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Astron. Astrophys. 354, 847-852 (2000)

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3. The analysis of ampligrams and time scale spectra

The present study was therefore aimed at isolating the weak components of the X-ray photon trains using the technique described in Paper I and then searching for the differences between the two groups of sources. In order to ensure negligible influence from the apparent brightness, the brightest sources were removed from the data. Thus 3C 273 was removed from the QSO group and that only 8 S1 type sources were used: NGC 6814, NGC 6251, NGC 5506, NGC 7314, NGC 4151, NGC 4051, NGC 3227 and MCG-2-58. The subgroup selected according to the above limitations is described in Table 2.


Table 2. Subgroup of objects with low X-ray counting rates

All the data in the photon-count time series were converted into low-20 ampligrams, showing components corresponding to the lowest 20% of the maximum wavelet coefficient amplitude (see Paper I for detailed description of the method). The ampligrams were again wavelet-transformed to obtain a time-scale spectrum for each individual 1024-second sample. Time-scale spectra were then used to construct both the average time-scale spectrum for each category of sources and the corresponding graph of standard deviations, these are presented in Fig. 3.

[FIGURE] Fig. 3. A comparison of time-scale average spectra and their standard deviations for both categories of objects

There are obvious differences between the categories, especially for S1 and QSO. As can be seen, the structures in the S1 time-scale spectrum have larger extent in the vertical direction than those for the QSO time-scale spectrum. That means more deterministic structure in S1 variability data. Another difference is that the levels in the time-scale spectra and their standard deviations are about 2.5 times larger for QSO than for S1, which indicates a significantly larger variability for QSOs. As we have removed apparent brightness effects, this is an inherent characteristic of QSOs.

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© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000