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Kazuki Takahashi, Hajime Inoue, Tadayasu Dotani, Origin of the “Disk-Line” Feature in the X-Ray Energy Spectrum of a Seyfert Galaxy, NGC 4151, Publications of the Astronomical Society of Japan, Volume 54, Issue 3, 25 June 2002, Pages 373–386, https://doi.org/10.1093/pasj/54.3.373
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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 |$1.5 \times 10^6 \,\mathrm{s}$|. 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 |$10^{17} \,\mathrm{cm}$|. 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, |$10^{17} \,\mathrm{cm}$|, 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 (|$\sim 2 \,\mathrm{keV}$|) and the upper limit of its width (|$\sigma < 92 \,\mathrm{eV}$|) 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.
