. Points plotted as filled circles correspond to the remaining part of the observation, when the total band flux was decreasing. For clarity, in this and in the next 6 figures, we do not plot the errors of the points. Instead, we indicate in the right lower part of the HR plot the average errors with solid crosses.

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In the text

Figure 6: As Fig. 5 but for the Small Window mode data of orbit 084. In the HR plot, the crosses correspond to the first 4 ksec of the observation, when the source flux was increasing. Filled circles correspond to the following 18 ksec part of the observation when the total band count rate was decreasing. The crosses in the left part of the HR plot correspond to the 4 ksec long part of the observation (22 ksec after the start of the observation) when the source flux increased again, while open squares show the HR variations during the last part of the observation when the source flux decreased once more.

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In the text

Figure 7: The same as Fig. 5 but for orbit 165. The open squares in the HR plot correspond to the first 5 ksec part of the observation, where a flare appears in both the hard and soft bands (as clearly visible in the upper panel). Crosses show the HR variations for the following 20 ksec, during which the total band flux increased. Finally, the filled circles correspond to the last 10 ksec part of the observation, during which the total count rate decreased slightly.

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In the text

Figure 8: The normalized hard and soft band light curves (upper panel) and HR plot for orbit 171-1 (lower panel). This observation shows the strongest HR variations. Filled circles in the upper right part of the HR plot correspond to the first 15 ksec part of the observation ("Part A" in the upper panel) when the total band count rate was decreasing. During "Part B", the total count rate increased, and the corresponding HR variations are shown by open squares. Filled circles and crosses in the lower left part of the HR plot correspond to the the first and second part part of the last 17 ksec of the observation, when the source flux decreased and then increased again.

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In the text

Figure 9: The normalized hard and soft band light curves and HR plot for the orbit 171-2 observation (upper and lower panels, respectively). Open squares in the HR plot correspond to the HR variations during the first part of the observation (indicated as "Part A" in the upper panel). Crosses and filled circles show the HR variations during the rising and decaying part, respectively, of the prominent hard band flare which appears in the last part of the observation.

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In the text

	Figure 10: The same as the previous figures, for orbit 259. Crosses and filled circles in the HR plot correspond to the rising and decaying flux parts in the beginning and towards the end of the observation, respectively. Open squares show the HR variations during the middle part of the observation, when the total flux remained roughly stable.
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In the text

	Figure 11: Normalized light curves and HR plot for the orbit 440-1 observation (upper and lower panels, respectively). The open squares in the HR plot show that during the beginning of the observation ("Part A" in upper panel) the source does not show spectral variations. Filled circles correspond to the last part of the observation ("Part B") when the source flux was decreasing.
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In the text

	Figure 12: Normalized light curves and HR plot for the orbit 440-2 observation (upper and lower panels, respectively). The source is at a very low intensity state, and the light curves in both bands (upper panel) do not show any statistically significant variations. As a result, no spectral variability is observed either, as seen in the HR plot.
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In the text

Figure 13: CCF plots for the orbit 84 (timing mode) observation. In the upper panel we plot the CCF estimated using the 8 s binned soft and hard band light curves. In the lower two panels we plot the CCF estimated using Part A and B light curves, indicated with the underlying arrows in Fig. 5. In all plots, positive lags correspond to the soft band leading the hard band variations. The solid lines in the lower panels indicate the best fitting skew Gaussian model curves to the CCFs (see text for details).

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In the text

	Figure 14: Same as Fig. 13 for the data of orbit 84 (Small Window mode).
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In the text

	Figure 15: Same as Fig. 13 for the data of orbit 165.
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In the text

	Figure 16: The CCF plots for the orbit 171-1 light curves. As in the previous figures, the lower panels show the CCF of various parts of the observation.
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In the text

	Figure 17: Same as with the previous CCF figures, for the orbit 171-2 light curves. In this case, in the middle panel we also plot the auto-correlation function of the "Part A" soft band light curve (dashed line).
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In the text

	Figure 18: Same as with Fig. 17 for the orbit 259 light curves. The dashed line in the middle panel shows the ACF of the "Part A" soft band light curve.
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In the text

	Figure 19: The CCF plots for the whole length hard and soft band light curves of orbit 440-1 (upper panel), and for Parts A and B of the light curves (middle and lower panel, respectively) shown in Fig. 11.
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In the text

	Figure 20: Plot of the hard and soft band PSDF of Mrk 421 (empty and filled circles, respectively). The solid line shows the average ASCA/RXTE power spectrum of the source (from Kataoka et al. 2001).
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In the text

	Figure 21: Rate of spectral variability as a function of the count rate in the 2.4-10 keV energy band; open circles indicate time periods where the spectrum softens, filled circles when it hardens.
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In the text

	Figure 22: Spectral evolution of three particular flares as function of the total count rate. The flares start at the labels "1''; filled circles indicate the rising times of the flares, open circles the decaying times.
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In the text