Discovery of a broad and skewed line feature from Seyfert galaxies is one of the most remarkable results obtained by ASCA. The line feature has been interpreted as an iron fluorescent line originating from the inner most region of the accretion disk around the super-massive black hole. The broad and skewed feature may be produced by the superposition of emission lines from various regions of the accretion disk, which have different gravitational and Doppler shifts (so-called "disk line" model). However, it is sometimes difficult to separate the broad line feature from the continuum, and the disk line parameters are model dependent. Therefore, it is desired to analyze the broad feature in a model-independent manner, which is the goal of this thesis.
NGC4151 is a bright nearby (z=0.0032) Seyfert 1.5 galaxy. Presence of the broad and skewed line feature has been known in 4.5-7.5 keV from the past observations. Thus it is suited for the detailed analysis of the feature. X-ray data from the ASCA and RXTE satellites have been analyzed in this thesis; ASCA observed NGC4151 for 2 weeks continuously and RXTE observed it every 5 days covering more than a half year.
We confirmed the presence of the excess flux in 4.5--7.5 keV after the subtraction of the continuum, which is approximated by a power law modified by the partial covering absorption with two different column densities (NH ∼ 1.5×1023 and 5×1022 atoms/cm2). The excess flux consists of a narrow peak at 6.4 keV and a broad wing-like feature extended between 4.5--7.5 keV. We detected significant variations of the flux and the absorption column densities in the continuum spectra (0.7-24 keV) on time scale of 1.8×105 sec. On the other hand, the excess flux was found to be variable only on the time scale longer than 106 sec. Time variations of the excess flux were such that the profile did not change and only the normalization did.
Then, we investigated whether the light curve of the excess flux in 4.5-7.5 keV is a result of a smearing and/or a delay of the continuum flux. We found that if we introduced a smearing and a delay on a time scale of 3 ×106sec, the excess flux light curve became agreeable to the modified light curve of the continuum flux. Hence, the region emitting the excess flux in 4.5-7.5 keV should have an extent with a size of 1017cm.
Whereas, the column density of the order of ∼1023 atoms/cm2 varied significantly on a time scale of 105-106 sec with an amplitude of 20 %. Taking account of the percentage of the variation, the size of the absorber is indicated to be no larger than 1017 cm, which is just the size of the line emitting region. The absorber should be around or inside the line emitting region.
If we interpret the excess flux with the so-called disk line, we face some difficulties:
We propose an alternative explanation for the excess flux. Because the narrow line and the broad wing vary hand-in-hand, these two components are considered to have a strong physical connection. One of such models is the reflection model. In this model, cool matter with a sufficiently large column density is irradiated by X-rays to produce narrow fluorescence lines and reflected continuum. The latter shows highly absorbed profile, roughly approximated by the absorption column of 1024 atoms/cm2. We found that the reflection model can really reproduce the overall energy spectra, and avoid the difficulties in (1)-(3). The equivalent width (EW) of the narrow line with respect to the reflected continuum of ∼1 keV is also explained by this model. From these results and considerations, we conclude that the disk line model is difficult to explain the spectral variations of NGC4151, and the reflection model is much more plausible.