2001ApJ...563..124Y

We present Chandra X-ray observations of the nearby Seyfert 1.5 galaxy NGC 4151. The images show the extended soft X-ray emission on the several hundreds of parsec scale with better sensitivity than previously obtained. We show that the hard X-ray component (greater than 2 keV) is spatially unresolved. The spectrum of the unresolved nuclear source may be described by a heavily absorbed (N_H≃3x10²² cm^–3), hard power law (Γ≃0.3) plus soft emission from either a power-law (Γ≃2.6) or a thermal (kT≃0.6 keV) component. The flux of the high-energy component has decreased from that observed by ASCA in 1993, and the spectrum is much harder (Γ∼0.3 in 2000 vs. Γ≃1.5 in 1993). The large difference between the soft and hard spectral shapes does not favor the partial covering or scattering model of the ``soft excess.'' Instead, it is likely that the hard and soft nuclear components represent intrinsically different X-ray sources. The stronger nuclear emission lines of those seen by the Chandra HETGS spectrum are detected. Spectra of the extended emission to almost 1 kpc northeast and southwest of the nucleus have also been obtained. The spectra of these regions may be described by either thermal bremsstrahlung (kT≃0.4-0.7 keV) or power-law (Γ≃2.5-3.2) continua plus three emission lines. There is an excellent correlation between the extended X-ray and [O III] λ5007 line emissions. We discuss the nature of the extended X-ray emission. Because there is no extended electron-scattered hard X-ray emission, an upper limit to the electron scattering column can be obtained. This upper limit is much too low for the soft X-rays to be electron-scattered nuclear radiation, unless the nucleus radiates soft X-rays much more strongly toward the extended regions than toward Earth, a situation we consider unlikely. We favor a picture in which the extended X-ray-emitting gas is heated in situ by the nuclear radiation. Some of the X-rays may originate from a hot phase that confines the warm ionized gas seen optically, although X-ray emission produced via photoionization by the nucleus is also likely. A faint, probably background, compact X-ray source lies ≃2'.2 from the nucleus to the southwest, approximately along an extension of the extended southwest X-ray and [O III] emission.