Another interpretation of the disk-line profile of the Seyfert galaxy, MCG—6-30-15 Original Research Article

Time variabilities of the broad and skewed feature around 5–7 keV (so called “disk-line feature”) observed from the Seyfert galaxy, MCG—6-30-15 were studied based on the ASCA long observation of this source in 1999. The RMS 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 RMS 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 s to ∼105 s. 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 s. Whereas, the RMS variabilities in the continuum bands other than 5.0–6.6 keV do not decrease so largely as in 5.0–6.6 keV. This suggests a presence of another time variation on a time scale longer than 105 s, which appears only in the continuum bands other than the disk-line band. This energy-dependent time variation could be interpreted by introducing a time variation of absorption on a time scale longer than 105 s. 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 (almost free from absorption) in the energy spectrum tends to be a depressed part in a plot of the RMS variability as a function of energy. In fact, the energy-resolved RMS variability on a time-bin of 2 × 105 s shows a depression around 5 – 7 keV and the degree of the depression as a function of energy is similar to the disk-line profile in the energy spectrum. This suggests, alternatively to the disk-line model, that the disk-line feature could be a combination of a fairly narrow line at 6.4 keV and a broad hump around 5 – 7 keV as 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 s.