Origin of the "Disk-Line" Feature in the X-Ray Energy Spectrum of a Seyfert Galaxy, NGC4151

We have studied the origin of the broad and skewed feature at 4.5-7.5 keV in the energy spectra of NGC4151 using the ASCA and RXTE data. The feature consists of a narrow peak at 6.4 keV and a broad wing extended between 4.5-7.5 keV. An analysis of the long-term variations revealed that the feature became variable only on a time scale longer than 1.5x10^6 s. Through a comparison with the continuum variabilities, we found that the emission region of the excess flux at 4.5-7.5 keV has an extent of 10^17 cm. The broad and skewed feature at 4.5-7.5 keV may be explained by the so-called ``disk-line'' model. If so, the size of the line-emitting region, 10^17 cm, should be equal to several or ten-times the Schwarzschild radius of the central black hole. This results in a black hole mass of 10^11 solar mass, which may be too large for NGC4151. We propose an alternative explanation for the broad and skewed feature, i.e. a ``reflection'' model, which can also reproduce the overall energy spectra very well. In this model, cold matter with a sufficiently large column density is irradiated by X-rays to produce a reflected continuum, which constitutes the broad wing of the feature, and narrow fluorescent lines. The equivalent width of the iron fluorescent line (~2 keV) and the upper limit of its width (sigma < 92 eV) are also consistent with this model. From these results and considerations, we conclude that the ``disk-line'' model has difficulty to explain the spectral variations of NGC4151, and the reflection model is more plausible.