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Astron. Astrophys. 363, 863-868 (2000)

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5. Comparison with previous observations

The behaviour of Mrk 766 during the BeppoSAX observation is rather different than during the ASCA-ROSAT observation, performed on December 1993 and discussed by Leighly et al. (1997). The source was at an average flux level very similar in the two observations. While in both cases there is significant flux and spectral variability on time scales of thousands of seconds, the origin of the spectral variability appears to be different. In the ASCA-ROSAT observation, it is driven by significant changes in the spectral index of the power law, while in our observation it seems to be mostly related to changes in the warm absorber properties. Indeed, in our observation the power law index is consistent with being constant, but at a value significantly steeper (i.e. [FORMULA]2.2) than those found by Leighly et al. ([FORMULA]1.6-2). Moreover, in the ASCA-ROSAT observations a soft excess is strongly required by the data, while in our observation this component may be absent. (It is worth noting than even in the ASCA-ROSAT observations the soft excess is marginally visible when the power law is the steepest, because the soft component is almost constant during the observation; it may therefore be not surprising that in our observation the soft component is hardly detectable). A further difference is the presence, in the ASCA data and during the high flux phase, of an iron line with [FORMULA]100 eV equivalent width, while in the BeppoSAX data such a line is not visible (but the upper limit on the equivalent width is marginally consistent with the abovementioned value).

To summarize, in both observations the source underwent large and complex variability, but the pattern and origin of the variations seem to be very different in the two cases.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000