We have studied the origin of the broad and skewed feature at 4.5–7.5 keV in the energy spectra of NGC 4151 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 1.5 \times 10 ^ { 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 } M _ { \odot } , which may be too large for NGC 4151 .
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 ( \sim 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 NGC 4151 , and the reflection model is more plausible .