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H. Inoue, C. Matsumoto (ISAS)
Time variabilities of the broad and skewed feature around 5 -- 7 keV observed from MCG-6-30-15 were studied based on the ASCA long observation of this source in 1999. The relative variability defined as the standard deviation divided by the mean for a sequence of X-ray counts in a time bin was calculated as a function of energy, with changing the width of the time bin. An interesting finding is that the relative variability in the disk-line energy band (5.0--6.6 keV) decreases more largely than those in the other energy bands as the bin width increases from 104 sec to 105 sec. This variability decrease in the 5.0--6.6 keV band in association with the bin-width increase is consistent with a presence of a random variation of the flux on a time scale around 104 sec. However, those in the continuum bands other than 5.0--6.6 keV need another variation on a time scale longer than 105 sec. These variability changes can be interpreted by introducing a time variation of absorption on a time scale longer than 105 sec. If the absorption changes in time, the time variability of the flux should become larger in the energy range suffering larger absorption. This predicts that an excess part (free from absorption) in the energy spectrum tends to be a depressed part in the time variability. This is consistent with the depression of the time variability in the disk line energy band. This suggests, alternatively to the disk-line model, that the disk-line feature could be a result of a transparent spectral-window of warm absorbers on the line of sight which change their column densities on a time scale longer than 105 sec.